Method in constructor?

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

possible duplicate of [Event Driven Thread](http://stackoverflow.com/questions/5339025/event-driven-thread) — Bombe, Mar 17 '11 at 14:06
This is similar to your other question, [How does this method work](http://stackoverflow.com/questions/5335161/how-does-this-method-work), as well as your question that Bombe pointed out. When an answer doesn't make sense, leave a comment asking for clarification; don't keep posting the same question — Zach L, Mar 17 '11 at 15:02

score 3 · Answer 1 · answered Mar 17 '11 at 14:02

3

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

answered Mar 17 '11 at 14:02

Peeter

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score 2 · Answer 2 · answered Mar 17 '11 at 14:02

2

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)

answered Mar 17 '11 at 14:02

Brian Roach

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"easier" as in amount of code you have to write; the effects at runtime are identical – matt b Mar 17 '11 at 14:03

score 1 · Accepted Answer · answered Mar 17 '11 at 14:05

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());

in this clean.addActionListener(new ActionListener(){ public void actionPerformed(ActionEvent ae) { cleanActionPerformed(ae); } }); what does new ActionListener() denote?? — Suhail Gupta, Mar 17 '11 at 14:39

score 0 · Answer 4 · answered Mar 17 '11 at 14:03

0

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

answered Mar 17 '11 at 14:03

Manoj

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score 0 · Answer 5 · answered Mar 17 '11 at 14:03

0

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.

answered Mar 17 '11 at 14:03

Robby Pond

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Method in constructor?

5 Answers5