Question

I have the following situation:

        public abstract class A {
            private Object superMember;
            public A() {
                superMember = initializeSuperMember();
                // some additional checks and stuff based on the initialization of superMember (***)
            }

            protected abstract Object initializeSuperMember();
        }

        class B extends A {
            private Object subMember;
            public B(Object subMember) {
                super();
                subMember = subMember;
            }

            protected Object initializeSuperMember() {
                // doesn't matter what method is called on subMember, just that there is an access on it
                return subMember.get(); // => NPE
            }
        }

The problem is that I get a NPE on a new object B creation.
I know I can avoid this by calling an initializeSuperMember() after I assign the subMember content in the subclass constructor but it would mean I have to do this for each of the subclasses(marked * in the code).
And since I have to call super() as the first thing in the subclass constructor I can’t initialize subMember before the call to super().

Anyone care to tell me if there’s a better way to do this or if I am trying to do something alltogether wrong?

Answer 1

Two problems:

First, you should never call an overrideable member function from a constructor, for just the reason you discovered. See this thread for a nice discussion of the issue, including alternative approaches.

Second, in the constructor for B, you need:

this.subMember = subMember;

The constructor parameter name masks the field name, so you need this. to refer to the field.