请高人解释上，TestCase 是一个接口，为什么AndroidTestCase类用extends 而不是用 implements 呢

/**
 * Extend this if you need to access Resources or other things that depend on Activity Context.
 */
public class AndroidTestCase extends TestCase {

    protected Context mContext;
    private Context mTestContext;

    @Override
    protected void setUp() throws Exception {
        super.setUp();
    }

@Deprecated
public interface TestCase extends Runnable
{
    /**
     * Called before run() is called.
     */
    public void setUp(Context context);

    /**
     * Called after run() is called, even if run() threw an exception, but
     * not if setUp() threw an execption.
     */
    public void tearDown();
}