Question

Let’s say I have a Java class A, which requires a helper class B. That helper class is only used in A, and has no other purpose. Also, B doesn’t need to use A in any way (don’t call methods or access fields).

So, the question is: where to put B?

There are the following options:

1. Static nested class. In my opinion, it just makes code less clear (much more indentation and such).

   public class A {
       ...
       private static class B { ... }
   }
   

2. Non-public class in the same source. I like this option.

   public class A {
       ...            
   }
   
   class B { 
       ... 
   }
   

3. Non-public class in the separate source. Looks like this option has a little overhead, though.

   // A.java
   public class A {
       ...            
   }
   
   // B.java
   class B { 
       ... 
   }
   

For now, I prefer the 2nd option. What are your thoughts on it? What is the best practice?
Are there any authoritative sources on it?

Answer 1

I strongly vote for option (1). The idea is, that class B is only needed by class A and option (1) is the only alternative that clearly expresses that intention: class B is part of class A.