CEP. 5

21.Difference between abstract class and interface

Abstract class and interface both are used to achieve abstraction where we can declare the abstract methods. Abstract class and interface both can't be instantiated.

But there are many differences between abstract class and interface that are given below.

Abstract class	Interface
1) Abstract class can have abstract and non-abstract methods.	Interface can have only abstract methods. Since Java 8, it can have default and static methods also.
2) Abstract class doesn't support multiple inheritance.	Interface supports multiple inheritance.
3) Abstract class can have final, non-final, static and non-static variables.	Interface has only static and final variables.
4) Abstract class can provide the implementation of interface.	Interface can't provide the implementation of abstract class.
5) The abstract keyword is used to declare abstract class.	The interface keyword is used to declare interface.
6) An abstract class can extend another Java class and implement multiple Java interfaces.	An interface can extend another Java interface only.
7) An abstract class can be extended using keyword "extends".	An interface can be implemented using keyword "implements".
8) A Java abstract class can have class members like private, protected, etc.	Members of a Java interface are public by default.
9)Example: public abstract class Shape{ public abstract void draw(); }	Example: public interface Drawable{ void draw(); }

Simply, abstract class achieves partial abstraction (0 to 100%) whereas interface achieves fully abstraction (100%).

Example of abstract class and interface in JavaLet's see a simple example where we are using interface and abstract class both.

1. //Creating interface that has 4 methods  
2. interface A{  
3. void a();//bydefault, public and abstract  
4. void b();  
5. void c();  
6. void d();  
7. }  
8.   
9. //Creating abstract class that provides the implementation of one method of A interface  
10. abstract class B implements A{  
11. public void c(){System.out.println("I am C");}  
12. }  
13.   
14. //Creating subclass of abstract class, now we need to provide the implementation of rest of the methods  
15. class M extends B{  
16. public void a(){System.out.println("I am a");}  
17. public void b(){System.out.println("I am b");}  
18. public void d(){System.out.println("I am d");}  
19. }  
20.   
21. //Creating a test class that calls the methods of A interface  
22. class Test5{  
23. public static void main(String args[]){  
24. A a=new M();  
25. a.a();  
26. a.b();  
27. a.c();  
28. a.d();  
29. }}  

Output

       I am a
       I am b
       I am c
       I am d

22.Event and Listener (Java Event Handling)

Changing the state of an object is known as an event. For example, click on button, dragging mouse etc. The java.awt.event package provides many event classes and Listener interfaces for event handling.

Java Event classes and Listener interfaces

Event Classes	Listener Interfaces
ActionEvent	ActionListener
MouseEvent	MouseListener and MouseMotionListener
MouseWheelEvent	MouseWheelListener
KeyEvent	KeyListener
ItemEvent	ItemListener
TextEvent	TextListener
AdjustmentEvent	AdjustmentListener
WindowEvent	WindowListener
ComponentEvent	ComponentListener
ContainerEvent	ContainerListener
FocusEvent	FocusListener

Steps to perform Event Handling

Following steps are required to perform event handling:

1. Register the component with the Listener

Registration Methods

For registering the component with the Listener, many classes provide the registration methods. For example:

• Button
  • public void addActionListener(ActionListener a){}
• MenuItem
  • public void addActionListener(ActionListener a){}
• TextField
  • public void addActionListener(ActionListener a){}
  • public void addTextListener(TextListener a){}
  TextArea
• • public void addTextListener(TextListener a){}
• Checkbox
  • public void addItemListener(ItemListener a){}
• Choice
  • public void addItemListener(ItemListener a){}
• List
  • public void addActionListener(ActionListener a){}
  • public void addItemListener(ItemListener a){}
  • Java Event Handling Code

We can put the event handling code into one of the following places:

1. Within class
2. Other class
3. Anonymous class

Java event handling by implementing ActionListener

1. import java.awt.*;  
2. import java.awt.event.*;  
3. class AEvent extends Frame implements ActionListener{  
4. TextField tf;  
5. AEvent(){  
6.   
7. //create components  
8. tf=new TextField();  
9. tf.setBounds(60,50,170,20);  
10. Button b=new Button("click me");  
11. b.setBounds(100,120,80,30);  
12.   
13. //register listener  
14. b.addActionListener(this);//passing current instance  
15.   
16. //add components and set size, layout and visibility  
17. add(b);add(tf);  
18. setSize(300,300);  
19. setLayout(null);  
20. setVisible(true);  
21. }  
22. public void actionPerformed(ActionEvent e){  
23. tf.setText("Welcome");  
24. }  
25. public static void main(String args[]){  
26. new AEvent();  
27. }  
28. }