Question

Since i can access private variables of an inner class directly and … the only way to access inner class is via the outer class .. is there any value in having public accessor/ mutator methods for an inner class?

public class Outer {
    Inner   i;

    Outer() {
        i.v = 5;
    }

    private class Inner {
        private int v;
    }  
}

For the purposes of this question, assume that no lazy instantiation or other data manipulation is needed upon access.

Answer 1

Well, making the fields non-private would avoid synthetic methods being created to access those private members. In your example, the compiled Outer$Inner class contains a synthetic method like this:

static int access$002(Outer$Inner, int);
   Code:
      0: aload_0
      1: iload_1
      2: dup_x1
      3: putfield      #1                  // Field v:I
      6: ireturn

Maybe that will be inlined down to nothing, maybe it won’t… but unless there’s any reason to force its existence, why not just give v the default access?

As for using accessor methods rather than using the fields directly… that’s more of a matter of taste. You can’t prevent the outer class having direct access to the fields, so it’s a little like asking whether you should create accessors for private fields within a class.

As mentioned in another answer, using accessors makes it more refactoring-friendly. Is the nested class just an implementation detail which will never need to move? If so, maybe that’s fine. I find that if I’ve got a class like that which is just encapsulating fields with no other behaviour, I make the fields public (within a private class, don’t forget) to advertise “Hey, I’m not checking anything here…”