Question

(This is a hypothetical question for discussion, I have no actual problem).

Say that I’m making an implementation of SortedSet by extending LinkedHashMap:

class LinkedHashSortedMapThing extends LinkedHashMap implements SortedSet {
 ...
}

Now programmers who use this class may do

LinkedHashMap x = new LinkedHashSortedMapThing();

But what if I consider the extending of LinkedHashMap an implementation detail, and do not want it to be a part of the class’ contract? If people use the line above, I can no longer freely change this detail without worrying about breaking existing code.
Is there any way to prevent this sort of thing, other than favouring composition over inheritance (which is not always possible due to private/protected members)?

Answer 1

I think the easiest way would be to make a private inner class that extends LinkedHashMap, and have LinkedHashSortedMapThing keep a reference to that and point all its methods there.

class LinkedHashSortedMapthing implements SortedSet {
    private class Foo extends LinkedHashMap {
        ...
    }

    private Foo foo;

    public void clear() {foo.clear();}
    public boolean containsValue(Object value) {return foo.containsValue(value);}
    ...
}