Java Swing - Listeners
TOPIC 01 – MOUSELISTENER

LESSON NOTE

INTRO TO EVENT DRIVEN PROGRAMMING

Event driven programs are programs that run a sequence of code when an event (such as a mouse click or a key press) occurs.

To do this in java, we simply need to include what is a called an event listener (or just listener). A listener is an object that is responsible for watching (listening) for events. When an event occurs, that object contains code that is executed.

INTERFACES

Interfaces are simply special classes that have a list of methods without method bodies. Classes can then implement interfaces and provided some or all of the functionality intended by the interface.

MOUSELISTENER INTERFACE

If you look up MouseListener in the java documentation, you will find that it is an interface with the following methods:

void mouseClicked(MouseEvent e)

void mouseEntered(MouseEvent e)

void mouseExited (MouseEvent e)

void mousePressed(MouseEvent e)

void mouseReleased(MouseEvent e)

We can create an object that will listen for mouse events described by the method names above (mouse click, mouse enters an area, mouse exits an area, mouse pressed, mouse released). All it has to do is implement the MouseListener interface. Inside that class, the five methods above must be implemented.

WHO LISTENS?

The question is, which object should listen? Should we create a separate object to do this? We can! And it would keep things nice and neat.

However, if we want to respond to events by drawing something on the screen, we will have to access information that is also accessible by the paint method of our JPanel.

So, the easiest way to implement the MouseListener is to do so in your JPanel class. Therefore, your code for the class name will now look like this:

public class AwesomePanel extends JPaneld implements MouseListener

WHAT WILL THIS LOOK LIKE?

This might seem a little messy but its not too bad once you start working with it. The green text shows you the new code you are adding.

public class MousePanel extends JPanel implements MouseListener

{

public void paint(Graphics g)

{…}

public void mouseClicked(MouseEvent e)

{…}

public void mouseEntered(MouseEvent e)

{…}

public void mouseExited (MouseEvent e)

{…}

public void mousePressed(MouseEvent e)

{…}

public void mouseReleased(MouseEvent e)

{…}

}

MOUSE EVENT

You probably noticed that every method in the MouseListener interface has a MouseEvent argument. This is simply an object that is automatically created that contains information about the event.

The methods you will most likely use from this object are getX() and getY() which return the x and y coordinates of the event.

EXAMPLE 1 – SHOWING MOUSE EVENTS

import javax.swing.JFrame;

public class MouseListenerTester

{

public static void main(String[] args)

{

JFrame jf = new JFrame();

jf.setSize(600,400);

jf.setVisible(true);

jf.setTitle("MouseListener Events");

//Create my panel and add it to JFrame object

MouseListenerPanel pan = new MouseListenerPanel();

jf.add(pan);

}

}

import java.awt.Graphics;

import java.awt.event.MouseEvent;

import java.awt.event.MouseListener;

import javax.swing.JPanel;

public class MouseListenerPanel extends JPanel implements MouseListener

{

//datafields

String clicked = "";

String released = "";

String pressed = "";

String entered = "";

String exited = "";

public MouseListenerPanel()

{

this.addMouseListener(this);

}

public void paint(Graphics g)

{

super.paint(g); //required to repaint bg everytime

g.drawString("Clicked: " + clicked, 10, 20);

g.drawString("Released:" + released, 10, 40);

g.drawString("Pressed:" + pressed, 10, 60);

g.drawString("Entered:" + entered, 10, 80);

g.drawString("Exited:" + exited, 10, 100);

}

public void mouseClicked(MouseEvent e)

{

clicked = "(" + e.getX() + "," + e.getY() + ")";

repaint();

}

public void mouseReleased(MouseEvent e)

{

released = "(" + e.getX() + "," + e.getY() + ")";

repaint();

}

public void mousePressed(MouseEvent e)

{

pressed = "(" + e.getX() + "," + e.getY() + ")";

repaint();

}

public void mouseEntered(MouseEvent e)

{

entered = "(" + e.getX() + "," + e.getY() + ")";

repaint();

}

public void mouseExited(MouseEvent e)

{

exited = "(" + e.getX() + "," + e.getY() + ")";

repaint();

}

}

The above code will create the following window that will update the coordinates of different mouse events. Of course, you need to run it to appreciate what is happening.

EXAMPLE 2 - USING CLICKS TO CHANGE PICTURES

Here’s an example that allows you to draw different information that is changed every time the mouse is clicked.

import javax.swing.JFrame;

public class MouseClickerTester

{

public static void main(String[] args)

{

JFrame jf = new JFrame();

jf.setSize(600,400);

jf.setVisible(true);

jf.setTitle("MouseListener Events");

//Create my panel and add it to JFrame object

MouseClickerPanel pan = new MouseClickerPanel();

jf.add(pan);

}

}

import java.awt.Graphics;

import java.awt.event.MouseEvent;

import java.awt.event.MouseListener;

import javax.swing.JPanel;

public class MouseClickerPanel extends JPanel implements MouseListener

{

//datafields

int turn = 0;

public MouseClickerPanel()

{

this.addMouseListener(this);

}

public void paint(Graphics g)

{

super.paint(g); //required to repaint bg everytime

if (turn == 0)

{

g.drawRect(50, 100, 200, 300);

g.drawString("Chapter 1", 10, 10);

}

else if (turn == 1)

{

g.drawLine(50, 100, 200, 300);

g.drawString("Chapter 2", 10, 10);

}

else if (turn == 2)

{

g.drawLine(150, 80, 220, 100);

g.drawString("Chapter 3", 10, 10);

}

else if (turn == 3)

{

g.drawOval(150, 80, 50, 100);

g.drawString("Chapter 4", 10, 10);

}

else if (turn >= 4)

{

g.drawRect(250, 148, 306, 80);

g.drawString("Chapter 5 (last one)", 10, 10);

}

}

public void mouseClicked(MouseEvent e)

{

turn++;

repaint();

}

public void mouseReleased(MouseEvent e)

{ }

public void mousePressed(MouseEvent e)

{ }

public void mouseEntered(MouseEvent e)

{ }

public void mouseExited(MouseEvent e)

{ }

}