What are the purposes of inner classes

Question

I am reviewing the concept of inner classes in java. so far from what I\'ve understood and applied java inner classes has a link or access to the methods and fields of its o

Answer 1

A classic use for an inner class is the implementation of an iterator inside a container (ArrayList, for example - look for class Itr). All the container wants to expose to the rest of the world is an Iterator. However, it has to create some concrete implementation of that iterator, possibly familiar with the internals of the container. Using an inner class hides the implementation, while keeping it close to the container's implementation. And being inner (i.e. non-static), it is bound to a specific instance of that container, which lets it access private container members.

There are a few types of inner classes - non-static nested class, local classes and anonymous classes. Each one has a somewhat different purpose, so when asking about an inner class, you should specify what kind are you talking about.

Assuming you're referring to non-static inner classes, I'd say the reason to use them is the same as using regular classes (namely abstraction and dividing code into logical units), but there's no reason to make this use of classes visible to the rest of the world. You can also make nested classes public, of course, in which case you'd make them nested instead of independent in order to express their tight relation with the outer class.