Question

I have often come across this snippet :

General form of invokeLater is – static void invokeLater(Runnable obj)

But i have often come across these types of codes:

{
    SwingUtilities.invokeLater(new Runnable() {
   public void run() {
     new tester();
   }
 });

}

or—–(example of another type)

{
   clean.addActionListener(new ActionListener(){
   public void actionPerformed(ActionEvent ae) {
     cleanActionPerformed(ae);
   }
 });

}

Now what is this?
There is method in the constructor!! I am unable to understand the way of writing these snippets.
Explain very clearly

Answer 1

What you see are anonymous inner classes. They are one-shot classes that you won’t use anywhere else.

Take a look at this piece:

clean.addActionListener(new ActionListener(){
   public void actionPerformed(ActionEvent ae) {
     cleanActionPerformed(ae);
   }
 });

addActionListener is expecting an implementation of an ActionListener, which is an interface.

Now, you could write a whole new class that implements it and then put an instance of it as an argument of addActionListener, but this way is faster to write (and a bit harder to read) if you won’t use such a class anywhere else.

The same effect could have been achieved with this:

class SomeActionListener implements ActionListener {
   public void actionPerformed(ActionEvent ae) {
       cleanActionPerformed(ae);
   }
}

...

clean.addActionListener(new SomeActionListener());