JAVA: More Examples of Handling Events

<< JAVA: Event Handling

JAVA: Problem in Last Code Example >>

Web Design & Development CS506

Lesson 12

More Examples of Handling Events

Handling Mouse Event

Mouse events can be trapped for any GUI component that inherits from Component class. For example,

JPanel, JFrame & JButton etc.

To handle Mouse events, two types of listener interfaces are available.

MouseMotionListener

MouseListener

The class that wants to handle mouse event needs to implement the corresponding interface and

needs to provide the definition of all the methods in that interface.

MouseMotionListener interface

Used for processing mouse motion events

Mouse motion event is generated when mouse is moved or dragged

MouseMotionListener interfaces is defined in JDK as follows

public interface MouseMotionListener {

public void mouseDragged (MouseEvent me);

public void mouseMoved (MouseEvent me);

}

MouseListener interface

Used for processing "interesting" mouse events like when mouse is:

Pressed

Released

Clicked (pressed & released without moving the cursor)

Enter (mouse cursor enters the bounds of component)

Exit (mouse cursor leaves the bounds of component)

MouseListener interfaces is defined in JDK as follows

public interface MouseListener {

public void mousePressed (MouseEvent me);

public void mouseClicked (MouseEvent me);

public void mouseReleased (MouseEvent me);

public void mouseEntered (MouseEvent me);

public void mouseExited (MouseEvent me);

}

Example Code: Handling Mouse Events

Example to show Mouse Event Handling .Every time mouse is moved, the coordinates for a new

place is shown in a label.

Web Design & Development CS506

1. import java.awt.*;

2. import javax.swing.*;

3. import java.awt.event.*;

4. public class EventsEx implements MouseMotionListener{

5. JFrame frame;

6. JLabel coordinates;

7. // setting layout

8. public void initGUI ( ) {

// creating event generator

10.

frame = new JFrame();

11.

Container cont = frame.getContentPane();

12.

cont.setLayout(new BorderLayout( ) );

13.

coordinates = new JLabel ();

14.

cont.add(coordinates, BorderLayout.NORTH);

15.

// registring mouse event handler with generator

16.

frame.addMouseMotionListener(this);

17.

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

18.

frame.setSize(350, 350);

19.

frame.setVisible(true);

20.

} // end initGUI method

21.

//default constructor

22.

public EventsEx ( ) {

23.

initGUI();

24.

}

// MouseMotionListener event hadler handling dragging

25.

public void mouseDragged (MouseEvent me) {

26.

int x = me.getX();

27.

int y = me.getY();

28.

coordinates.setText("Dragged at [" + x + "," + y + "]");

29.

}

// MouseMotionListener event handler handling motion

30.

public void mouseMoved (MouseEvent me) {

31.

int x = me.getX();

32.

int y = me.getY();

33.

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

100

Web Design & Development CS506

34.

}

35.

public static void main(String args[]) {

36.

EventsEx ex = new EventsEx();

37.

}

38.

} // end class

Another Example: Handling Window Events

Task

We want to handle Window Exit event only

Why?

When window is closed, control should return back to command prompt.

But we have already achieved this functionality through following line of code

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

But, what if we want to display some message (Good Bye) before exiting?

To handle window events, we need to implement "WindowListner" interface.

"WindowListner" interface contains 7 methods We require only one i.e.

windowClosing

But, We have to provide definitions of all methods to make our class a concrete class

WindowListener interface is defined in the JDK as follows

public interface WindowListener {

101

Web Design & Development CS506

public void windowActivated(WindowEvent we);

public void windowClosed(WindowEvent we);

public void windowClosing(WindowEvent we);

public void windowDeactivated(WindowEvent we);

public void windowDeiconified(WindowEvent we);

public void windowIconified(WindowEvent we);

public void windowOpened(WindowEvent we);

}

public void windowClosing(WindowEvent we is our required method

Example Code: WindowExitHandler

This example code is modification of the last code example i.e. EventsEx.java

import java.awt.*;

import javax.swing.*;

import java.awt.event.*;

public class EventsEx implements MouseMotionListener ,

WindowListener {

JFrame frame;

JLabel coordinates;

// setting layout

public void initGUI ( ) {

// 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);

// registring mouse event handler with generator

13.

frame.addMouseMotionListener(this);

// registering window handler with generator

14.

frame.addWindowListener(this);

15.

frame.setSize(350, 350);

16.

frame.setVisible(true);

17.

} // end initGUI method

//default constructor

18.

public EventsEx ( ) {

19.

initGUI();

20.

}

102

Web Design & Development CS506

// MouseMotionListener event hadler handling dragging

21.

public void mouseDragged (MouseEvent me) {

22.

int x = me.getX();

23.

int y = me.getY();

24.

coordinates.setText("Dragged at [" + x + "," + y + "]");

25.

}

// MouseMotionListener event handler handling motion

26.

public void mouseMoved (MouseEvent me) {

27.

int x = me.getX();

28.

int y = me.getY();

29.

30.

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

31.

}

// window listener event handler

32.

public void windowActivated (WindowEvent we) { }

33.

public void windowClosed (WindowEvent we) { }

34.

public void windowClosing (WindowEvent we) {

35.

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

36.

System.exit(0);

37.

}

38.

public void windowDeactivated (WindowEvent we) { }

39.

public void windowDeiconified (WindowEvent we) { }

40.

public void windowIconified (WindowEvent we) { }

41.

public void windowOpened (WindowEvent we) { }

42.

public static void main(String args[]) {

43.

EventsEx ex = new EventsEx();

44.

}

45.

} // end class

103

Table of Contents: