java - 将额外内容放入 bundle 中会导致奇怪的冲突和应用程序崩溃......为什么？

Question

我在主应用程序 Activity 中遇到了一个奇怪的错误。

我的 Activity 中有一个 FragmentStatePagerAdapter (作为内部类)，其 instantiateItem(ViewGroup container, intposition) 方法被重写以处理方向更改。

此方法工作正常，直到我通过 onSaveInstanceState(Bundle outState) 将字符串放入 Bundle 中。当像这样实现该方法时，不会调用 instantiateItem:

 @Override
protected void onSaveInstanceState(Bundle outState){
    if (myObject != null) {
        outState.putString(MY_OBJECT_KEY, myObject.getName());
    }
    super.onSaveInstanceState(outState);
}

当我删除 outState.putString(MY_OBJECT_KEY, myObject.getName()) 时，每次更改方向时都会毫无问题地调用 instantiateItem。为什么是这样？非常感谢任何帮助。

我得到的错误如下:

E/AndroidRuntime: FATAL EXCEPTION: main
   Process: com.android.miki.rpiplantsapp, PID: 22292                                     
   java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android.miki.rpiplantsapp/com.android.miki.rpiplantsapp.PlantStatsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.android.miki.rpiplantsapp.MoistureFragment.setOptimalStatText(java.lang.String)' on a null object reference
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
       at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:4077)
       at android.app.ActivityThread.-wrap15(ActivityThread.java)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1350)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:148)
       at android.app.ActivityThread.main(ActivityThread.java:5422)
       at java.lang.reflect.Method.invoke(Native Method)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.android.miki.rpiplantsapp.MoistureFragment.setOptimalStatText(java.lang.String)' on a null object reference
       at com.android.miki.rpiplantsapp.PlantStatsActivity$ViewPageAdapter.updateCurrentFragsOptimal(PlantStatsActivity.java:733)
       at com.android.miki.rpiplantsapp.PlantStatsActivity.loadPlantItemTabs(PlantStatsActivity.java:426)
       at com.android.miki.rpiplantsapp.PlantStatsActivity.onCreate(PlantStatsActivity.java:225)
       at android.app.Activity.performCreate(Activity.java:6251)
       at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
       at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:4077) 
       at android.app.ActivityThread.-wrap15(ActivityThread.java) 
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1350) 
       at android.os.Handler.dispatchMessage(Handler.java:102) 
       at android.os.Looper.loop(Looper.java:148) 
       at android.app.ActivityThread.main(ActivityThread.java:5422) 
       at java.lang.reflect.Method.invoke(Native Method) 
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

instantiateItem 方法如下所示:

@Override
    public Object instantiateItem(ViewGroup container, int position){
        switch (position){
            case 0:
                MoistureFragment moistureTab = (MoistureFragment) super.instantiateItem(container, position);
                currentMoistureFragment = moistureTab;
                return currentMoistureFragment;

            case 1:
                LightFragment lightTab = (LightFragment) super.instantiateItem(container, position);
                currentLightFragment = lightTab;
                return currentLightFragment;
            case 2:
                TemperatureFragment tempTab = (TemperatureFragment) super.instantiateItem(container, position);
                currentTempFragment = tempTab;
                return currentTempFragment;
            default:
                return null;
        }
    }

updateCurrentFragsOptimal() 看起来像这样:

   public void updateCurrentFragsOptimal(double moistureValue, double lightValue, double tempValue){
        currentMoistureFragment.setOptimalStatText(String.valueOf(moistureValue));
        currentLightFragment.setOptimalStatText(String.valueOf(lightValue));
        currentTempFragment.setOptimalStatText(String.valueOf(tempValue));
    }

Answer 1

嗯，这比我想象的要简单...我发现当 savedInstanceState 不为 null 时，我在 onCreate() 方法中调用了一个方法，那调用了另一种方法。此方法做了几件事，阻止了 instantiateItem 被调用，因此 fragment 最终为 null 并导致 NullPointerException。只是一个简单的逻辑错误。