Java Notes

Top-level Listeners

This is an example of top-level Listeners in Java.

In this example you will learn how to create top level-listeners in swing applications.

Using this as a listener

A common way to write simple applets is to use the use the applet itself as a listener (referred to as this).

For example

...
public class Hello extends JApplet implements ActionListener {
   JButton b;
   ...
   public void init() {
      JButton b = new JButton("Hello");
      b.addActionListener(this);
      ...
   }
   
   public void actionPerformed(ActionEvent e) {
      message = "Hi";
      repaint();
   }
   ...
}

This doesn't seem to be a problem for small programs, but what happens if there is more than one button? There can be only one actionPerformed() method in a class.

Problem: One listener for many components

Inside the listener method, it's possible to check the parameter to find out which component caused the event. For example,

JButton b1, b2;
. . .
public MyClass() {  // constructor
    ...
    b1.addActionListener(this);
    ...
    b2.addActionListener(this);  // SAME listener!
    ...
}//end constructor
 
public void actionPerformed(ActionEvent e) {
    Object obj = e.getSource();    // get the control that caused the event
    if (o instanceof JButton) {    // make sure it's a button.
        JButton b = (JButton)obj;  // downcast to a button
        if (b == b1) {   // UGLY, DON'T DO THIS
           // do something for button b1
        } else if (b == b2) {
           // do something for button b2
        }
    } else if (o instanceof JTextField) {
       ...
    ...

Using one listener makes the response slower, and forces all events to be handled in the same place. This uses the event model which was introduced in Java 1.1, but it has all the problems of the old Java 1.0 event model. Altho you will see this style us