How to implement only certain methods of an abstract class?

Question

In my concrete class I need to implement (set public) only certain methods of an abstract class. So I cannot extend it beacause all the abstract methods are public.

I could use composition, and forward just methods I need, but I obviously need to implement the abstract class. So, I'm wondering, is it a good approach to implement an abstract class in a private class and then forward in the parent class only the needed methods of the inner class?

For example:

public class ConcreteClass{

    private InnerClass inner = new InnerClass();

    public String fwMethod() {
        return inner.method1();
    }

    private class InnerClass extends AbstractClass {
        public String method1(){ ...}
        public String method2(){ ...}
    }
}

Is there some better approach?

Answer 1

A concrete subclass of an abstract class must provide at least minimal implementations of the abstract methods.

• You could implement the methods that you don't want to implement to throw an unchecked exception. The UnsupportedOperationException is good choice, or you could implement your own class with a more specific meaning.

• You could declare the subclass as abstract ... though that means you won't be able instantiate it.

• You could refactor your base classes and interfaces so that your concrete class has fewer abstract methods to implement.

Answer 2

Why don't you modify the abstract class to inherit from another class, and you implements only the one with the methods you need ?

Example if you need only aa, ab instead of aa, ab, aaa, aab: class A, methods aa, ab class B, methods aaa, aab B inherits from A.

Your concrete class inherits from A and has access only to aa and ab, avoinding implements of aaa, aab.

Answer 3

Implement the methods you need, and throw an UnsopportedOperationException on any method you don't want to implement.

http://docs.oracle.com/javase/7/docs/api/java/lang/UnsupportedOperationException.html