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Inner classes

So what are inner classes good for anyway?

By Tony Sintes, JavaWorld.com, 07/27/01

QSo what are inner classes good for anyway?

ABelieve it or not, there are advantages to Java's inner classes. But before we go into that, I'll provide a short background on inner classes.

Inner classes nest within other classes. A normal class is a direct member of a package, a top-level class. Inner classes, which became available with Java 1.1, come in four flavors:

  • Static member classes
  • Member classes
  • Local classes
  • Anonymous classes


Let's take a quick look at each in turn.

Briefly, a static member class is a static member of a class. Like any other static method, a static member class has access to all static methods of the parent, or top-level, class.

Like a static member class, a member class is also defined as a member of a class. Unlike the static variety, the member class is instance specific and has access to any and all methods and members, even the parent's this reference.

Local classes are declared within a block of code and are visible only within that block, just as any other method variable.

Finally, an anonymous class is a local class that has no name.

To answer your specific question, I'll focus on the member and anonymous inner classes since those are the ones you'll likely encounter and use. To me, the advantages of inner classes can be divided into three categories: an object-oriented advantage, an organizational advantage, and a call-back advantage.

The object-oriented advantage

In my humble opinion, the most important feature of the inner class is that it allows you to turn things into objects that you normally wouldn't turn into objects. That allows your code to be even more object-oriented than it would be without inner classes.

Let's look at the member class. Since its instance is a member of its parent instance, it has access to every member and method in the parent. At first glance, this might not seem like much; we already have that sort of access from within a method in the parent class. However, the member class allows us to take logic out of the parent and objectify it. For example, a tree class may have a method and many helper methods that perform a search or walk of the tree. From an object-oriented point of view, the tree is a tree, not a search algorithm. However, you need intimate knowledge of the tree's data structures to accomplish a search.

An inner class allows us to remove that logic and place it into its own class. So from an object-oriented point of view, we've taken functionality out of where it doesn't belong and have put it into its own class. Through the use of an inner class, we have successfully decoupled the search algorithm from the tree. Now, to change the search algorithm, we can simply swap in a new class. I could go on, but that opens up our code to many of the advantages provided by object-oriented techniques.

The organizational advantage

Object-oriented design isn't everyone's thing, but luckily, inner classes provide more. From an organizational point of view, inner classes allow us to further organize our package structure through the use of namespaces. Instead of dumping everything in a flat package, classes can be further nested within classes. Explicitly, without inner classes, we were limited to the following hierarchy structure:

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