Defining inner class outside java file

I want to create a class, ClassB, as inner class of ClassA, but I want to write down outside ClassA.java file.

How can I do this?

It will be a lot of inner class, and ClassA.java file will be enormous.

UPDATE
What I really want to do is define ten classes that they will be only accessible by one class. All of them are defined inside the same package.

Thanks.