错误“Junit 测试中没有公共 TestCase(String name) 或 TestCase()

我是 junit test android 的初学者。我正在跟进本教程 http://developer.android.com/training/activity-testing/activity-basic-testing.html但得到这个错误

junit.framework.AssertionFailedError: Class com.example.projectfortest.test.MainActivityTest has no public constructor TestCase(String name) or TestCase()
at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:191)
at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:176)
at android.test.InstrumentationTestRunner.onStart(InstrumentationTestRunner.java:554)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1701)

这是我的代码：

public class MainActivityTest extends
        ActivityInstrumentationTestCase2<MainActivity> {

    private MainActivity mMainActivity;
    private TextView mFirstTestText;
    private String expected, actual;

    public MainActivityTest(Class<MainActivity> activityClass) {
        super(activityClass);
    }

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

        mMainActivity = getActivity();
        mFirstTestText = (TextView) mMainActivity.findViewById(R.id.txt1);

        expected = mMainActivity.getString(R.string.txt1_test);
        actual = mFirstTestText.getText().toString();
    }

    public void testPreconditions() {
        assertNotNull("mMainActivity is null", mMainActivity);
        assertNotNull("mFirstTestText is null", mFirstTestText);
    }

    public void testMyFirstTestTextView_labelText() {
        assertEquals(expected, actual);
    }
}

我没有发现我的代码有任何问题。请帮忙

正如异常所说，向您的类添加默认构造函数。这是测试框架初始化所必需的。替换你的构造函数：

public MainActivityTest(Class<MainActivity> activityClass) {
    super(activityClass);
}

通过以下方式：

public MainActivityTest() {
    super(MainActivity.class);
}

该构造函数没有框架所需的参数，并显示在代码清单中你的教程 http://developer.android.com/training/activity-testing/activity-basic-testing.html#fixture: