Use methods declared in implementation that are not defined in interface

Question

I have a class defined by an interface

public interface Test {
    void testMethod();
}

Test test = new TestImpl();

public class TestImpl implements Test {         


        

Answer 1

The problem is with the following line:

Test test = new TestImpl();

This tells the compiler to forget that the new object is a TestImpl and treat it as a plain old Test. As you know, Test does not have anotherMethod().

What you did is called "upcasting" (casting an object to a more general type). As another poster has said, you can fix your problem by not upcasting:

TestImpl test = new TestImpl();

If you're sure that a Test object is really a TestImpl, you can downcast it (tell the compiler it is a more specific type):

Test test = new TestImpl();
:
((TestImpl) test).anotherMethod();

This is generally a bad idea, however, since it can cause ClassCastException. Work with the compiler, not against it.