Why have public static class inside a class

Question

I was going through some code and I saw this:

public class A {
    public A(SomeObject obj) {
      //Do something
    }
    //Some stuff
  public static class B {
    //Some other stuff
  }
}

I was wondering since even the inner class is public why have it as nested and not a separate class? Also, can I do this here: new A.B(SomeObject) ? I feel this defeats the purpose of a static class but I saw this implementation as well so wanted to know.

Answer 1

I was wondering since even the inner class is public why have it as nested and not a separate class?

That's really a matter to ask whoever wrote the class. It can allow the outer class to act as a "mini-namespace" though - if the nested class is only useful in the context of the outer class, it seems reasonable. It indicates deliberate tight coupling between the two classes. I most often see this in the context of the builder pattern:

Foo foo = new Foo.Builder().setBar(10).build();

Here it makes sense to me to have Foo.Builder nested within Foo rather than as a peer class which would presumably be called FooBuilder.

Note that it also gives some visibility differences compared with just unrelated classes.

Also, can I do this here: new A.B(SomeObject) ?

No, because B doesn't have a constructor with a SomeObject parameter - only A does (in the example you've given).

I feel this defeats the purpose of a static class

You should try to work out exactly what you deem the purpose of a static class to be, and in what way this defeats that purpose. Currently that's too vague a statement to be realistically discussed.

Answer 2

You would have an inner class like this so

• you can keep an class which only exists to support the outer class encapsulated.
• you want to be able to access private members of the outer class or other nested classes.
• you want a nested class with static fields (a weak reason I know ;)
• you have a class with a very generic name like Lock or Sync which you wouldn't want to be mixed with other classes of the same name used by classes in the same package.

can I do this here: new A.B(SomeObject) ?

You can.

I feel this defeats the purpose of a static class

It takes getting used to but once you start you may have trouble not turning your entire program into one file.java ;)