CODEWITHSUNDEEP
javainner-classes
Suhail Gupta
Suhail Gupta

Reputation: 23206

Method in constructor?

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

Upvotes: 2

Views: 144

Answers (5)

darioo
darioo

Reputation: 47183

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());

Upvotes: 1

Robby Pond
Robby Pond

Reputation: 73484

These are just anonymous classes or a local class without a name. Anonymous classes are usually used when you will only reference a class once.

Instead of doing addListener(this) and having your class implement some interface.

Upvotes: 0

Manoj
Manoj

Reputation: 5612

Its called anonymous Inner class, where object is declared and initialized at the same time.

Upvotes: 0

Peeter
Peeter

Reputation: 9382

They are called anonymous classes, you can read up on them at http://download.oracle.com/javase/tutorial/java/javaOO/innerclasses.html

Upvotes: 3

Brian Roach
Brian Roach

Reputation: 76898

They're anonymous classes.

You could write them as a class in their own file,instantiate it, and pass it as the argument... but for one-off use that is the easier way to do it.

(As Matt notes in a comment below, "easier" in that you don't have to create the files and write out the classes, etc)

Upvotes: 2

Related Questions