Question

我在以下方法中获得了NullPointerException，并且我已经尝试了所有我无法想到的事情。

   private void showCustomListView(int year, int month, int day) {

        List<Transaction> mTransactionList = new ArrayList<Transaction>();
        mTransactionList = AppManager.getTransactionsByDay(year, month, day);
        DailyTransactionAdapter mAdapter = new DailyTransactionAdapter(getActivity(), 0, mTransactionList);
        dailyTransactionListView.setAdapter(mAdapter);
        }

以下是上下文：我有一个活动：Activity_A托管Fragment_A。从Fragment_A的onResume()回调中调用上述方法，这是o nResume()方法

  @Override
    public void onResume() {
        super.onResume();
        //When the Fragment resumes set the shown date to today
        mYear = selectedDate.get(Calendar.YEAR);
        mMonth = selectedDate.get(Calendar.MONTH);
        mDay = selectedDate.get(Calendar.DAY_OF_MONTH);
        tvDisplayDate.setText(new StringBuilder()
                .append(mMonth + 1).append("-").append(mDay).append("-")
                .append(mYear).append(" "));

        showCustomListView(mYear, mMonth, mDay);
        btnChangeDate.setOnClickListener(new View.OnClickListener() {
            @Override
            //After the view is created set the onclick lister for the
            //Change date button
            public void onClick(View view) {
                showDatePicker();
            }
        });

    }

此适配器的数据来自内存列表，如此。这是 AppManager

中的代码部分

   public static List<Transaction> getTransactionsByDay(int year, int month, int day){
        List<Transaction> dayTransactions = new ArrayList<Transaction>();
        List<Transaction> allTransactions = getTransactionList();
        Log.i(TAG, "Size: " + allTransactions.size());
        for (Transaction tempTransaction : allTransactions){
            if (tempTransaction.getTransactionDate().get(Calendar.YEAR) == year &&
                    tempTransaction.getTransactionDate().get(Calendar.MONTH) == month &&
                    tempTransaction.getTransactionDate().get(Calendar.DAY_OF_MONTH) == day)
            {
                dayTransactions.add(tempTransaction);
            }

        }
        return dayTransactions;
    }

当我启动应用程序时，这可以正常运行，就像在IDE（Android Studio）中运行应用程序一样。问题出现在我启动另一个Activity - Activity_B，在上面的列表运行并且可见之后，在Activity_B中我将一个项目添加到内存列表中，列表的大小从10到11开始。

当我现在从Activity_B转到Fragment_A时，应用程序在上面的showCustomListView()方法中与NPE崩溃。我试图清除列表，我已经尝试了adapter.notifydatasetchanged()但都没有成功，只有在我在应用程序运行时添加到列表后才会崩溃，只有在我重新启动应用程序后它才会正常运行。< / p>

我还应该尝试什么：这里有一些NPE

11-04 09:36:19.147    6464-6464/com.valuecardnfc.valuecardnfc E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.valuecardnfc.valuecardnfc, PID: 6464
    java.lang.NullPointerException
            at com.valuecardnfc.valuecardnfc.Models.AppManager.getTransactionsByDay(AppManager.java:80)
            at com.valuecardnfc.valuecardnfc.Fragments.DailyTransactionFragment.showCustomListView(DailyTransactionFragment.java:160)
            at com.valuecardnfc.valuecardnfc.Fragments.DailyTransactionFragment.onResume(DailyTransactionFragment.java:102)
            at android.support.v4.app.Fragment.performResume(Fragment.java:1521)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:963)
            at android.support.v4.app.FragmentManagerImpl.performPendingDeferredStart(FragmentManager.java:823)
            at android.support.v4.app.FragmentManagerImpl.startPendingDeferredFragments(FragmentManager.java:1128)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1477)
            at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:472)
            at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:1068)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
            at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
            at android.view.View.measure(View.java:16497)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at android.view.View.measure(View.java:16497)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
            at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
            at android.view.View.measure(View.java:16497)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)

Answer 1

tempTransaction.getTransactionDate().get(Calendar.YEAR)

NullPointerException有两种可能性：

tempTransaction为null。
getTransactionDate()返回null。

由于您在多个位置使用getTransactionDate()的返回值，因此您应该创建一个新变量：

Calendar transactionDate = tempTransaction.getTransactionDate();

现在，您可以检查此transactionDate是否为null，并且可以在if条件下重复使用它。您还会立即知道tempTransaction是否为null，因为此行会在这种情况下抛出NullPointerException。

Android中无法解释的空指针异常

1 个答案: