How to rectify? — “both define getObjectCopy(), but with unrelated return types” — but it's *one* function

Question

I have the following interface heirarchy (with all non-relevant functions stripped out). I am getting this error when trying to compile it:

types ValidLineGettable

Answer 1

Assuming all the return values extend Copyable, have all versions of getObjectCopy() return Copyable. For example:

public interface ValidateValue extends Copyable
{
     // Other functions...

     @Override
     Copyable getObjectCopy();
}

public Blammy implements ValidateValue
{
    // Other functions...

     @Override
    public Copyable getObjectCopy()
    {
        SomethingThatExtendsCopyable blammy = new SomethingThatExtendsCopyable();

        return (Copyable)blammy;
    }
}

Edit

In your code above the error is caused by the fact that the "getObjectCopy" method has a different return value in the ValidateValue and ValidLineGettable interfaces, but the calling signature is the same. In java, you do not get polymorphism by changing only the return value; this results in a compile error.

If you change the return value to Copyable then the TextLineValidator no longer gains value by extending both of its parent interfaces. A simpler approach is to have one interface (Copyable) and multiple classes that implement that interface, each of which returns a Copyable value which may be an instance of a class that extends (or implements) Copyable.