Java 7th Sem Ashokkumar 44h64

[email protected]

=i;k--) System.out.print("\t"); for (int j=0;j<=i;j++) System.out.print("*\t"); System.out.println("\n"); } } }

Program to print the diamond pattern. class Six { public static void main (String arg[]) {

Mr. Ashok Kumar K | 9742024066 | [email protected]

www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. int n=5; for (int i=0; i =0;i--) { for (int j=0;j
Write a method named lastDigit that returns the last digit of an integer. For example, lastDigit(3572) should return 2. It should work for negative numbers as well. For example, lastDigit(-947) should return 7. class Seven { public static void main (String arg[]) { int n = -7676; int ld = lastDigit(n); System.out.println("Last Digit in "+n+" is .... "+ld); n = 3489; ld = lastDigit(n);


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. System.out.println("Last Digit in "+n+" is .... "+ld); } public static int lastDigit(int n) { return Math.abs(n%10); } }

Write a method named firstDigit that returns the first digit of an integer. For example, firstDigit(3572) should return 3. It should work for negative numbers as well. For example, firstDigit(-947) should return 9.

class Eight { public static void main (String arg[]) { int n = -7676; int ld = firstDigit(n); System.out.println("First Digit in "+n+" is .... "+ld); n = 3489; ld = firstDigit(n); System.out.println("First Digit in "+n+" is .... "+ld); } public static int firstDigit (int n) { n = Math.abs(n); while (n > 9) { n = n/10; } return n; } }


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest.

Write a recursive function in java to print all natural numbers from 1 up to (n-1) public class Nine { public static void main(String args[]) { int n = 10; series(n); } public static void series(int x) { if (x > 0) series(--x); if (x != 0) System.out.print(x + " "); } }

Write a Java program to print the random number in between 0 and n import java.util.Random; public class Ten { public static void main(String args[]) { Random r = new Random(); int n = 100; int ran = r.nextInt(n); System.out.println("Random number in betwwen 0 and "+n+" is .... "+ran); n = 200; ran = r.nextInt(n); System.out.println("Random number in betwwen 0 and "+n+" is .... "+ran);


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. n = 300; ran = r.nextInt(n); System.out.println("Random number in betwwen 0 and "+n+" is .... "+ran); n = 400; ran = r.nextInt(n); System.out.println("Random number in betwwen 0 and "+n+" is .... "+ran); } }

Write a Java program to print the command line arguments public class Eleven

{

public static void main (String arg[]) { System.out.println("Command line arguments are .. "); for (String s : arg) { System.out.println(s); } } }

Write a program to calculate the average among the elements {4,5,7,8}, using for each in java. How for each is different from for loop? public class Twelve {


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. public static void main(String args[]) { int arr[] = {4,5,7,8}; int sum = 0; for (int ele : arr) { sum = sum + ele; } System.out.println("Average = "+ (float) sum/arr.length); } }

1.5 Variables and Data types 1.5.1 Variables Variable is the name or the identifier given to the known memory location. It is used to store some program data/value which might be referenced in future. To put it in the other way, the memory location will be accessed through the variable name. Java defines the following type of variables 







Instance Variables (Non static) Variable for which a separate memory space will be created for each instance of a class is called a Instance variable. i.e., its value will be unique to each instance of a class. Instance variables will not be having 'static' keyword prefixed. They are also called non static variables. Class Variables (Static) Variable which actually shares the value among all the instances of a class is called a Class Variable. It is created by prefixing the keyword 'static' while declaring a variable. It tells the compiler that there is exactly one copy of this variable in existence regardless of how many times the class has been instantiated. It is also called static variable. Local Variables Any method in java will store its temporary state in a local variable. A variable which can be accessed only within its defining block can be called as a local variable for that block. It can't be accessed anywhere else outside this block. There is no special keyword to designate a variable as local. A variable by default becomes local if it's been defined within any block (open brace and closing brace) Parameters Parameters are the input variables for a method within a class.

1.5.2 Data types Mr. Ashok Kumar K | 9742024066 | [email protected]

www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. A Data type is the classification identifying one of the various types of data, which determines the possible values for that type. There are eight primitive data types and three reference data types in Java as described by the figure.

Fig: Data types in Java

Primitive Data types The Primitive data types are predefined data types, which always hold the value of the same data type, and the values of a primitive data type don't share the state with other primitive values. These data types are named by a reserved keyword in Java programming language. 

byte: The byte data type can be useful for saving memory in large arrays, where the memory savings actually matters



short: As with byte, the same guidelines apply: you can use a short to save memory in large arrays, in situations where the memory savings actually matters.



int: For integral values, this data type is generally the default choice unless there is a reason (like the above) to choose something else. This data type will most likely be large enough for the numbers your program will use, but if you need a wider range of values, use long instead



long: Use this data type when you need a range of values wider than those provided by int.



float: The float data type is a single-precision 32-bit IEEE 754 floating point. As with the recommendations for byte and short, use a float (instead of double) if you need to save memory in large arrays of floating point numbers.



double: The double data type is a double-precision 64-bit IEEE 754 floating point. For decimal values, this data type is generally the default choice. As mentioned above, this data type should never be used for precise values, such as currency.



char: The char data type is a single 16-bit Unicode character. It has a minimum value of '\u0000' (or 0) and a maximum value of '\uffff' (or 65,535 inclusive).



boolean: The boolean data type has only two possible values: true and false. Use this data type for simple flags that track true/false conditions. Data type

Size

Possible values

byte

8 bits (1 byte)

-128 to +127


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. short

16 bits (2 bytes)

-32768 to +32767

int

32 bits (4 bytes)

-2147483648 to +2147483647

long

64 bits (8 bytes)

-9223372036854775808 to +9223372036854775807

float

36 bits (4 bytes)

1.40129846432481707e-45 to 3.40282346638528860e+38

double

64 bits (8 bytes)

4.94065645841246544e-324d to 1.79769313486231570e+308d

char

16 bits (2 bytes)

0 to 65,535

boolean

1 bit

true or false

String data type In addition to the eight primitive data types listed above, the Java programming language also provides special for character strings via the java.lang.String class. Enclosing your character string within double quotes will automatically create a new String object; For example, String s = "Ashok Kumar"; String objects are immutable, which means that once created, their values cannot be changed. The String class is not technically a primitive data type, but considering the special given to it by the language; you'll probably tend to think of it as such.

Reference Data types A Reference data type is a variable that can contain the reference or an address of dynamically created object. These types of data type are not predefined like primitive data type. The reference data types are arrays, classes and interfaces that are made and handled according to a programmer need. 

Array: An array is a special kind of object that contains collection of similar type of elements. The java array enables the to store the values of the same type in contiguous memory allocations. The elements in an array are identified by an integer index which initially starts from 0 and ends with one less than number of elements available in the array. All elements of an array must contain the same type of value i.e. if an array is a type of integer then all the elements must be of integer type.

It is a reference data type because the class named as Array implicitly extends

java.lang.Object. There are two different ways of declaring an array as shown below: datatype[] arr_name = new datatype[arraysize]; Ex: int[] arr = new int[50]; datatype[] arr_name = {element 1, element 2, ................. , element n} Ex: int[] arr = {5, 10, 15, 20, 25}; 

Class: The name of a class is treated as a type in a java program, so that you can declare a variable of an object-type, and a method which can be called using that object- type variable. Whenever a variable is created, a reference to an


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. object is also created using the name of a class for its type i.e. that variable can contain either null or a reference to an object of that class. It is not allowed to contain any other kind of values. Such type is called reference types in Java. The object becomes an instance when the memory is allocated to that object using new keyword. Ex: class Box { int h, w, l; double getArea() { } void setValues (int h, int w, int l) { } } Box b = new Box(); // here b is an object/instance, Box is a class type 

Interface: The name of an interface can be used to specify the type of a reference. A value is not allowed to be assigned to a variable declared using an interface type until the object implements the specified interface. Hold on here, you will learn more about interfaces in later chapters.

1.5.3 Default values It's not always necessary to assign a value when a variable (field) is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler. Data Type

Default Value

byte

0

short

0

int

0

long

0L

float

0.0f

double

0.0d

char

'\u0000'

String (or any object)

null

boolean

false

1.5.4 Literals The new keyword isn't used when initializing a variable of a primitive type. Primitive types are special data types built into the language; they are not objects created from a class. A literal is the source code representation of a fixed value; literals are represented directly in your code without requiring computation.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. As shown below, it's possible to assign a literal to a variable of a primitive type: boolean result = true; char capitalC = 'C'; byte b = 100; short s = 10000; int i = 100000; 

Integer Literal: An integer literal is of type long if it ends with the letter L or l; otherwise it is of type int. Please use the upper case letter L because the lower case letter l is hard to distinguish from the digit 1. Following example shows the syntax for creating the literals in binary and hexadecimal number system. // The number 26, in decimalint decVal = 26; // The number 26, in hexadecimal int hexVal = 0x1a; // The number 26, in binary int binVal = 0b11010;



Floating-point Literal: A floating-point literal is of type float if it ends with the letter F or f; otherwise its type is double and it can optionally end with the letter D or d. The floating point types (float and double) can also be expressed using E or e (for scientific notation), F or f (32-bit float literal), and D or d (64-bit double literal; this is the default and by convention is omitted). Examples: double d1 = 123.4; // same value as d1, but in scientific notation double d2 = 1.234e2; float f1 = 123.4f;



Character and String Literals: Literals of types char and String may contain any Unicode (UTF-16) characters. If your editor and file system allow it, you can use such characters directly in your code. If not, you can use a "Unicode escape" such as '\u0108' (capital C with circumflex), or "S\u00ED Se\u00F1or" (Sí Señor in Spanish). Always use 'single quotes' for char literals and "double quotes" for String literals.



There is a special null literal that can be used as a value for any reference type. null may be assigned to any variable, except variables of primitive types. null is often used in programs as a marker to indicate that some object is unavailable.



Finally, there is another literal called a class literal, formed by taking a type name and appending ".class"; for example, String class. This refers to the object (of type Class) that represents the type itself.

1.5.5 Escape Characters Java s a few special escape sequences for char and String literals: \b (backspace), \t (tab), \n (line feed), \f (form feed), \r (carriage return), \" (double quote),


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. \' (single quote), and \\ (backslash).

1.5.6 Creating and Destroying Objects We know that, a class is a template (blue print) for an object, and an object is an instance of the class.

Creating Objects (Object instantiation) Consider a class definition as follows, class Box { int w; int l; int h; // This is a constructor Box(int w, int l, int h) { this.w = w; this.h = h; this.l = l; } long

getArea() { return l*w*h;

} } Object of this class can be created as follows: Box b1 = new Box(5, 10, 15); The object creation statement above have three parts:   

Declaration: LHS of the above statement constitutes the declaration. It is associated with variable name and object type. Instantiation: It is the new keyword in java that creates the object Initialization: The new operator is followed by a call to a constructor, which initializes the new object.

This object creation can be visualized as follows:

Declaring a variable to refer to another object: A variable can be created to refer to another existing object as follows: Box b2; b2 = b1;


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. This can be visualized as follows:

Destroying Objects Java provides a wonderful concept of Garbage Collection which performs automatic memory management, so you don't need to delete the object that you've created. JVM does it for you automatically through Garbage Collection. We will see the concept of Garbage Collection in deep at later chapters.

1.5.7 Access specifiers in Java One of the concepts in object-oriented programming is encapsulation. It concerns about the hiding of data in a class and making this class available only through methods. In this way the chance of making accidental mistakes in changing values is minimized. Java allows you to control access to classes, methods, and fields via so-called access specifiers. Java provides four different types of access specifiers

public public classes, methods, and fields can be accessed from everywhere. The only constraint is that a file with Java source code can only contain one public class which should be a main class.

protected protected methods and fields can only be accessed within the same class to which the methods and fields belong, also within its subclasses but not from anywhere else. You use the protected access level when it is appropriate for a class's subclasses to have access to the method or field, but not for unrelated classes.

private private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses. So, the private access specifier is opposite to the public access specifier. It is mostly used for encapsulation

default If you do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package.



1.6 Operators and Expressions 1.6.1 Operators Operators are such symbols that perform some operations on one or more operands. Operators are used to manipulate primitive data types. Once we declare and initialize the variables, we can use operators to perform certain tasks like asg a value, adding the numbers etc. Operators in Java are classified as follows: The list below is arranged in the order of their precedence (Highest to Lowest) 

Postfix Operators exp++, exp--



Unary Operators ++expr, --expr, +expr, -expr, ~, !



Multiplicative Operators *, /, %



Additive Operators +, -



Shift Operators <<, >>, >>>



Relational Operators <, >, <=, >=, instanceof



Equality Operators ==, !=



Bitwise Operators Bitwise AND (&) Bitwise EXCLUSIVE OR (^) Bitwise OR (|)



Logical Operators Logical AND (&&) Logical OR (||)



Ternary Operators ?:



Assignment Operators =, +=, -=, *=, /=, %=, &=, ^=, |=, <<=, >>=, >>>=

Shift Operators 

The signed left shift operator "<<" shifts a bit pattern to the left, and



The signed right shift operator ">>" shifts a bit pattern to the right.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. The bit pattern is given by the left-hand operand, and the number of positions to shift by the right-hand operand. 

The unsigned right shift operator ">>>" shifts a zero into the leftmost position, while the leftmost position after ">>" depends on sign extension.

Example: public class Example { public static void main(String[] args) { int num1 = -5; num1 = num1 >> 2; System.out.println(num1); int num2 = 5; num2 = num2 >>> 2; System.out.println(num2); int num3 = 5; num3 = num3 << 2; System.out.println(num3); } }

Bitwise Operators 

The bitwise & operator performs a bitwise AND operation.



The bitwise ^ operator performs a bitwise exclusive OR operation.



The bitwise complement operator ~ inverts a bit pattern; it can be applied to any of the integral types, making every "0" a "1" and every "1" a "0". For example, a byte contains 8 bits; applying this operator to a value whose bit pattern is "00000000" would change its pattern to "11111111".

Example public class Example { public static void main(String[] args) { int num1 = -5; num1 = num1 & 2; System.out.println(num1); int num2 = 5; num2 = num2 & 2;


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. System.out.println(num2); int num3 = 5; num3 = ~num3; System.out.println(num3); } }

Ternary Operator The ternary operator (also known as the conditional operator) can be used as an alternative to the Java if/then/else syntax, but it goes beyond that, and can even be used on the right hand side of Java statements. Syntax (condition) ? (truth block) : (false block) ; The condition gets evaluated first. If the condition evaluates to true, the truth block gets executed and returns, else the false block gets executed and returns. Example public class Example { public static void main(String[] args) { int a = 10; int b = 20; int min = (a < b) ? a: b; System.out.println("Min value is .. "+min); } }

Logical Operators public class Example { public static void main(String args[]) {


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. boolean a = true; boolean b = false; System.out.println("a && b = " + (a && b)); System.out.println("a || b = " + (a || b)); System.out.println("!(a && b) = " + !(a && b)); } }

1.6.2 Expressions An expression is a construct made up of variables, operators, and method invocations, which are constructed according to the syntax of the language, which evaluates to a single value. Some examples of expressions are illustrated in bold below: int cadence = 0; anArray[0] = 100; System.out.println("Element 1 at index 0: " + anArray[0]); int result = 1 + 2; // result is now 3 if (value1 == value2) System.out.println("value1 == value2"); The data type of the value returned by an expression depends on the elements used in the expression. The expression cadence=0 returns an int because the assignment operator returns a value of the same data type as its left-hand operand; in this case, cadence is an int. As you can see from the other expressions, an expression can return other types of values as well, such as boolean or String. The Java programming language allows you to construct compound expressions from various smaller expressions as long as the data type required by one part of the expression matches the data type of the other. Here's an example of a compound expression: 1 * 2 * 3 In this particular example, the order in which the expression is evaluated is unimportant because the result of multiplication is independent of order; the outcome is always the same, no matter in which order you apply the multiplications. However, this is not true of all expressions. For example, the following expression gives different results, depending on whether you perform the addition or the division operation first: x + y / 100

// ambiguous

You can specify exactly how an expression will be evaluated using balanced parenthesis: ‘(‘ and ‘)’. For example, to make the previous expression unambiguous, you could write the following: (x + y) / 100

// unambiguous, recommended

If you don't explicitly indicate the order for the operations to be performed, the order is determined by the precedence assigned to the operators in use within the expression. Operators that have a higher precedence get evaluated first. For example, the division operator has a higher precedence than does the addition operator. Therefore, the following two statements are equivalent: x + y / 100


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. x + (y / 100) // unambiguous, recommended When writing compound expressions, be explicit and indicate with parentheses which operators should be evaluated first. This practice makes code easier to read and to maintain.

1.7 Control Statements Java Control statements control the order of execution in a java program, based on data values and conditional logic. There are three main categories of control flow statements; Selection statements 

If



if-else



switch

Iteration statements 

while



do-while



for

Jump statements 

break



continue



return

1.7.1 Selection Statements

The if statement The if statement executes a block of code only if the specified expression is true. If the value is false, then the if block is skipped and execution continues with the rest of the program. You can either have a single statement or a block of code within an if statement. Note that the conditional expression must be a Boolean expression. Syntax: if ( ) { <statements> } Example: public class Example {


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. public static void main(String[] args) { int a = 10, b = 20; if (a > b) System.out.println("a > b"); if (a < b) System.out.println("b > a"); } }

The if –else statement The if-else statement is an extension of the if statement. If the statements in the if statement fails, the statements in the else block are executed. You can either have a single statement or a block of code within if-else blocks. Note that the conditional expression must be a Boolean expression. Syntax: if ( ) { <statements> } else { <statements> } Example: public class Example { public static void main(String[] args) { int a = 10, b = 20; if (a > b) System.out.println("a > b"); else System.out.println("b > a"); } }

The switch statement The switch case statement is a multi-way branch with several choices. A switch is easier to implement than a series of if/else statements. Structure of switch case: 

The switch statement begins with a keyword, followed by an expression that equates to a no long integral value.



Following the controlling expression is a code block that contains zero or more labeled cases.



Each label must equate to an integer constant and each must be unique.

Working of switch case: 

When the switch statement executes, it compares the value of the controlling expression to the values of each case label.



The program will select the value of the case label that equals the value of the controlling expression and branch down that path to the end of the code block.



If none of the case label values match, then none of the codes within the switch statement code block will be executed. Java includes a default label to use in cases where there are no matches.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. 

We can have a nested switch within a case block of an outer switch.

Syntax: switch (<non-long integral expression>) { case label1: <statement1> ; break; case label2: <statement2> ; break; … case labeln: <statementn> ; break; default: <statement> } Example: public class Example { public static void main(String[] args) { int a = 10, b = 20, c = 30; int status = -1; if (a > b && a > c) { status = 1; } else if (b > c) { status = 2; } else { status = 3; } switch (status) { case 1: System.out.println("a is the greatest"); break; case 2: System.out.println("b is the greatest"); break; case 3: System.out.println("c is the greatest"); break; default: System.out.println("Cannot be determined"); } } }

1.7.2 Iteration Statements

The while loop The while statement is a looping construct control statement that executes a block of code while a condition is true. You can either have a single statement or a block of code within the while loop. The loop will never be executed if the testing expression evaluates to false. The loop condition must be a boolean expression. Syntax: while ( ) {


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. <statements> } Example: public class Example { public static void main(String[] args) { int count = 1; System.out.println("Printing Numbers from 1 to 10"); while (count <= 10) { System.out.println(count++); } } }

The do-while loop The do-while loop is similar to the while loop, except that the test is performed at the end of the loop instead of at the beginning. This ensures that the loop will be executed at least once. Syntax: do { } while ( ); Example: public class Example { public static void main(String[] args) { int count = 1; System.out.println("Printing Numbers from 1 to 10"); do { System.out.println(count++); } while (count <= 10); } }

The for loop The for loop is a looping construct which can execute a set of instructions a specified number of times. It’s a counter controlled loop. Syntax: for ( ; ; ) { } Example: public class Example { public static void main(String[] args) {


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. System.out.println("Printing Numbers from 1 to 10"); for (int count = 1; count <= 10; count++) { System.out.println(count); } } }

1.7.3 Jump Statements

The break statement The break statement transfers control out of the enclosing loop (for, while, do or switch statement). You use a break statement when you want to jump immediately to the statement following the enclosing control structure. You can also provide a loop with a label, and then use the label in your break statement. The label name is optional, and is usually only used when you wish to terminate the outermost loop in a series of nested loops. Syntax: break; // the unlabeled form break ; // the labeled form Example: public class Example { public static void main(String[] args) { System.out.println("Numbers 1 - 10"); for (int i = 1;; ++i) { if (i == 11) break; // Rest of loop body skipped when i is even System.out.println(i + "\t"); } } }

The continue statement A continue statement stops the iteration of a loop (while, do or for) and causes execution to resume at the top of the nearest enclosing loop. You use a continue statement when you do not want to execute the remaining statements in the loop, but you do not want to exit the loop itself. You can also provide a loop with a label and then use the label in your continue statement. The label name is optional, and is usually only used when you wish to return to the outermost loop in a series of nested loops. Syntax: continue; // the unlabeled form continue ; // the labeled form Example: public class Example {


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. public static void main(String[] args) { System.out.println("Odd Numbers"); for (int i = 1; i <= 10; ++i) { if (i % 2 == 0) continue; // Rest of loop body skipped when i is even System.out.println(i + "\t"); } } }

The return statement The return statement exits from the current method, and control flow returns to where the method was invoked. Syntax: The return statement has two forms:  One that returns a value return val; 

one that doesn't returns a value return;

Example: public class Example { public static void main(String[] args) { int res = sum(10, 20); System.out.println(res); } private static int sum(int a, int b) { return (a + b); } }




VTU 7th sem B.E (CSE/ISE)

JAVA/ J2EE Notes prepared by

Mr. Ashok Kumar K 9742024066 | [email protected]


Unit 2:

Classes, Inheritance, Exceptions, Applets

Mr. Ashok Kumar K 9742024066 | [email protected]


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. 15 reasons to choose VTUPROJECTS.COM for your final year project work 1. Training from the scratch We train our students on all the languages and technologies required for developing the projects from the scratch. No prerequisites required. 2. Line by Line code explanation Students will be trained to such an extent where they can explain the entire project line by line code to their respective colleges. 3. Study Materials We provide the most efficient study material for each and every module during the project development 4. Trainers Each faculty in AKLC will be having 6+ years of corporate Industry experience and will be a subject matter expert in strengthening student's skillset for cracking any interviews. He will be having a thorough experience in working on both product and service driven industries. 5. Reports and PPTs Project report as per the university standards and the final presentation slides will be provided and each student will be trained on the same. 6. Video manuals Video manuals will be provided which will be useful in installing and configuring various softwares during project development 7. Strict SDLC Project development will be carried out as per the strict Software Development model 8. Technical Seminar topics We help students by providing current year's IEEE papers and topics of their wish for their final semester Technical seminars 9. Our Availability We will be available at our centers even after the class hours to help our students in case they have any doubts or concerns. 10. Weightage to your Resume Our students will be adding more weightage to their resumes since they will be well trained on various technologies which helps them crack any technical interviews 11. Skype/ Team viewer In case the student needs an emergency help when he/she is in their colleges, we will be helping out them through Skype/ Team viewer screen sharing 12. Practical Understanding Each and module in the project will be implemented and taught to the students giving practical real world applications and their use. 13. Mock demo and presentations Each student will have to prepare for mock demo and presentations every week so that he/she will be confident enough to demonstrate the project in their respective colleges 14. Communication & Soft skills Training We provide communication and soft skills training to each students to help improve their presentation and demonstration skills. 15. Weekly monitoring Each student will be monitored and evaluated on the status of the project work done which helps the students to obtain thorough understanding on how the entire project will be developed



2.1 CLASSES AND OBJECTS Here, in this topic we are going to explore the classes and objects concepts which are the basic building blocks of any object oriented programming language..

2.1.1 Basics Class A class is a blueprint or prototype from which objects are created. It's just a template for an object and describes an object’s behavior to the JVM.

Object An object is a real instance of a class. It can be any real world entity we come across in our life. Example an Animal is an Object, also a Bank is an Object, a Human is an Object etc. A class just describes how a Dog looks like. (Say, a dog has 4 legs, it barks, it eats etc.), but an Object refers to a real Dog

Syntax A class can be declared through the following syntax: class { type1 var1; type2 var2; ... typeN varN; type1 method1() { } type2 method2() { } .... typen methodN() { } } An object of the class can be created using the following syntax obj_name = new ( );

Example


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. Creating a class class Student { int regno; String name; double score; public void setStudentDetails(int a, String s, double d) { regno = a; name = s; score = d; } public void printStudentDetails() { System.out.println("Student regno : " + regno); System.out.println("Student name : " + name); System.out.println("Student score : " + score + "%"); } }

Creating an object Student stud = new Student();

Object's can be accessed using a dot operator as follows stud.setStudentDetails(24, "Arun", 45.89); stud.printStudentDetails();

Observations



Memory allocation for an object For understanding purpose, lets break down the object creation statement into two steps: Step 1: Creating a variable of class type that can refer to an object Student stud; Here ‘stud’ is just like a normal local variable (allocated in stack memory) for the defining block. It will be initially pointing to a null object (literally meaning NOTHING).

Step 2: Instantiation and Initialization stud = new Student(); Here the object is instantiated with using a new keyword in java. A dynamic memory will be created in the heap memory and the reference for this will be assigned to the local variable created in step 1. The object will also be initialized by JVM. It does so by making a call to the object's constructor (discussed later). You can arguments to the constructor within the parenthesis following the


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. class name on RHS of the above statement. These two steps can be visualized as follows



Instance Variables and Class Variables Variables within the class created without the keyword 'static' prefixed are called instance variables or object variables or non-static variables. A separate memory space for these types of variables will be created for each instance of a class. Variables within the class created with the keyword 'static' prefixed are called class variables or static variables. This type of variables actually shares the value among all the instances of a class. We will see in detail with an example, a bit later.



We learnt how to create an object. Then, who destroys them? , it's JVM. You don't need to worry about object destruction. It will be done through a process called Garbage Collection (discussed later).



Class name can be any legal Identifier.



A single .java file can contain any number of classes within it, but only one class can be made as public and it should be the main class (main class is the one which contains the main function).



The name of the .java file should be the name of the main class.

Example program for Classes and Objects // Class Definition.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. class Student { // Instance variable int regno; String name; double score; // Methods public void setStudentDetails(int a, String s, double d) { regno = a; name = s; score = d; } public void printStudentDetails() { System.out.println("Student regno : " + regno); System.out.println("Student name : " + name); System.out.println("Student score : " + score + "%"); } } // Main Class. public class Example1 { // Main function public static void main(String[] args) { // Object creation Student s1 = new Student(); s1.setStudentDetails(24, "Arun", 45.89); s1.printStudentDetails(); } } Output

2.1.2 Constructors Definition: A constructor is a special method that is used to initialize a newly created object and is called just after the memory is allocated for the object. It can be used to initialize the objects to required values at the time of object creation. It is not mandatory for the coder to write a constructor for the class. If you don't write a constructor, Java provides a default constructor that initializes the object to its default values. Data Type


Default Value

www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. byte

0

short

0

int

0

long

0L

float

0.0f

double

0.0d

char

'\u0000'

String (or any object)

null

boolean

false

A class contains constructors that are invoked to create objects from the class blueprint. Constructor declarations look like method declarations—except that they use the name of the class and have no return type.

Example Student has one constructor:

class Student { Student (int rno, String nm, double sc) { regno = rno; name = nm; score = sc; } }

Observations



Parameterized Constructor You can create more than one constructor for a class each having different signatures from one another. Sounds something familiar? Right, it implies the constructors can be overloaded. Purpose of having the constructor overloaded is to provide more than one ways of initializing an object. So, the of the object can initialize an object using any of the constructors provided. , if you don't provide any constructor, Java will provide a default zero argument constructors. Look at the below example, class Student { int regno; String name; double score; // Three argument constructor Student (int rno, String nm, double sc) { ....


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. } // Two argument constructor Student (String nm, double sc) { .... } // Zero argument constructor Student () { .... } } If the of a class has all the three values (name, regno, and score) for a particular student, he/she can use three argument constructor to create an object initializing it to these values, as follows: Student stud1 = new Student(009, "Neel", 89.56); If the of a class has only two values (name, and score) and the regno should be auto generated by the class itself, then he/she can use two argument constructor to create an object, as follows: Student stud1 = new Student( "Neel", 89.56); If the of a class doesn't have any idea about the initial values, he can simply go ahead and use the default zero argument constructor to create an object, and he can assign the values at any time later on. Student stud1 = new Student();



"this" keyword Java defines the 'this' keyword to be used within any method to refer to the current object. Within an instance method or a constructor, this is a reference to the current object — the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using 'this'. Uses of "this" keyword There are many uses of 'this' keyword, but we are very young at this point of time to explore everything. So let's see few uses of 'this' keyword which can be understood now: 

'this' keyword can be used to call a constructor within another constructor of the same class. If used, it must be the first statement in the constructor. class Student { int regno; String name; double score; // Three argument constructor Student (int rno, String nm, double sc) { this(); // call zero argument constructor this(nm, sc); // Call two argument constructor .... } // Two argument constructor Student (String nm, double sc) { ....


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. } // Zero argument constructor Student () { .... } } 

'this' keyword can be used to resolve the name space collision. Name space collision is the situation where the member variables of a class and the local variables within a constructor have same names. class Student { int regno; String name; double score; // Three argument constructor Student(int regno, String name, double score) { this.regno = regno; this.name = name; this.score = score; } }





'this' can be used to return an object, and 'this' can be used as a parameter for an object. class Student { .... public Student getStudent () { return this; } }

Constructor Chaining Like, 'this' keyword is actually a reference for the current object being used; the 'super' keyword is actually a reference to the parent class of the current object being used. If a class does not extend any parent class, then by default it's going to extend an Object class (present in java.lang package). So you can think Object class as the default parent (base class) for all the classes in Java. We are going to deal with 'super' keyword in later point of time. For now, you can think of 'super' keyword as the way to make a call to base class constructor from its derived class. The of a class is invoking a class' constructor, and this class' is ing the constructor call to its base class' constructor, and the base class' constructor in turn can the call to its base class' constructor, and so on. This chain of constructor calls starting from an object continuing till Object class' constructor can be referred as Constructor Chaining. class Student { Student() { // the constructor call to its base class' constructor super(); } }



2.1.3 Garbage Collection , when you instantiate an object using 'new' keyword, the memory for this object will be allocated in JVM heap.

Concept and Definition: Garbage collection is a mechanism provided by JVM to reclaim heap space from objects which are eligible for Garbage collection. Garbage collection relieves java programmer from memory management and gives more time to focus on business logic. Garbage Collection in Java is carried by a daemon thread called Garbage Collector. You cannot force Garbage collection in Java; it will only trigger if JVM thinks it needs a garbage collection based on Java heap size.

When an object becomes eligible for Garbage Collection? An object could be eligible for Garbage Collection in any of these scenarios.  All references of that object explicitly set to null.  Object is created inside a block and reference goes out scope once control exit that block.  Parent object set to null, if an object holds reference of another object and when you set container object's reference null, child or contained object automatically becomes eligible for garbage collection.

Observations Java Heap is divided into three generation for sake of garbage collection. These are  Young generation (further divided into three parts: Eden space, Survivor 1, and Survivor 2 space)  Tenured or Old generation, and  Perm area. New objects are created into young generation and subsequently moved to old generation. String pool is created in Perm area of Heap, garbage collection can occur in perm space but depends upon JVM to JVM. Minor garbage collection is used to move object from Eden space to Survivor 1 and Survivor 2 space and Major collection is used to move object from young to tenured generation. Whenever Major garbage collection occurs application threads stops during that period which will reduce application’s performance and throughput.

2.1.4 "finalize()" method Concept and Definition Every object in Java can have a finalize() method defined within it. Syntax to define a finalize() method is: protected void finalize() { // finalization code here } finalize() method is a special method in Java which is called just before the Garbage Collector reclaims this object. The intent is for finalize() to release system resources such as open files, open sockets, open database connections etc, before getting collected. The keyword protected is a specifier that prevents access to finalize() by code defined outside its class.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. Example: class Student { public void sayHello() { System.out.println("Hello world .."); } // finalize method protected void finalize() { System.out.println("Inside finalize method .. "); } } public class Example { // Main function public static void main(String[] args) { Student s1 = new Student(); s1.sayHello(); } }

Can you really guess the output of this program? If you are expecting the output to be the execution of both 'sayHello()' and 'finalize()' method, then your understanding on finalize() concept is not perfect yet. Below is the output of the above program.

As you can see, the finalize() method is not called here. , finalize() is not like a destructor in C++. finalize() method will not be called simply because the object moves out of scope, instead it will be called only when JVM performs a Garbage collection which is an undetermined process. There is an alternative to invoke finalize() method, you can initiate a Garbage Collection manually by making a call to System.gc() method. Replace the main function in above example with the one below: public class Example { // Main funnction public static void main(String[] args) { Student s1 = new Student(); s1.sayHello(); System.gc(); // Initiates Garbage Collection } } Output:



2.1.5 “static” Keyword It is possible to create a member of a class that can be used by itself, without reference to a specific instance. To create such a member, precede its declaration with the keyword 'static'. When a member is declared static, it can be accessed before any objects of its class are created, and without reference to any object. The keyword 'static' in Java can be prefixed to any of these:   

Variables Methods Blocks

Static Variables Static variables are also called class variables which actually shares the value among all the instances of a class. It is created by prefixing the keyword 'static' while declaring a variable. It tells the compiler that there is exactly one copy of this variable in existence regardless of how many times the class has been instantiated. Memory for static variables will be allocated in Code Segment of JVM memory. Example: class Student { int regno; String name; double score; static String principalName; Student(int r, String n, double s) { regno = r; name = n; score = s; } } In the main function, let’s create an object 'stud1', Student.principalName = “Hemanth”; Student stud1 = new Student(7, "Ajay", 76.45); Memory allocation at this point, looks like,



Let’s create another object 'stud2', and let’s change the principal’s name. Student stud2 = new Student(9, "Arun", 87.24); Student.principalName = “Chandrashekhar”; The memory allocation now looks like,

You can now realize from the above figure, the static variable 'principalName' is actually shared among 'stud1' and 'stud2', meaning, if you reassign a value to 'principalName' w.r.t 'stud2', this change also reflects in 'stud1' as well.

Static Methods Conceptually, static methods are same as static variables, i.e., they are independent of any specific instance of an object. They are bound at class level and not at object level. Static methods can be invoked without creating an instance of its defining class. The most familiar example for static method is our main function. We prefix the main function with static keyword thus allowing JVM to execute this method without instantiating main class. Example

class Student { int regno; String name; double score;


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. // Static method.. !! This can be called without creating an object public static void sayHello() { System.out.println("Hello everybody, Good Morning .. "); } // Non Static method.. !! This requires an object public void printStudentDetails() { System.out.println("Reg no .. " + regno); System.out.println("Name .. " + name); System.out.println("Score .. " + score); } } public class Test { // Main funnction public static void main(String[] args) { Student.sayHello(); // Right Student.printStudentDetails();

Student s1 = new Student(); s1.printStudentDetails();

// Wrong: Compiler throws an error here

// Right

} }

Restrictions on static methods   

They can only call other static methods. They must only access static data. They cannot refer to 'this' or 'super' in any way.

Static Blocks Static blocks are also called Static initialization blocks. A static initialization block is a normal block of code enclosed in braces, { }, and proceeded by the static keyword. Syntax: static { // Logic } A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The JVM guarantees that static initialization blocks are called in the order that they appear in the source code. , this code will be executed when JVM loads the class. JVM combines all these blocks into one single static block and then executes. Restriction on static methods also applies to static blocks. Example: class Student { int regno; String name; double score; static { System.out.println("I am the static block in the Student class");


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. } public static void sayHello() { System.out.println("Hello everybody, Good Morning .. "); } public void printStudentDetails() { System.out.println("Reg no .. " + regno); System.out.println("Name .. " + name); System.out.println("Score .. " + score); } static { sayHello(); } } public class Example { // Main funnction public static void main(String[] args) { System.out.println("I am the main function"); Student stud = new Student(); } static { System.out.println("I am the static block in the main class"); } } Output

2.1.6 Inner Classes and Nested Classes Java allows you to define a class within another class. Such a class is called a nested class as shown below: class OuterClass { class NestedClass { .... } .... } Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes.



class OuterClass { static class StaticNestedClass { .... } class InnerClass { .... } .... }

A nested class is a member of its enclosing class. Non-static nested classes (inner classes) have access to other of the enclosing class, even if they are declared private. Static nested classes do not have access to other of the enclosing class. Advantages of Nested Classes  

It increases encapsulation It constitutes to more readable and maintainable code

Example: class OuterClass { class InnerClass { public void makeCall() { sayHello(); sayGoodMorning(); } } static class StaticNestedClass { public void makeCall() { sayHello(); sayGoodMorning();

// RIGHT // RIGHT

// RIGHT // WORNG: static nested class can't access non static member

} } public static void sayHello() { System.out.println("Hello World .. "); } public void sayGoodMorning() { System.out.println("Good Morning .. "); } }



2.2 INHERITANCE Here, we are going to explore one of the most important characteristics of any object oriented programming language – Inheritance.

2.2.1 Basics Definition and Concept Generally, a process by which a child class acquires the properties (state and behavior) of its parent class is referred to as Inheritance. For example, Hyundai is a parent class whose properties are inherited by the classes named iTen, iTwenty, Verna, Getz etc. Inheritance can also be referred to the concept of Generalization, which is the process of extracting common characteristics (states and behavior) from two or more classes, and combining them into a generalized super class. A class that is derived from another class is called a subclass (also a derived class, extended class, or child class). The class from which the subclass is derived is called a superclass (also a base class or a parent class). Excepting java.lang.Object, which has no superclass, every class in Java has one and only one direct superclass. In the absence of any explicit superclass, every class is implicitly a subclass of java.lang.Object. Classes can be derived from classes that are derived from classes that are derived from classes, and so on, and ultimately derived from the topmost class, java.lang.Object. Such a class is said to be descended from all the classes in the inheritance chain stretching back to java.lang.Object. Below figure helps us to visualize the concept of Inheritance.

Syntax Inheritance in Java can be done by using 'extends' keyword likewise in the following syntax. class DerivedClass extends BaseClass { }

Example class Animal { public void eat() { System.out.println("EATING THRICE A DAY"); } public void sleep() { System.out.println("SLEEPING FOR 8 HOURS"); }


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. } class Dog extends Animal { public void bark() { System.out.println("Bow Bow"); } } class Cat extends Animal { public void meow() { System.out.println("Meow Meow"); } }

Inheritance provides code reuse Idea behind inheritance is, when you have the same piece of code repeated in more than one class, you can avoid this duplication of code by creating a general class that contains the duplicated code and having those classes extending this general class. Thus no need to write the duplicated code in all the subclasses, you are writing it only once in the general class. From the above example you can see that eat() and sleep() are the codes that are common to both Dog and a Cat. The duplication of the code is avoided by defining a general class Animal and having the eat() and sleep() method within it. Cat and Dog just extends the Animal class. Thus, we can conclude that Inheritance provides Code Reusability.

Observations



A subclass inherits all the public and protected (fields, methods, and nested classes) from its superclass. Constructors are not , so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.



A subclass does not inherit the private of its parent class. However, if the superclass has public or protected methods for accessing its private fields, these can also be used by the subclass.



Superclass variable can reference a subclass object, but not vice-versa. public class Example { public static void main(String[] args) { Animal animal = new Dog(); // RIGHT, because all Dogs are Animals animal.eat(); animal.sleep(); Dog dog = new Animal();

// WRONG, because all Animals are not Dogs

} }



Using super 

super can be used here to make a call to superclass constructor from the derived class constructor. Example:


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. class Animal { int num_of_legs; String type_of_animal; Animal (int num_of_legs, String type_of_animmal) { this.num_of_legs = num_of_legs; this.type_of_animal = type_of_animmal; } public void eat() { System.out.println("EATING THRICE A DAY"); } public void sleep() { System.out.println("SLEEPING FOR 8 HOURS"); } } class Dog extends Animal { String name_of_dog; Dog (int num_of_legs, String type_of_animmal, String name_of_dog) { super(num_of_legs,type_of_animmal ); // Invokes superclass constructor this.name_of_dog = name_of_dog; } public void bark() { System.out.println("Bow Bow"); } } class Cat extends Animal { String name_of_cat; Cat (int num_of_legs, String type_of_animmal, String name_of_cat) { super(num_of_legs,type_of_animmal ); // Invokes superclass constructor this.name_of_cat = name_of_cat; } public void meow() { System.out.println("Meow Meow"); } } public class Test { // Main funnction public static void main(String[] args) { Dog dog = new Dog(4, "Cornivorous", "Jimmy"); dog.eat(); dog.sleep(); dog.bark(); } } 

super can also be used to refer to superclass member when there is a name collision between superclass member and a derived class member. Example: class A { int i; }


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. class B extends A { int i; B (int a, int b) { this.i = a; super.i = b; } }



// Refers to derived class member // Refers to superclass member

Multilevel Inheritance As discussed before, Java does allows a class to extend another class which further extends another class and so on, eventually the top most class will be java.lang.Object. This type of inheritance is called as Multilevel Inheritance (more than one level of inheritance). Figure on the right gives an example for multilevel inheritance, where Object is the topmost superclass and class D is at the leaf level. The point of interest here is the order of execution of constructors when we create an object of the class at the leaf level. class A { A() { System.out.println("A's Constructor"); } } class B extends A { B() { System.out.println("B's Constructor"); } } class C extends B { C() { System.out.println("C's Constructor"); } } class D extends C { D() { System.out.println("D's Constructor"); } } public class Test { // Main funnction public static void main(String[] args) { D d1 = new D(); } } Output


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. We can conclude from the output that, the order of execution of constructors is in the same order as the derivation, from superclass to subclass. This is because super(); will be the first statement in every class' constructor: when you create an Object of class D, the D's class constructor will be invoked which es the call to its super class constructor C() which further es to B(), and eventually it es on the call to Object() where the initialization of the memory takes place to this program after which the constructor A() will be executed continuing till D() in the order of derivation.



Multiple Inheritance in Java No, Multiple inheritance is NOT allowed in Java. Multilevel inheritance is when a class inherits from more than one superclass. For example, Here, Pegasus inherits from both horse and a flying bird. This kind of inheritance given rise to many problems in C++ which can be thought of as a reason for Java discouraging it. We can think of two reasons why Java doesn't allows multiple inheritance  To avoid ambiguity error Ambiguity error arises when Horse and Bird both defines a function with the same name and Pegasus runs into a situation of dilemma upon invoking that function w.r.t its object.  Multiple inheritance will be rarely used.

How multiple inheritance is achieved in Java? Java s a limited form a multiple inheritance by allowing a class to inherit from one other class and an unlimited number of interfaces. Meaning, a class can extend only one class but it can implement unlimited number of interfaces. Example:


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. Class A is a base class which has been inherited by class A along with implementing the other three interfaces namely P, Q, and R. Programming Example: Suppose if you are a newly ed student in car driving school, assume you are allowed only to use the steering wheel of the car on day 1 while only the trainer will be having access to brake and the accelerator. The question here is how to implement the access restriction to the student on the car object? Interface comes as a solution for this. Idea behind interface is, rather than using the object directly, use it through an interface, so that you can only use that feature of an object which is defined in the interface. Definition: An interface is a reference type, similar to a class that can contain only constants, method signatures, and nested types. There are no method bodies. Interfaces cannot be instantiated—they can only be implemented by classes or extended by other interfaces

class Car { void start() { System.out.println("Starting Car .. "); } void killEngine() { System.out.println("Engine Killled .. "); } } interface Trainer { void moveRight(); void moveLeft(); void accelerate(); void brake(); } interface Student { void moveLeft(); void moveRight(); } class Hyundai extends Car implements Trainer, Student { public void moveRight() { System.out.println("Moving Right .. "); } public void moveLeft() { System.out.println("Moving Left .. "); } public void accelerate() { System.out.println("Accelerating .. "); } public void brake() { System.out.println("Applying brake .. "); } } public class Example { public static void main(String [] args) { Trainer trainer = new Hyundai(); trainer.moveLeft(); // RIGHT


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. trainer.moveRight(); trainer.accelerate(); trainer.brake();

// RIGHT // RIGHT // RIGHT

Student student = new Hyundai(); student.moveLeft(); // RIGHT student.moveRight(); // RIGHT student.accelerate(); // ERROR: student have no access to accelerate() student.brake(); // ERROR: student have no access to brake() } }

2.2.2 Overloading and Overriding The word "Polymorphism" refers to the ability to take more than one form. In of programming, the polymorphism refers to the process in which a member function of a class behaves differently for different inputs At this point of time, you can think of two different ways in which Java provides the Polymorphism feature.

Method Overloading Concept: Consider a Chess application, here we will have to implement the move() operation for all the pawns. Are we going to select 9 different names for move() function of different pawns? How confusing it is. To make it so simple and achieve polymorphism, we will define a different move() functions for different pawns but the name of all these functions will be the same, move(). Only thing that differs is the argument list within the parenthesis. We will 'Queen' object to call move() on queen, we will 'King' object to call move() on king, and so on. This way the of the object invokes only one function move(), and will observe different results depending on type of input. This is what we call method overloading. Definition: When you have more than one method with the same name but different arguments, the methods are said to be overloaded. The arguments list within the parenthesis should be differing either by type of parameters or the number of parameters. Please note, the return type of the functions will not come into picture here. Meaning, if two functions have same function name and argument list but with different return types, you can't call them as overloaded functions because return type alone can't decide it. Example: class A { public void sum (int a, int b) { System.out.println("Sum of "+a+" and "+b+" is .. "+(a+b)); }


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. public void sum(String a, String b) { System.out.println("Concatenation of "+a+" and "+b+" is .. "+a+b); } } public class Test { public static void main(String [] args) { A a1 = new A (); a1.sum(7, 9); a1.sum("AKLC ", "JMASTER"); } } Output

More on Overloading:  

Method overloading is called Static polymorphism because the call to the overloaded functions will be resolved at compile time. These are not overloaded functions, instead they lead to ambiguity errors void sum(int a) and int sum(int a) void sum(int a, float b) and void sum (float a, int b)

Method Overriding Concept: Consider our very first example on Inheritance, Dog and Cat extends Animal. Here we can see the Animal class defines the common (general) characteristics of all its subclasses, for instance sleep() method which makes the Animal to sleep for 8 hours. Assume a situation where all the animals in our application sleep for 8 hours but only the Dog sleeps for 4 hours. How will you implement this change in sleep() function which should reflect only in Dog class alone? Overriding is a solution for this. Overriding is nothing but simply redefining the sleep() function again the subclass (Dog class) so that the new definition hides the previous definition for sleep() function in parent class (Animal class). Definition: An instance method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as an instance method in the superclass overrides the superclass's method. Note that an overriding method can also return a subtype of the type returned by the overridden method. This is called a covariant return type.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. Example: class Animal { public void eat() { System.out.println("EATING THRICE A DAY"); } // Overridden method public void sleep() { System.out.println("SLEEPING FOR 8 HOURS"); } } class Dog extends Animal { // Overriding method public void sleep() { System.out.println("DOG SLEEPING FOR 4 HOURS"); } public void bark() { System.out.println("Bow Bow"); } } class Cat extends Animal { public void meow() { System.out.println("Meow Meow"); } } public class Test { public static void main(String [] args) { Dog dog = new Dog(); Cat cat = new Cat(); dog.sleep(); cat.sleep(); } } Output

More on Overriding:  

Method overriding is called Dynamic polymorphism because a call to overridden functions is resolved at runtime. Method overriding occurs only when the names and the type signatures of the two methods are identical. If they are not, then the two methods are simply overloaded.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. Dynamic Method Dispatch Concept: Here we will focus on method overriding in bit detail. We know that a base class reference can be given to a derived class object. The simple question I am going to answer in this topic is what if we call an overridden method on this object? Example: class Animal { public void eat() { System.out.println("EATING THRICE A DAY"); } // Overridden method public void sleep() { System.out.println("SLEEPING FOR 8 HOURS"); } } class Dog extends Animal { // Overriding method public void sleep() { System.out.println("DOG SLEEPING FOR 4 HOURS"); } public void bark() { System.out.println("Bow Bow"); } } class DomesticDog extends Dog { public void sleep() { System.out.println("DOG SLEEPING FOR 6 HOURS"); } } class WildDog extends Dog { public void sleep() { System.out.println("DOG SLEEPING FOR 2 HOURS"); } } public class Example { public static void main(String [] args) { Animal animal1 = new Animal(); Animal animal2 = new Dog(); Animal animal3 = new DomesticDog(); animal1.sleep(); animal2.sleep(); animal3.sleep();

// calls Animal's sleep() // calls Dog's sleep() // calls DomesticDog's sleep()

} }


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. Output

We can conclude from the above program, it is the type of the object being referred to (not the type of the reference variable) that determines which version of an overridden method will be executed. Therefore, if a superclass contains a method that is overridden by a subclass, then when different types of objects are referred to through superclass reference variable, different versions of the method are executed. Since this decision should be made base on the type of the object, the JVM can make this decision only at runtime since the object will not be available at the compile time. Definition: Dynamic method dispatch is the mechanism by which a call to an overridden method is resolved at run time, rather than compile time. Note: If you want to invoke an overridden method (base class method) from the overriding method (derived class method), you should use 'super' keyword as illustrated in below example. class DomesticDog extends Dog { public void sleep() { super.sleep(); // calls Dog' sleep() System.out.println("DOG SLEEPING FOR 6 HOURS"); } } , it’s not mandatory that your class should override the base class method. It is completely dependent on your implementation. Two questions for you at this point of time. 1. How will you enforce a restriction that a particular method in base class should never be overridden in the derived class? 2. How will you enforce a restriction that a particular method in base class should mandatorily be overridden in the derived class? Yes, Java provides a feature to enforce such restrictions. Following couple of topics deals with them.

2.2.3 "final" Keyword in Java You can use final keyword in java for three different uses.   

Variables can be made final Methods can be made final Class can be made final

'final' variables Concept and Definition A final variable can be only once assigned a value. This value cannot be changed latter. If final variable is used


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. in a class then it must assigned a value in a class constructor. Attempting to change the value of final variable/field will generate error. Syntax: final ; Example class Animal { int age = 8; final int legs = 4; // RIGHT: final variables can be assigned a value here final int tails; A () { tails = 1; // RIGHT: final variable can be assigned a value in constructor } void increment() { age = age + 10; legs = legs + 1; // ERROR: Can't modify the final variable tails = tails + 1; // ERROR: Can't modify the final variable } }

'final' methods: To prevent Overriding Concept and Definition If you want to enforce a restriction like, a particular method in the base class should never be overridden in the derived class, simply prefix that method definition with the 'final' keyword. A final method cannot be overridden by sub class. Example: class Animal { // This is a final method final public void eat() { System.out.println("EATING THRICE A DAY"); } public void sleep() { System.out.println("SLEEPING FOR 8 HOURS"); } } class Dog extends Animal { // WRONG: Cannot override a final method public void eat() { System.out.println("EATING TWICE A DAY"); } // RIGHT public void sleep() { System.out.println("SLEEPING FOR 4 HOURS"); } }

'final' classes: To prevent Inheritance Concept and Definition


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. Unless a class definition has been prefixed with 'final', any other class can override it. There is no restriction on that. Java provides a feature where you can restrict a class from being inherited. This can be done by prefixing a class definition with 'final' keyword. A class declared final cannot be sub classed. Other classes cannot extend final class. It provides some benefit to security and thread safety. Example class Animal { } // RIGHT final class Dog extends Animal { } // WRONG: Cannot subclass the final class Dog class DomesticDog extends Dog { }

2.2.4 Abstract classes in Java Concept There are situations in which you will want to define a superclass that declares the structure of a given abstraction without providing a complete implementation of every method. That is, sometimes you will want to create a superclass that only defines a generalized form that will be shared by all of its subclasses, leaving it to each subclass to fill in the details. To put it differently, you may encounter some situations where you need to enforce a restriction that some functions in the base class has to mandatorily overridden by the derived class. If you want to enforce this restriction on any method in base class, simply prefix its declaration with 'abstract' keyword. , you should not give the definition for abstract functions in the base class because mandatorily it will be defined in derived classes. If a class contains one or more abstract methods, it's like a class containing one or more methods without any definition (just the declaration), so it's obvious that the class is not complete until any other class inherits it and gives the definition for these methods. The conclusion is you cannot create a direct objects of the abstract class, all you can do is having its reference to the object of the derived class.

Definitions Abstract method An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon) like this: abstract void fun(int a); Abstract class An abstract class is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed. If a class includes abstract methods, the class itself must be declared abstract, as in: class A { abstract void fun(int a); }


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, the subclass must also be declared abstract

Example: // abstract class abstract class Animal { void sleep() { System.out.println("Sleeping for 8 hours"); } abstract void sound(); // abstract function }

class Dog extends Animal { public void sound() { System.out.println("BOW BOW"); } } class Cat extends Animal { public void sound() { System.out.println("MEOW MEOW"); } }

public class Example { public static void main(String [] args) { Animal animal1 = new Animal(); Animal animal2 = new Dog(); Animal animal3 = new Cat(); } }

// WRONG: Abstract class cannot be initiated // RIGHT // RIGHT



2.3 EXCEPTIONS 2.3.1 Basics Concept There's always a tendency for erroneous situations to occur either within a program or beyond it. For example consider a program which tries to divide a number 10 by an input number entered by the . What if the number entered by the is 0 (zero)? The program attempts to divide 10 by 0 (zero) which causes the application to crash. Another example for erroneous situation is when you try to create an object and there isn't enough space in the JVM heap to allocate the memory for it. These kind of erroneous situations are called Exceptions. Java provides a wonderful exception handling mechanism to avoid an application crash.

Definition Generally, Exceptions are such anomalous conditions which change the normal flow of execution of a program. Exceptions are used for signaling erroneous (exceptional) conditions which occur during the run time processing. In Java, Exception is an object that describes an exceptional condition that has occurred in a piece of code.

Java's default exception handling mechanism Let's see with the help of below example, what happens in Java if something goes wrong in the runtime. public class Test { public static void main(String[] args) { int a = 10; int b = 0; System.out.println("Before Division"); double res = a/b; System.out.println("After Division"); } } When an exception occurs within a method, the method creates (throws) an object to describe the exception. This object is called as Exception Object, which contains the complete information about what went wrong here, ex: type of exception, state of program, line number in the source where this exception occurred and so on. After a method throws an object, it will be caught by the block of code written to handle it. If you have not handled an exception in your program, the exception object will be caught by the runtime system and it handles it in its own way. i.e., default exception handling will come into picture which only does two things  

Print the error message in the console Terminate the program

You can realize this in the following output to the program shown above


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. You can see a short description of what went wrong and then the program has been terminated. The S.O.P statement written after the division has not been executed. The description of the error printed in the console is called the stack trace of the exception which prints the method call hierarchy. You can see the method call stack for the modified version of above program. You should clearly observe the type of object that's been created is ArithmeticException. It is one of the inbuilt classes in Java used to represent an exception caused during arithmetic operations. There are various other types of classes in Java to represent different scenarios where an exception occurs. public class Test { public static void main(String[] args) { System.out.println("Before Division"); fun1(); System.out.println("After Division"); } public static void fun1() { fun2(); } public static void fun2 () { fun3(); } public static void fun3 () { fun4(); } public static void fun4() { int a = 10; int b = 0; double res = a/b; } } Output

2.3.2 Different Type of Exceptions Generally, Exceptions can be broadly classified into Checked Exceptions and Unchecked Exceptions 

Checked Exceptions are also called as Compile time Exceptions which are checked by compiler to make sure the programmer has written the handler for these. If not, it will throw a compile time error.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. 

Unchecked Exceptions are also called as Runtime Exceptions which are not known to the compiler at compile time. Hence, compiler is not able to check them for the handlers.

Exception classes There may be various scenarios where an exception might occur. Arithmetic exception which we saw in previous topic is one among them. For exception in Java, there is a built in class used to represent the scenario. Hence, whenever an exception occurs, JVM will throw an object of the corresponding class which well describes the exception scenario. Below figure shows the exception class hierarchy. You can see, Throwable is the root of all the exceptions.

Throwable, declared in java.lang, is the root of entire exception family. It has two children, Exception and Error classes. 



Exception class: Object of its subclasses are created and thrown whenever unusual condition occur which can be caught and handled it programmatically. It can be either checked exceptions or unchecked exceptions. Example: ArithmeticException Error class: Error class will be created and thrown when a serious problem occurs beyond the application scope which can't be controlled within an application. Example: OutOfMemoryError will be thrown when JVM is running short of memory.

Checked Exception classes directly inherit the Exception class while Unchecked Exception inherits from a subclass of Exception called RuntimeException. Below table shows the complete list of all Checked Exception classes in Java Exception class ClassNotFoundException

Meaning Class not found.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. CloneNotedException IllegalAccessException InstantiationException InterruptedException NoSuchFieldException NoSuchMethodException

Attempt to clone an object that does not implement the Cloneable interface. Access to a class is denied. Attempt to create an object of an abstract class or interface. One thread has been interrupted by another thread. A requested field does not exist. A requested method does not exist.

Below table shows the complete list of all Unchecked Exception classes in Java Exception class ArithmeticException ArrayIndexOutOfBoundsException ArrayStoreException ClassCastException EnumConstantNotPresentException IllegalArgumentException IllegalMonitorStateException IllegalStateException IllegalThreadStateException IndexOutOfBoundsException NegativeArraySizeException NullPointerException NumberFormatException SecurityException StringIndexOutOfBounds TypeNotPresentException UnedOperationException

Meaning Arithmetic error, such as divide-by-zero. Array index is out-of-bounds. Assignment to an array element of an incompatible type. Invalid cast. An attempt is made to use an undefined enumeration value. Illegal argument used to invoke a method. Illegal monitor operation, such as waiting on an unlocked thread. Environment or application is in incorrect state. Requested operation not compatible with current thread state. Some type of index is out-of-bounds. Array created with a negative size. Invalid use of a null reference. Invalid conversion of a string to a numeric format. Attempt to violate security Attempt to index outside the bounds of a string. Type not found. An uned operation was encountered.

2.3.3 Java's Exception handling in brief Java provides five keywords for exception handling. They are dealt in detail in later topics. Here we will see them in brief.

try Program statements that you need to monitor for exception will be placed inside try block. If any exception occurs within a try block, the corresponding exception class is created and thrown.

catch A try block should be mandatorily followed by a catch block which will be executed if the exception object is thrown from the try block. Technically, we say that catch block catches the exception thrown by a try block.

throw A programmer can use throw keyword to manually throw an exception.

throws


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. All the checked exceptions thrown out of a particular method will be specified as such using the throws clause in the function signature.

finally If you want some piece of code to be executed without depending on whether a exception occurs or not, you can place such piece of code inside a finally block.

2.3.4 Using try and catch Concept Use try block to enclose the piece of code which you need to monitor for exceptions. Exceptions thrown from the try block will be caught by the corresponding catch block. A catch block is mandatory for every try block. , a single try block can have more than one catch block.

Syntax try { // Piece of code which you are monitoring for exception } catch (Exception_Class ex) { // This block will be executed if an exception occurs in above try block }

Example public class Example { public static void main(String[] args) { try { int a = 10; int b = 2; System.out.println("Before Division"); int res = a/b; System.out.println("After Division"); } catch (ArithmeticException e) { System.out.println("Exception occured"); } System.out.println("Bye"); } } public class Example { public static void main(String[] args) { try { int a = 10; int b = 0; System.out.println("Before Division"); int res = a/b; System.out.println("After Division"); } catch (ArithmeticException e) { System.out.println("Exception occured"); } System.out.println("Bye"); }


Output

Output

www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. } You can observe the above examples. In the first example there is no exception being thrown from the try block, hence the catch block will simple be skipped. In the second example there is an ArithmeticException thrown from the try block which is caught by the catch block which is capable of catching ArtithmeticException.

Observations

Displaying the description of the exception Exception description can be printed in the catch block using e.getMessage() as follows. catch (ArithmeticException e) { System.out.println("Exception occured"); System.out.println(e.getMessage()); } Output

Displaying the method call stack (stack trace) of the exception Call stack can be printed using e.printStackTrace() as follows. catch (ArithmeticException e) { System.out.println("Exception occured"); e.printStackTrace(); } Output

Catching Generic Exception If you are not sure about what kind of exception is thrown from a try block, you are still able to catch it using a generic catch clause as follows, which catches any type of exception thrown. try { int a = 10; int b = 0; int res = a/b;


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. System.out.println("No Exception"); } catch (Exception e) { System.out.println("Exception occured"); e.printStackTrace(); } Output

Multiple catch clauses As mentioned earlier, in the scenario where more than one type of exception will be thrown from a single try block, you can have more than one catch blocks, one corresponding to each type of exception object thrown. Example, below try block is capable of throwing ArithmeticException and ArrayIndexOutOfBoundsException. Hence I have written two catch blocks one for each of these exception. try { int arr[] = {2,4,5,6}; arr[10] = 90; int a = 10; int b = 0; int res = a/b; System.out.println("No Exception"); } catch (ArithmeticException e) { System.out.println("Exception occured"); e.printStackTrace(); } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Exception occured"); e.printStackTrace(); } Output

Unreachable code error When you use multiple catch statements, it is important to that exception subclasses must come before any of their super classes. This is because a catch statement that uses a superclass will catch exceptions of that type plus any of its subclasses. Thus, a subclass would never be reached if it came after its superclass. This is called unreachable code error in Java. Example


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. try { int arr[] = {2,4,5,6}; arr[10] = 90; int a = 10; int b = 0; int res = a/b; System.out.println("No Exception"); } catch (Exception e) { System.out.println("Exception occured"); e.printStackTrace(); } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Exception occured"); e.printStackTrace(); }

Unreachable catch block for ArrayIndexOutOfBoundsExceptio n. It is already handled by the catch block for Exception

try { int arr[] = {2,4,5,6}; arr[10] = 90; int a = 10; int b = 0; int res = a/b; System.out.println("No Exception"); } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Exception occured"); e.printStackTrace(); } catch (Exception e) { System.out.println("Exception occured"); e.printStackTrace(); }

Nested try catch blocks You can write a try block within another try block. This kind of try blocks is called Nested try blocks. Each time a try statement is entered, the context of that exception is pushed on the stack. If an inner try statement does not have a catch handler for a particular exception, the stack is unwound and the next outer try statement’s catch handlers are inspected for a match. This continues until one of the catch statements succeeds, or until the entire nested try statements are exhausted. If no catch statement matches, then the Java run-time system will handle the exception. Example try { int arr[] = { 2, 4, 5, 6 }; arr[2] = 90; try { int a = 10; int b = 20; int res = a / b; String s = null; System.out.println(s.length()); } catch (ArithmeticException e) {


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. System.out.println("Exception occured"); e.printStackTrace(); } System.out.println("No Exception"); } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Exception occured"); e.printStackTrace(); } catch (NullPointerException e) { System.out.println("Exception occured"); e.printStackTrace(); }

2.3.5 throw, throws, and finally

throw keyword Concept Regardless of who throws an exception (JVM or a programmer) it's always thrown with the throw statement. So far you have only been catching an exception thrown by Java runtime system. It's also possible for you to throw an exception manually using throw statement. Example try { ArithmeticException e = new ArithmeticException("This is thrown by me"); throw e; } catch (Exception e) { e.printStackTrace(); } Output

Observations 

Not all the objects can be thrown. , only those objects which are subclasses of Throwable can be thrown. Example public class Example { public static void main(String[] args) { try { Example ex = new Example();


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. throw ex; // ERROR: can't throw this object. } catch (Exception e) { e.printStackTrace(); } } } 

throw keyword will be mainly used for two purposes:  Re-throwing an exception object from the catch block  Throwing a custom exception (dealt later) Example for re-throwing an exception public class Example { public static void main(String[] args) { try { fun(); } catch (ArithmeticException e) { System.out.println("Exception caught in main function"); } } private static void fun() { try { int a = 10; int b = 0; int res = a/b; } catch (ArithmeticException e) { System.out.println("Exception caught in sub function"); throw e; } } } output

throws keyword Concept Exception thrown by a method can be handled either within the same method or in the caller of that method. We generally prefer the later one since caller of the method will be given full freedom to handle the exception in its own way. In this case, the method which throws an exception should indicate to the compiler that the exception handling is not done within it but it's done in the caller of that method, else it will give a compilation error.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. A method can indicate to the compiler that the exception handling is done in the caller of the method using throws clause in the method declaration. Example public class Example { public static void main(String[] args) { try { fun(); } catch (IOException e) { e.printStackTrace(); } } private static void fun() throws IOException { // This tries to open a file 'sample.txt' in the class path PrintWriter out = new PrintWriter(new FileWriter("sample.txt")); out.close(); } } Observations



Since compiler is aware of only checked exceptions, it will give a compilation error only for unhandled checked exceptions. Therefore, only checked exceptions should be specified in the throws clause mandatorily. However, there is no restriction like you should not specify unchecked exceptions in the throws clause

finally keyword Concept The finally block always executes when the try block exits. This ensures that the finally block is executed regardless of exception occurs or not. Putting cleanup code in a finally block is always a good practice, even when no exceptions are anticipated. Cleanup code meaning, releasing the external resources (like SQL connection, hard disk files etc). Example public class Test { public static void main(String[] args) { try { int a = 10; int b = 0; int res = a/b; System.out.println("No Exception occured"); } catch (Exception e) { System.out.println("Exception occured"); } finally { System.out.println("Executing finally block"); } } } public class Test {


Output

Output

www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. public static void main(String[] args) { try { int a = 10; int b = 10; int res = a/b; System.out.println("No Exception occured"); } catch (Exception e) { System.out.println("Exception occured"); } finally { System.out.println("Executing finally block"); } } }

2.3.6 Custom Exceptions Concept Java provides a wonderful feature where you can create your own exception class. This is definitely required in your application where you wish to handle certain abnormal situation in your own way. Example, in a banking application, when a person tries to withdraw the amount even though he have lesser than the minimum balance. Java doesn't have any built in exception class that describes this scenario. You need to define one.

Syntax: All you want to do is create a new class to represent the custom exception and extend it either directly the Throwable class or the Exception class or the RuntimeException class

Example: class LowBalanceException extends Exception { double amt; public LowBalanceException(double amt) { this.amt = amt; } public String toString() { return "HOLD ON! LOW ON BALANCE ! YOU CAN ONLY WITHDRAW "+amt+ " AT THIS TIME"; } } class { long accnum; static final double minBalance = 5000; double balance = 0; (long accnum, int openingBalance) { this.accnum = accnum; balance = openingBalance; }


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. void deposit(double amt) { balance = balance + amt; } void withdraw(double amt) throws LowBalanceException { if ((balance - amt) < minBalance) { LowBalanceException e = new LowBalanceException (balance minBalance); throw e; } else { balance = balance - amt; } } double getBalance() { return balance; } } public class Example { public static void main(String[] args) { try { acc = new (1234, 9000); System.out.println("OPENING BALANCE : " + acc.getBalance()); System.out.println("DEPOSITING 5000/-"); acc.deposit(5000); System.out.println("NEW BALANCE: " + acc.getBalance()); System.out.println("WITHDRAWING 3000/-"); acc.withdraw(3000); System.out.println("NEW BALANCE: " + acc.getBalance()); System.out.println("WITHDRAWING 12000/-"); acc.withdraw(12000); } catch (LowBalanceException e) { System.out.println(e); // Calls e.toString() } } } Output

You can observe the above program. class LowBalanceException extends Exception class thus it is a custom exception defined by us and it has overridden toString() method in Throwable class to print the appropriate exception message.



2.3.7 Chained Exceptions Concept It is possible to associate one exception with another exception, thus recording the exact cause for an exception. That is, one exception causing another exception is called Exception Chaining. These are the methods and constructors added in Throwable class to exception chaining: Throwable getCause() Throwable initCause(Throwable) Throwable(String, Throwable)

Throwable(Throwable)

This method returns the underlying exception This method associates an exception with another exception that actually caused this exception. This Constructor associates an exception with another exception that actually caused this exception. String argument is the description of cause exception This Constructor associates an exception with another exception that actually caused this exception.

Example class LowBalanceException extends Exception { public String toString() { return "HOLD ON! LOW ON BALANCE ! "; } } class EMIPaymentFailedException extends Exception { public String toString() { return "EMI PAYMENT FAILED DUE TO SOME REASONS."; } } class { long accnum; static final double minBalance = 5000; double balance = 0; (long accnum, int openingBalance) { this.accnum = accnum; balance = openingBalance; } void deposit(double amt) { balance = balance + amt; } double getBalance() { return balance; } void autoEMIPayment(double amt) throws EMIPaymentFailedException { try { if ((balance - amt) < minBalance) { EMIPaymentFailedException e = new throw e; } else { balance = balance - amt;


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. } } catch (EMIPaymentFailedException e) { LowBalanceException e2 = new LowBalanceException(); e.initCause(e2); throw e; } } } public class Test { public static void main(String[] args) { acc = new (1234, 9000); System.out.println("OPENING BALANCE : " + acc.getBalance()); try { acc.autoEMIPayment(2000); System.out.println("First EMI Payment Successful"); acc.autoEMIPayment(2000); System.out.println("Second EMI Payment Successful"); acc.autoEMIPayment(2000); System.out.println("Third EMI Payment Successful"); } catch (EMIPaymentFailedException e) { System.out.println("Exception during EMI payment"); System.out.println("Toplevel Exception .. "+e); System.out.println("Cause Exception .. "+e.getCause()); } } } Output

The above example is a slight modification of the previous example. I have created another custom exception and added a new method within class. You can see from the output, EMIPaymentFailedException is associated with LowBalanceException. That is, these two exceptions are in chain.



2.3 APPLETS 2.3.1 Basics Concept and Definition Applet is a Java program that can be embedded into HTML pages. Java applets run on the Java enables web browsers such as Mozilla and Internet explorer. Applet is designed to run remotely on the client browser, so there are some restrictions on it. Applet can't access system resources on the local computer. Applets are used to make the web site more dynamic and entertaining. The class Applet provides the foundation for creating applets in Java. The class Applet is contained in java.applet package

Two types of Applet 1. 2.

The first are those based directly on the Applet class. These applets use the Abstract Window Toolkit (AWT) to provide the graphic interface. The second types are those based on the Swing class JApplet.

Because JApplet inherits Applet, all the features of Applet are also available in JApplet.

Observations 

All applets are subclasses (either directly or indirectly) of Applet.



Applets are not stand-alone programs. Instead, they run within either a web browser or an applet viewer.



Execution of an applet does not begin at main( ). Instead, execution of an applet is started and controlled with an entirely different mechanism,



To use an applet, it is specified in an HTML file. One way to do this is by using the <APPLET> tag.



The applet will be executed by a Java-enabled web browser when it encounters the <APPLET> tag within the HTML file.

The Applet class The list of all methods contained within Applet class is given below along with the description. Return type

Method name and description

void

destroy() Called by the browser or applet viewer to inform this applet that it is being reclaimed and that it should destroy any resources that it has allocated.

AccessibleContext getAccessibleContext() Gets the AccessibleContext associated with this Applet.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. AppletContext

getAppletContext() Determines this applet's context, which allows the applet to query and affect the environment in which it runs.

String

getAppletInfo() Returns information about this applet.

AudioClip

getAudioClip(URL url) Returns the AudioClip object specified by the URL argument.

AudioClip

getAudioClip(URL url, String name) Returns the AudioClip object specified by the URL and name arguments.

URL

getCodeBase() Gets the base URL.

URL

getDocumentBase() Gets the URL of the document in which this applet is embedded.

Image

getImage(URL url) Returns an Image object that can then be painted on the screen.

Image

getImage(URL url, String name) Returns an Image object that can then be painted on the screen.

Locale

getLocale() Gets the locale of the applet.

String

getParameter(String name) Returns the value of the named parameter in the HTML tag.

String[][]

getParameterInfo() Returns information about the parameters that are understood by this applet.

void

init() Called by the browser or applet viewer to inform this applet that it has been loaded into the system.

Boolean

isActive() Determines if this applet is active.

Boolean

isValidateRoot()


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. Indicates if this container is a validate root. static AudioClip newAudioClip(URL url) Get an audio clip from the given URL. void

play(URL url) Plays the audio clip at the specified absolute URL.

void

play(URL url, String name) Plays the audio clip given the URL and a specifier that is relative to it.

void

resize(Dimension d) Requests that this applet be resized.

void

resize(int width, int height) Requests that this applet be resized.

void

setStub(AppletStub stub) Sets this applet's stub.

void

showStatus(String msg) Requests that the argument string be displayed in the "status window".

void

start() Called by the browser or applet viewer to inform this applet that it should start its execution.

void

stop() Called by the browser or applet viewer to inform this applet that it should stop its execution.

2.3.2 Applet Architecture Applets are event driven. An Applet waits until an event occurs. The AWT notifies the applet about an event by calling event handler that has been provided by the applet. The applet takes appropriate action and then quickly return control to AWT. All swing components descend from the AWT container class. initiates interaction with the applet (and not the other way around) An Applet skeleton import java.awt.*; import java.applet.*; /*


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. */ public class AppletSkel extends Applet { // Called first. public void init() { // initialization } /* Called second, after init(). Also called whenever the applet is restarted. */ public void start() { // start or resume execution } // Called when the applet is stopped. public void stop() { // suspends execution } /* Called when applet is terminated. This is the last method executed. */ public void destroy() { // perform shutdown activities } // Called when an applet's window must be restored. public void paint(Graphics g) { // redisplay contents of window } }

When an applet begins, following methods are called in sequence. 1. 2. 3.

init () start() paint()

When an applet is terminated, following methods are called in sequence. 1. 2.

stop() destroy()

init() The init() method is the first method to be called. This is where you should initialize variables. This method is called only once during the run time of your applet. start() The start( ) method is called after init(). It is also called to restart an applet after it has been stopped. Whereas init() is called once - the first time an applet is loaded, start( ) is called each time an applet’s HTML document is displayed onscreen. So, if a leaves a web page and comes back, the applet resumes execution at start(). paint()


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. The paint() method is called each time your applet’s output must be redrawn.The paint() method has one parameter of type Graphics. This parameter will contain the graphics context, which describes the graphics environment in which the applet is running. stop() The stop() method is called when a web browser leaves the HTML document containing the applet - when it goes to another page. destroy() The destroy() method is called when the environment determines that your applet needs to be removed completely from memory. At this point, you should free up any resources the applet may be using. The stop() method is always called before destroy().

2.3.3 A simple applet that sets the foreground and background colors and outputs a string. To output a string to an applet, use drawString( ), which is a member of the Graphics class. Typically, it is called from within either update( ) or paint( ). It has the following general form: void drawString(String message, int x, int y) Here, message is the string to be output beginning at x,y.

import java.awt.*; import java.applet.*; /* */ public class One extends Applet { String msg; public void init() { setBackground(Color.blue); setForeground(Color.white); msg = "Inside init( ) --"; } public void start() { msg += " Inside start( ) --"; } public void paint(Graphics g) { msg += " Inside paint( ) --"; g.drawString(msg, 10, 30); } } Output:



2.3.4 Requesting Repainting The repaint() method is defined by the AWT. It causes the AWT run-time system to execute a call to your applet’s update() method, which, in its default implementation, calls paint(). The repaint() method has four forms. 1. 2. 3. 4.

void void void void

repaint() repaint(int left, int top, int width, int height) repaint(long maxDelay) repaint(long maxDelay, int x, int y, int width, int height)

Here, left & top are the co-ordinates height & width are the region's height and width maxDelay specifies the maximum number of milliseconds that can elapse before update() is called.

Example: Simple Banner Applet import java.awt.*; import java.applet.*; /* */ public class SimpleBanner extends Applet implements Runnable { String msg = " A Simple Moving Banner."; Thread t = null; int state; boolean stopFlag;


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. public void init() { setBackground(Color.blue); setForeground(Color.white); } // Start thread public void start() { t = new Thread(this); stopFlag = false; t.start(); } // Entry point for the thread that runs the banner. public void run() { char ch; for (;;) { try { repaint(); Thread.sleep(250); ch = msg.charAt(0); msg = msg.substring(1, msg.length()); msg += ch; if (stopFlag) break; } catch (InterruptedException e) { } } } // Pause the banner. public void stop() { stopFlag = true; t = null; } // Display the banner. public void paint(Graphics g) { g.drawString(msg, 50, 30); } }

2.3.5 Using the status window. showStatus() function can be used to print the message in the status bar of an applet

Example: import java.awt.*; import java.applet.*; /* */ public class StatusWindow extends Applet { public void init() { setBackground(Color.blue); setForeground(Color.white); } public void paint(Graphics g) { g.drawString("This is in the applet window.", 10, 20); showStatus("This is shown in the status window.");


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. } } Output:

2.3.6 The HTML Tag Oracle currently recommends that the APPLET tag be used to start an applet from both an HTML document and from an applet viewer. Syntax of Applet tag is given below: < APPLET [CODEBASE = codebaseURL] CODE = appletFile [ALT = alternateText] [NAME = appletInstanceName] WIDTH = pixels HEIGHT = pixels [ALIGN = alignment] [VSPACE = pixels] [HSPACE = pixels] > [< PARAM NAME = AttributeName VALUE = AttributeValue>] [< PARAM NAME = AttributeName2 VALUE = AttributeValue>] ... [HTML Displayed in the absence of Java]     

CODEBASE is an optional attribute that specifies the base URL of the applet code, which is the directory that will be searched for the applet’s executable class file CODE is a required attribute that gives the name of the file containing your applet’s compiled .class file. ALT tag is an optional attribute used to specify a short text message that should be displayed if the browser recognizes the APPLET tag but can’t currently run Java applets. NAME is an optional attribute used to specify a name for the applet instance. WIDTH and HEIGHT are required attributes that give the size (in pixels) of the applet display area.


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest.   

ALIGN is an optional attribute that specifies the alignment of the applet. VSPACE and HSPACE are optional. VSPACE specifies the space, in pixels, above and below the applet. HSPACE specifies the space, in pixels, on each side of the applet. PARAM NAME and VALUE: The PARAM tag allows you to specify applet-specific arguments in an HTML page.

2.3.7 ing parameters to Applets The <APPLET> tag in HTML allows you to parameters to your applet. To retrieve a parameter, use the getParameter() method. It returns the value of the specified parameter in the form of a String object. Example: import java.awt.*; import java.applet.*; /* <param name=fontName value=Courier> <param name=fontSize value=14> <param name=leading value=2> <param name=Enabled value=true> */ public class ParamDemo extends Applet { String fontName; int fontSize; float leading; boolean active; public void start() { String param; fontName = getParameter("fontName"); if (fontName == null) fontName = "Not Found"; param = getParameter("fontSize"); try { if (param != null) fontSize = Integer.parseInt(param); else fontSize = 0; } catch (NumberFormatException e) { fontSize = -1; } param = getParameter("leading"); try { if (param != null) leading = Float.valueOf(param).floatValue(); else leading = 0; } catch (NumberFormatException e) { leading = -1; } param = getParameter("Enabled"); if (param != null) active = Boolean.valueOf(param).booleanValue(); } public void paint(Graphics g) { g.drawString("Font name: " + fontName, 0, 10); g.drawString("Font size: " + fontSize, 0, 26);


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. g.drawString("Leading: " + leading, 0, 42); g.drawString(" Active: " + active, 0, 58); } }

2.3.8 getDocumentBase() and getCodeBase() getDocumentBase() returns the URL of the directory holding the HTML file that started the applet. getCodeBase() returns the URL of the directory from which the applet’s class file was loaded.

Example: import java.awt.*; import java.applet.*; import java.net.*; /* */ public class Bases extends Applet { public void paint(Graphics g) { String msg; URL url = getCodeBase(); // get code base msg = "Code base: " + url.toString(); g.drawString(msg, 10, 20); url = getDocumentBase(); // get document base msg = "Document base: " + url.toString(); g.drawString(msg, 10, 40); } } Output:

2.3.9 Playing Audio files using AudioClip Interface Below example is a Java applet which continuously plays an audio clip named “anthem.wav” loaded from applets parent directory. import java.applet.Applet;


www.vtuprojects.com | Final year IEEE project development and training from scratch by Mr. Ashok Kumar K. Registration started. 9742013378 or 9742024066 and lock your project at the earliest. import java.applet.AudioClip; import java.awt.Graphics; public class LoadSoundApplet extends Applet { AudioClip audioClip; public void init() { audioClip = getAudioClip(getCodeBase(), "anthem.wav"); } public void paint(Graphics g) { audioClip.loop(); } } Necessary HTML file to run the above applet is given here: Applet sound example 3h436x

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