Thanks. I think that the might achieve what I was after. Since there was some confusion about what I really want to do, and why, I'll try to give you an example, but I'm going to have to show it in pseudo-java.

public class Foo
{
    public void doSomething(Bar bar)
    { ... }
}
[download]

now, I want to write code that tests Foo.doSomething. I want that test to be independent of any problems that Bar might have. So, I write this...

public class FooTest
{
    testDoSomething()
    {
        class MockBar extends Bar
        {
            //    code to simulate a "Bar"
        }
        ...
        bar = new MockBar();
        foo = new Foo();
        foo.doSomething(bar);
        
        // check that the right things happened to bar    
    }    
}
[download]