Question

Continuing from this question: Why can't you reduce the visibility of a method in a Java subclass?

I need to create class B that is almost identical to class A, except that B cannot do certain things that A can.

Being a lazy programmer as I am, I tried to inherit A, only to greet with error that B cannot reduce the visibility of A methods. Duh!..

Now A is an API from a vendor, my intention is to encapsulate this API so that it is easier to use.

I wonder what is the best practice to work around this?

Answer 1

Two options:

If you need B to keep the same interface as A (so that client code can use any of the two without changes), you can override “forbidden” methods in B and have them throw an UnsupportedOperationException. For example:

public class A
{
    public int allowedMethod() { ... }
    public int forbiddenMethod() { ... }
}

public class B extends A
{
    public int forbiddenMethod()
    {
        throw new UnsupportedOperationException("Sorry, not allowed.");
    }
}

Or, if you really want the API of B to be a subset of the API of A, then just have B contain an instance of A, and delegate method calls appropriately.

    public class A
    {
        public int allowedMethod() { ... }
        public int forbiddenMethod() { ... }
    }

    public class B
    {
        private A a;

        public int allowedMethod()
        {
            return a.allowedMethod();
        }
    }