OOP design of little helper classes

Question

Lets say I have a class C that is doing some job.

For that I need a little very simple helper class H (e.g. representation of a pair or a 3-tuple). H is only needed in C.

I would put H inside of C.

class C
{
   void foo ()
   {
     // ... use H to do the job more easy ...
   }

   class H
   {
      // very simple and contained stuff
   }
}

Is putting H inside of C a good idea? I do it to have it contained and at the place I think it should be and nowhere else. But I am not sure it is good design.

Answer 1

That is a very good design. The visibility of the classes, variables or methods should always be limited to the only required scope, never more.
If later this class needs to be used outside the inner class, just move it somewhere else.

Answer 2

More of an addendum:

First: if H objects are supposed to exist without an owning C instance, then H should be static. You might even want to make it private.

Then, keep in mind: you have an inner class here, so H's full name is actually something like whatever.package.com.C.H. In other words: you really tied H to C.

What if you decide at some point that H has enough other aspects that make it worth to be "its own" thing? Then you might have to touch some more code, to turn your whatever.package.com.C.H into whatever.package.com.H.

Thus you could consider to make H a toplevel class, but to not make it public. So you keep it in C.java, but not as inner class!

( but of course, that really depends on "style", I think that the Google java coding standard tells you to only have one top level class per .java file)

Final words: if there is a way to turn your H class into something that could be used in other places, then I would go forward and right away make it a public class of its own. If re-using seems reasonable, write your code so it can be easily re-used! And a 3-value tuple doesn't sound like it is very specific to anything, so it sounds like a candidate for re-use.

Answer 3

Yes it is good design. What you're trying to do is called inner class.you can also make it private. If it doesn't need exclusive access to the members of the outer class, make it a static nested class , it will require less memory.