JAVA: Problem in Last Code Example

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Web Design & Development CS506

Lesson 13

Problem in Last Code Example

Problem

We were interested in windowClosing() method only

But have to provide definitions of all the methods, Why?

Because a class implementing an interface has to provide definitions of all methods

present in that interface.

Solution

To avoid giving implementations of all methods of an interface when we are not using these

methods we use Event Adapter classes

Adapter Classes

For listener interfaces containing more than one event handling methods, jdk defines adapter

classes. Examples are

For WindowListener Æ WindowAdapter

For MouseMotionListener Æ MouseMotionAdapter

and many more

Adapter classes provide definitions for all the methods (empty bodies) of their corresponding

Listener interface

It means that WindowAdapter class implements WindowListener interface and provide the

definition of all methods inside that Listener interface

Consider the following example of MouseMotionAdapter and its corresponding

MouseMotionListener interface

public interface MouseMotionListener {

public void mouseDragged (MouseEvent me);

public void mouseMoved (MouseEvent me);

}

public class MouseMotionAdapter implements MouseMotionListener{

public void mouseDragged (MouseEvent me) { }

public void mouseMoved (MouseEvent me) { }

}

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Available Adapter classes

How to use Adapter Classes

Previously handler class need to implement interface

public class EventsEx implements MouseMotionListener{...}

Therefore it has to provide definitions of all the methods inside that interface

Now our handler class will inherit from adapter class

public class EventsEx extends MouseMotionAdapter{...}

Due to inheritance, all the methods of the adapter class will be available inside our handler class

Since adapter classes has already provided definitions with empty bodies.

We do not have to provide implementations of all the methods again

We only need to override our method of interest.

Example Code 13.1: Handling Window Events using Adapter Classes

Here we are modifying the window event code in the last example to show the use of

WindowAdapter instead of WindowListener. Code related to MouseMotionListener is deleted to

avoid cluttering of code.

import java.awt.*;

import javax.swing.*;

import java.awt.event.*;

public class EventsEx extends WindowAdapter {

JFrame frame;

JLabel coordinates;

// setting layout

public void initGUI ( ) {

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// creating event generator

frame = new JFrame();

Container cont = frame.getContentPane();

10.

cont.setLayout(new BorderLayout( ) );

11.

coordinates = new JLabel ();

12.

cont.add(coordinates, BorderLayout.NORTH);

// registering window handler with generator

13.

frame.addWindowListener(this);

14.

frame.setSize(350, 350);

15.

frame.setVisible(true);

16.

} // end initGUI method

//default constructor

17.

public EventsEx ( ) {

18.

initGUI();

19.

}

// As you can see that we have only implemented

// our required method

20.

public void windowClosing (WindowEvent we) {

21.

JOptionPane.showMessageDialog(null, "Good Bye");

22.

System.exit(0);

23.

}

24.

public static void main(String args[]) {

25.

EventsEx ex = new EventsEx();

26.

}

27.

} // end class

Problem in Last Code Example

We have inherited from WindowAdapter

What if we want to use MouseMotionAdpater as well ? or what if our class already inherited form

some other class ?

Problem

-- Java allows single inheritance

Solution

-- Use Inner classes

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Inner Classes

A class defined inside another class

Inner class can access the instance variables and members of outer class

It can have constructors, instance variables and methods, just like a regular class

Generally used as a private utility class which does not need to be seen by others classes

Example Code13.2: Handling Window Event with Inner Class

Here we are modifying the window event code in the last example to show the use of

WindowAdapter as an inner class.

import java.awt.*;

import javax.swing.*;

import java.awt.event.*;

public class EventEx {

JFrame frame;

JLabel coordinates;

// setting layout

public void initGUI ( ) {

frame = new JFrame();

10.

Container cont = frame.getContentPane();

11.

cont.setLayout(new BorderLayout( ));

12.

coordinates = new JLabel ();

13.

cont.add(coordinates, BorderLayout.NORTH);

/* Creating an object of the class which is handling our

window events and registering it with generator */

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14.

WindowHandler handler = new Window Handler ();

15.

frame.addWindowListener(handler);

16. frame.setSize(350, 350);

17. frame.setVisible(true);

18. } // end initGUI

//default constructor

19. public EventEx ( ) {

20.

initGUI();

21. }

/* Inner class implementation of window adapter. Outer

class is free to inherit from any other class. */

22.

private class WindowHandler extends WindowAdapter {

// Event Handler for WindowListener

23.

public void windowClosing (WindowEvent we) {

24.

JOptionPane.showMessageDialog(null, "Good Bye");

25.

System.exit(0)

26.

}

27. } // end of WindowHandler class

28. public static void main(String args[]) {

29. EventEx e = new EventEx();

30. }

31.} // end class

Example Code 13.3: Handling Window and Mouse Events with Inner Class

Here we are modifying the window event code of the last example to handle window and mouse events

using inner classes. The diagram given below summarizes the approach.

Outer class for GUI and other code

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Inner class

Handling

Window

import java.awt.*;

import javax.swing.*;

import java.awt.event.*;

public class EventEx {

JFrame frame;

JLabel coordinates;

// setting layout

public void initGUI ( ) {

frame = new JFrame();

10.

Container cont = frame.getContentPane();

11.

cont.setLayout(new BorderLayout( ) );

12.

coordinates = new JLabel ();

13.

cont.add(coordinates, BorderLayout.NORTH);

/* Creating an object of the class which is handling our

window events and registering it with generator */

14.

WindowHandler whandler = new WindowHandler ();

15.

frame.addWindowListener(whandler);

/* Creating an object of the class which is handling our

MouseMotion events & registering it with generator */

16.

MouseHandler mhandler = new MouseHandler ();

17.

frame.addMouseMotionListener(mhandler);

18. frame.setSize(350, 350);

19. frame.setVisible(true);

20. }

//default constructor

21. public EventEx ( ) {

22. initGUI();

23. }

/* Inner class implementation of WindowAdapter. Outer class

is free to inherit from any other class. */

24. private class WindowHandler extends WindowAdapter {

// Event Handler for WindowListener

25. public void windowClosing (WindowEvent we) {

26.

JOptionPane.showMessageDialog(null, "Good Bye");

27.

System.exit(0)

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28.

}

29. } // end of WindowHandler

//Inner class implementation of MouseMotionAdapter

30. private class MouseHandler extends MouseMotionAdapter {

// Event Handler for mouse motion events

31. public void mouseMoved (MouseEvent me) {

32.

int x = me.getX();

33.

int y = me.getY();

34.

coord.setText("Moved at [" + x + "," + y + "]" );

35. }

36. } // end of MouseHandler

37. public static void main(String args[]) {

38. EventEx e = new EventEx();

39. }

40.} // end class

Example Code: Making Small Calculator using Inner classes

User enters numbers in the provided fields

On pressing "+" button, sum would be displayed in the answer field

On pressing "*" button, product would be displayed in the answer field

1. import java.awt.*;

2. import javax.swing.*;

3. import java.awt.event.*;

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4. public class SmallCalcApp implements ActionListener{

JFrame frame;

JLabel firstOperand, secondOperand, answer;

JTextField op1, op2, ans;

JButton plus, mul;

9. // setting layout

10. public void initGUI ( ) {

11.

frame = new JFrame();

12.

firstOperand = new JLabel("First Operand");

13.

secondOperand = new JLabel("Second Operand");

14.

answer

= new JLabel("Answer");

15.

op1 = new JTextField (15);

16.

op2 = new JTextField (15);

17.

ans = new JTextField (15);

18.

plus = new JButton("+");

19.

plus.setPreferredSize(new Dimension(70,25));

20.

mul = new JButton("*");

21.

mul.setPreferredSize(new Dimension(70,25));

22.

Container cont = frame.getContentPane();

23.

cont.setLayout(new FlowLayout());

24.

cont.add(firstOperand);

25.

cont.add(op1);

26.

cont.add(secondOperand);

27.

cont.add(op2);

28.

cont.add(plus);

29.

cont.add(mul);

30.

cont.add(answer);

31.

cont.add(ans);

/* Creating an object of the class which is handling

button events & registering it with generators */

32.

ButtonHandler bHandler = new ButtonHandler();

33.

plus.addActionListener(bHandler);

34.

mul.addActionListener(bHandler);

35.

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

36.

frame.setSize(200, 220);

37.

frame.setVisible(true);

38. }

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39. //constructor

40.

public SmallCalcApp ( ) {

41. initGUI();

42. }

//Inner class implementation of ActionListener

43. private class ButtonHandler implements ActionListener{

44. public void actionPerformed(ActionEvent event) {

45.

String oper, result;

46.

int num1, num2, res;

47.

if (event.getSource() == plus) {

48.

oper = op1.getText();

49.

num1 = Integer.parseInt(oper);

50.

oper = op2.getText();

51.

num2 = Integer.parseInt (oper);

52.

res = num1+num2;

53.

result = res+"";

54.

ans.setText(result);

}

56.

else if (event.getSource() == mul) {

57.

oper = op1.getText();

58.

num1 = Integer.parseInt(oper);

59.

oper = op2.getText();

60.

num2 = Integer.parseInt (oper);

61.

res = num1*num2;

62.

result = res+"";

63.

ans.setText(result);

}

65. } // end actionPerformed method

66. } // end inner class ButtonHandler

67.

public static void main(String args[]) {

68.

SmallCalcApp scApp = new SmallCalcApp();

69.

}

70. }// end class

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Anonymous Inner Classes

Has no name

Same as inner class in capabilities

much shorter

difficult to understand

Named vs. Anonymous Objects

Named

String s = "hello"; System.out.println(s);

"hello" has a named reference s.

Anonymous

System.out.println("hello");

We generally use anonymous object when there is just a one time use of a particular object but in

case of a repeated use we generally used named objects and use that named reference to use that objects

again and again.

Example Code 13.4 Handling Window Event with Anonymous Inner Class

Here we are modifying the window event code of 13.3 to show the use of anonymous inner class.

28.

import java.awt.*;

29.

import javax.swing.*;

30.

import java.awt.event.*;

31.

public class EventsEx extends WindowAdapter {

32.

JFrame frame;

33.

JLabel coordinates;

// setting layout

34.

public void initGUI ( ) {

// creating event generator

35.

frame = new JFrame();

36.

Container cont = frame.getContentPane();

37.

cont.setLayout(new BorderLayout( ) );

38.

coordinates = new JLabel ();

39.

cont.add(coordinates, BorderLayout.NORTH);

// registering event handler (anonymous inner class)

// with generator by using

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40. frame.addWindowListener (

41.

new WindowAdapter ( ) {

42.

public void windowClosing (WindowEvent we) {

43.

JOptionPane.showMessageDialog(null, "Good Bye");

44.

System.exit(0);

45.

} // end window closing

46. } // end WindowAdapter

47. ); // end of addWindowListener

48. frame.setSize(350, 350);

49. frame.setVisible(true);

50. } // end initGUI method

//default constructor

51. public EventsEx ( ) {

52. initGUI();

53. }

54. public static void main(String args[]) {

55. EventsEx ex = new EventsEx();

56. }

57. } // end class

Summary of Approaches for Handling Events

1. By implementing Interfaces

2. By extending from Adapter classes

To implement the above two techniques we can use

Same class

putting event handler & generator in one class

Separate class

1. Outer class

Putting event handlers & generator in two different classes

3. Inner classes

3. Anonymous Inner classes

References

Java A Lab Course by Umair Javed

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