Help regarding Java Swing

Question

Hi All,
I am a bit confused witht the shortcut syntax for adding action listeners for event handling.
In the Java tutorial it is mentioned that for implementing an event handler following are the three steps:
1. Create a class that implements ActionListener interface.
For example:
public class MyClass implements ActionListener {
2. Registers an instance of the event handler class as a listener on one or more components.
For example:
someComponent.addActionListener(instanceOfMyClass);
3. In the event handler class, a few lines of code implement the methods in the listener interface.
For example:
public void actionPerformed(ActionEvent e) {
...//code that reacts to the action...
In the tutorial a shortcut syntax is also given:
For Example:
Class TestApp {
JButton btn = new JButton("Test");
btn.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
...//code that reacts to the action...
My doubt is:
1. How are we allowed to create an instance of ActionListener which is an "interface". (doesnt addActionListener expect instance of a class that implements the ActionListener interface? )
2. How are we able to access local variables of class "TestApp" the inside the actionPerformed() function.
Could anyone explain more about this shortcut syntax of adding action listeners?
Thanks in regards
Arun

Answer 1

The example uses so-called "anonymous inner classes". ActionListener is an interface, of course. But if you use the construct
new ActionListener() {
   public void actionPerformed(ActionEvent e) {
   ...//code that reacts to the action...
}this is about equivalent to defining an inner class
private class MyActionListener implements ActionListener {
   public void actionPerformed(ActionEvent e) {
   ...//code that reacts to the action...
}and using that class. The difference is mainly, that the in nthe shortcut notation the class has no explicit name, it is anonymous.
There are advantages and disadvantages to that. The main disadvantage is, that you create a large amount of classes in this way. I've read somewhere, that SUN is planning to rewrite the examples without the use of anonymous inner classes.