J A V A SHASHI BHUSHAN

MAIN ISSUES Object Oriented Concepts Examples FAQs

Abstract Classes & Methods  An abstract class is a class that can only be sub-classed - it can not be instantiated.  It contains abstract methods with no implementation. Contd.

Abstract Classes & Methods  Used to create a template class or methods.  Similar to function prototypes in C or C++.

Abstract Classes & Methods abstract class GraphicsObject { int x, y; : void moveTo (int newX, int newY) { : } abstract void draw ( ) ; // to be implemented by all subclasses // } Contd.

Abstract Classes & Methods: Each non-abstract subclass of Graphic object such as circle and rectangle has to provide an implementation for the draw method. class Circle extends GraphicObject { void draw ( ) { : } Contd.

Abstract Classes & Methods: class Rectangle extends GraphicObject { void draw ( ) { : } Contd.

OOP : Interface  An interface is a collection of abstract methods (without implementations) and constant values  Interfaces add most of the functionality that is required for many applications which would normally resort to using multiple inheritance in a language such as C++. Contd. …

OOP : Interface Interfaces are useful for:  Capturing similarities between unrelated classes without forcing a class relationship.  Declaring methods that one or more classes are expected to implement.

Interfaces Do Not Provide Multiple Inheritance:  The interface hierarchy is independent of the class hierarchy – classes that implement the same interface may or may not be related through the class hierarchy. This is not true for multiple inheritance.

OOP : Interface Declaration  [Public] interface InterfaceName [extends list of SuperInterfaces]{ the interface body }

OOP : The Interface Body interface Collection { int MAXIMUM = 500; void add(Object obj); void delete(Object obj); Object find(Object obj); int currentCount( ); }

OOP : Implementing an Interface class FIFOQueue implements Collection { ……… void add(Object obj) { ……… } Contd.

OOP : Implementing an Interface void delete(Object obj) { ……… } Contd.

OOP : Implementing an Interface Object find (Object obj) { ……… } Contd.

OOP : Implementing an Interface int currentCount( ) { ……… }

Example 1 (The Math Class) Below is a small program that uses some of the Math class methods: public class MyMath { public static void main(String args[ ] ) { int x; double rand, y, z; float max; Contd.

Example 1 (The Math Class) rand = Math.random( ); x = Math.abs (-123); y = Math.round ( ); z = Math.pow (2, 4); Contd.

Example 1 (The Math Class) System.out.println(rand); System.out.println(x); System.out.println(y); System.out.println(z); System.out.println(max); } Contd.

Example 1 (The Math Class) This program produces the following output: $ java MyMath le+10

Example 2 (The Character Class) This program below uses some of the methods in the Character class: public class Mychar { public static void main (String args[ ]) { char c1 = ‘A’; char c2 = ‘z’; char c3 = ‘5’; Contd.

Example 2 (The Character Class) System.out.println(“Is character” + c1 +” uppercase?” + Character.isUpperCase(c1)); System.out.println(“Is character” + c2 +” uppercase?” + Character.isUpperCase(c2)); Contd.

Example 2 (The Character Class) System.out.println(“Is character” + c3 +” a digit? ” + Character.isDigit(c3)); System.out.println(“Convert” + c1 +” to lowerercase:” + Character.toLowerrCase(c1)); } Contd.

Example 2 (The Character Class) This program produces the following output: $ java Mychar Is character A uppercase? true Is character z uppercase? false Is character 5 a digit? true Convert A to lowercase: a

Example 3 (Using Command-Line Parameters) In this example, a program is written that will perform binary operations on integers. The program receives three parameters: an operator and two integers. For example, to add two integers, the following command could be used: java TestCommandParameters Contd.

Example 3 (Using Command-Line Parameters) This program will display the following output: = 5 Contd.

Example 3 (Using Command-Line Parameters) public class TestCommandParametrs { public static void main (String [ ] args) { int result = 0; if (args.length ! = 3) { System.out.println( “please use java operator operand1 operand2”); System.exit(0); } Contd.

Example 3 (Using Command-Line Parameters) switch (args[0].charAt(0)) { case ‘+’ : result = Integer.parseInt(args[1]) + Integer.parseInt(args[2]); break; { case ‘ - ’ : result = Integer.parseInt(args[1]) - Integer.parseInt(args[2]); break; Contd.

Example 3 (Using Command-Line Parameters) { case ‘ * ’ : result = Integer.parseInt(args[1]) * Integer.parseInt(args[2]); break; { case ‘ / ’ : result = Integer.parseInt(args[1]) / Integer.parseInt(args[2]); break; Contd.

Example 3 (Using Command-Line Parameters) System.out.println(args[1] + args[0] + args[2]+”=“ +result); }

Example 4 (Computing Fibonacci Numbers) public class TestFibonacci{ public static void main ( String args[ ] ) { int n = 5; System.out.println (“fibonacci number at index “+n” is+ fib (n)); }

Example 4 (Computing Fibonacci Numbers) public long fib (long n) { if (n == 0) || (n==1) return 1 else return fib(n-1) + fib(n-2); }

Recursive Calls fib(5) fib(4) fib(3) fib(2) fib(1) fib(2)fib(1) fib(0)fib(1)fib(0) fib(1)fib(0)

F A Q s What is the relationship between java and JavaScript? What is the difference between instance variables and methods and class variables and methods? Contd.

F A Q s Where can I learn more about java and find applets and applications to play? Contd.

F A Q s 1. obtaining source for java information for applets 3. Newsgroups *Comp.lang.java *Comp.lang.java.programme *Comp.lang.java.tech Contd.

F A Q s 1. If arrays are objects and you use new to create them and they have an instance variable length, where is the array class? Contd.

F A Q s 2.What are the differences between objects and primitive data types such as int and booleans ?