public interface MyInterface {
    public String getName();
}

public class B implements MyInterface {
...
}

public class BB implements MyInterface {
...
}

------解决方案--------------------
mockito  http://mockito.org/

Java code
 //You can mock concrete classes, not only interfaces
 LinkedList mockedList = mock(LinkedList.class);
 
 //stubbing
 when(mockedList.get(0)).thenReturn("first");
 when(mockedList.get(1)).thenThrow(new RuntimeException());

------解决方案--------------------
可以通过classloader来加载Mock类B，让Mock类B屏蔽掉实际的类B

------解决方案--------------------
new A(new B())...