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();    }

@Deprecatedpublic 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();}