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Newsgroups: comp.lang.java.programmer
Date: Sat, 9 Feb 2013 11:10:18 -0800 (PST)
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Subject: Re: Design Patterns
From: Lew <lewbloch@gmail.com>
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Xref: csiph.com comp.lang.java.programmer:22254

Arne Vajh=F8j wrote:
> A plain nested class does not have access to the local
> variables.
>=20
> If that is not needed then fine.
>=20
> If that is needed then it has to be either a local
> or anonymous class.

Nitpick: or inner class.

> And local classes are very rare in the real world.

Sort of. Named local classes are indeed rare. Anonymous local classes=20
are rather common.

 public List<File> getFiles()
 {
   final FileFilter filter =3D new FileFilter()=20
   {
     @Override boolean accept(File f) { return f.isDirectory(); }
   };
   // instance of an anonymous local class implementing FileFilter
   ... =20
 }

This is good Java idiom because the 'FileFilter' instance is GCable when=20
done. It hasn't got state so its cost is virtually nil. So the local class=
=20
feature gives you good control of a program's memory requirements.

In less educated programmers' code I've seen such implementors scoped at=20
instance or even (gasp) static member level. And then people wonder at the=
=20
heap requirements.

Java has a funky and nuanced type structure. If you take advantage of=20
the nuances you get a lot in return.

It helps to use JLS (Java Language Specification) terminology in its=20
exact, narrowest framing. Really. Some call all nested classes "inner"=20
classes. Nope. And anonymous classes can be member or local types. It's=20
all about scope and context.

The JLS has a detailed treatment, and somewhere out there floats a=20
brilliant infographic going back to 1.2-ish days, but here's a brief
taxonomy.

 top level * {public, package},
 nested {static, inner {member, local}} * {any access}

This is off the top of my head, so any corrections welcomed.

There's a lovely corner case involving

 public class Containing
 {
   private void doSomething() {...}

   public class Within extends Containing
   {
     // @Override // ?
     public void doSomething()=20
     {=20
       Containing.this.doSomething();
       // super.doSomething(); // ?
       ...
     }
   }
 }