意图传递日期结果为空

我搜索了一下,没有找到任何解释为什么null是意图的结果,例如:

How to get Date extra from an intent?

Android: Passing Date in putExra

看来Intent并没有在我的代码中传递任何东西,因为我的代码会导致NullPointerException(我正在研究Big Nerd Ranch的“Android编程”:

首先,我在else语句中声明并定义键EXTRA_DATE,并在CrimeFragment的代码中创建意图(注意:这是代码下面显示的LogCat输出中使用的标记):

      public static final String EXTRA_DATE = "date";
      private static Date date = null;
      ...

        if (small screen) {
             //small screen code
        } else {
            Log.d(TAG, "Large screen: activity");
            date = mCrime.getDate();
            Log.d(TAG, "date is " + date);
            Intent intent = new Intent(getContext(), DatePickerActivity.class);
            intent.putExtra(EXTRA_DATE, date);
            startActivityForResult(intent, REQUEST_DATE);
        }

从该代码段的LogCat输出(如下)中可以看出,“date”变量此时有一个值,就在用Intant发送之前:

CrimeFragment: Large screen: activity
CrimeFragment: date is Mon Oct 16 18:19:24 CDT 2017

startActivityForResult然后启动DatePickerActivity,并输入createFragment(Android Studio建议我添加导入,我做到了):

import static com.bignerdranch.android.criminalintent.DatePickerFragment.EXTRA_DATE;

public class DatePickerActivity extends SingleFragmentActivity {
    private final String TAG = "DatePickerActivity";
    private static final int REQUEST_DATE = 0;

@Override
protected Fragment createFragment() {
    Log.d(TAG, "Entered createFragment");
    Date date = (Date) getIntent().getSerializableExtra(EXTRA_DATE);
    Log.d(TAG,"date = " + date);
    return DatePickerFragment.newInstance(date);
}

如import语句中所述,DatePickerFragment在import语句中引用:

              public static DatePickerFragment newInstance(Date date){
                    Log.d(TAG, "Entered newInstance");
                    Log.d(TAG, "Date is " + date);
                    Bundle args = new Bundle();
                    args.putSerializable(EXTRA_DATE, date);
                    DatePickerFragment fragment = new DatePickerFragment();
                    fragment.setArguments(args);
                   return fragment;
               }

        public abstract class SingleFragmentActivity extends AppCompatActivity {
            protected abstract Fragment createFragment();
            private static final String TAG= "SingleFragmentActivity";

            @Override
            protected void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                Log.d(TAG, "Entered onCreate");
                setContentView(R.layout.activity_fragment);

                FragmentManager fm = getSupportFragmentManager();
                 //fragment_container is the top level FrameLayout in activity_fragment
                Fragment fragment = fm.findFragmentById(R.id.fragment_container);

                if (fragment == null){
                    fragment = createFragment();
                    fm.beginTransaction().add(R.id.fragment_container, fragment).commit();
                }
            }
        }

但意图立即失败,如LogCat行所示,其中“date=null”:

CrimeFragment: Entered onPause
CrimeFragment: Entered onPause
SingleFragmentActivity: Entered onCreate
DatePickerActivity: Entered createFragment
DatePickerActivity: date = null

此空值现在传递给DatePickerFragment:

public class DatePickerFragment extends DialogFragment {
    public static final String EXTRA_DATE = "com.bignerdranch.android.criminalintent.date";
    private static final String ARG_DATE = "Date";
    private static final String TAG = "DatePickerFragment";

    private DatePicker mDatePicker;

public static DatePickerFragment newInstance(Date date){
    Log.d(TAG, "Entered newInstance");
    Log.d(TAG, "Date is " + date);
    Bundle args = new Bundle();
    args.putSerializable(ARG_DATE, date);
    DatePickerFragment fragment = new DatePickerFragment();
    fragment.setArguments(args);
    return fragment;
}

public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container,
                       @Nullable Bundle savedInstanceState){
    Log.d(TAG, "Entered onCreateView");
    Date date = (Date) getArguments().getSerializable(EXTRA_DATE);
    Log.d(TAG, "Date equals: " + date);
    Calendar calendar = Calendar.getInstance();
    calendar.setTime(date);

直到发生NullPointerException:

java.lang.RuntimeException: Unable to start activity 
ComponentInfo{com.bignerdranch.android.criminalintent/com.bignerdranch. 
android.criminalintent.DatePickerActivity}: java.lang.NullPointerException:
Attempt to invoke virtual method ‘long java.util.Date.getTime()’ on a null 
object reference

如上面DatePickerActivity的LogCat输出所示,日期变量为null,因此从该点开始传递null,如上面onCreateView代码的以下LogCat片段所示。

DatePickerFragment: Entered newInstance
DatePickerFragment: Date is null
DatePickerFragment: Entered onCreateView
DatePickerFragment: Date equals: null
AndroidRuntime: Shutting down VM

通过我所有的阅读,包括stackoverflow中的所有帖子。com,似乎这应该是可行的,意图应该只是将日期值从CrimeFragment传递到DatePickerFragment,但它并不像“Date等于null”的值所示。

似乎我没什么可做的了,那个意图应该只是通过“日期”,但事实并非如此。