Some classes don't make make much sense on their own - they only make
sense in the context of another class. Inner classes are helpful at
defining this kind of relationship - they allow the developer to
express the relation between highly cohesive classes more explicitly.

As an example consider a Node in a Graph. A Node has a list of peer
Nodes which it can reach itself. It makes sense to define a Peer class
as an inner class of Node. For example:

public class Node
{
   private String name;
   private List peers = new ArrayList();

   private Node( String name )
   {
       this.name = name;
   }

   /**
    * Represents a peer node that is reachable from this node
    */
   class Peer
   {
       private Node node;
       private int distance;
   }
}

Other examples can be seen by browing the Javadocs or source, for
example HashMap and HashMap.Entry.

Paul

Not at all. See Roedy's very nice glossary:

 http://mindprod.com/jgloss/innerclasses.html

 Harald.

It may help to think of inner classes as /just/ syntactic sugar.  That is to
say, they provide /nothing/ that cannot be expressed (pretty directly) in Java
without using them.  All they do is provide a somewhat more lightweight way of
defining objects for some limited/local purpose that might seem not to justify
the apparent weight of a "full" standalone class.  Of course, the appearance is
deceptive, but if it help make the code clearer then that's a good thing even
if -- when all's said and done -- it's just an illusion.

   -- chris

Thanks to all who responded. It clearer to me now the purpose of the
inner class. I thought it was just for listerner or gui development but
it is more than that.

Never thought about the slim interface that Stefan mentioned and I
liked that synthetic analogy.

Everyone responded gives me a better picture. It makes sense now...

see http://mindprod.com/jgloss/innerclasses.html