Question

import java.awt.*;
import java.awt.event.*;
import javax.swing.*; 
public class test extends JFrame implements MouseListener
{
JFrame f;
JPanel p;
JButton b;
test()
{
    f=new JFrame("My Frame");
    p=new JPanel();
    f.add(p);
    b=new JButton("Button");
    p.add(b);
    f.setSize(100,100);
    f.setVisible(true);
    b.addMouseListener(new MouseListener()
    {
        public void mouseClicked(MouseEvent ae)
        {
            System.out.print("mouse clicked");
        }
    });
    b.addMouseListener(new MouseListener()
    {
        public void mouseEntered(MouseEvent ae)
        {
            System.out.print("mouse Entered");
        }
    });
    b.addMouseListener(new MouseListener()
    {
        public void mouseExited(MouseEvent ae)
        {
            System.out.print("mouse Exited");
        }
    });
    b.addMouseListener(new MouseListener()
    {
        public void mousePressed(MouseEvent ae)
        {
            System.out.print("mouse Pressed");
        }
    });
    b.addMouseListener(new MouseListener()
    {
        public void mouseReleased(MouseEvent ae)
        {
            System.out.print("mouse Released");
        }
    });
}
public static void main(String args[])
{
    new test();
}
}

Errors are as follows:

--------------------Configuration: <Default>--------------------
C:\Users\SHUBHENDU\Documents\test.java:19: <anonymous test$1> is not abstract and      does not override abstract method mouseExited(java.awt.event.MouseEvent) in        java.awt.event.MouseListener
        {
        ^
C:\Users\SHUBHENDU\Documents\test.java:26: <anonymous test$2> is not abstract and does not override abstract method mouseExited(java.awt.event.MouseEvent) in java.awt.event.MouseListener
        {
        ^
C:\Users\SHUBHENDU\Documents\test.java:33: <anonymous test$3> is not abstract and does not override abstract method mouseEntered(java.awt.event.MouseEvent) in java.awt.event.MouseListener
        {
        ^
C:\Users\SHUBHENDU\Documents\test.java:40: <anonymous test$4> is not abstract and does not override abstract method mouseExited(java.awt.event.MouseEvent) in java.awt.event.MouseListener
        {
        ^
C:\Users\SHUBHENDU\Documents\test.java:47: <anonymous test$5> is not abstract and does not override abstract method mouseExited(java.awt.event.MouseEvent) in java.awt.event.MouseListener
        {
        ^
5 errors

Answer 1

If you have a concrete Java class (non-abstract class) you have to implement all the methods that an interface has, of course if your class implements that interface.
So add unimplemented methods of MouseListener interface to your code. But you are adding a new MouseListener to the same button every time. That’s not the way it is used. You can/should implement all MouseListener methods in the same code body. Remove redundant MouseListener additions to your JButton b.