应用程序停止响应并在更改设备方向时发出NullPointerException。

我的应用程序工作正常，除非我旋转装置，在这种情况下，它给NullPointerException。要处理屏幕旋转我已经覆盖onSaveInstanceState。我检查的logcat也它说，在线42是mAnswerTextView.setText(R.string.false_button)Caused by: java.lang.NullPointerException 10-28 23:51:20.769: E/AndroidRuntime(25185): at com.bignerdranch.android.geoquiz.CheatActivity.onCreate(CheatActivity.java:42)代码。 java文件以及logcat详细信息都在下面发布。可能是什么原因？我是Android新手。应用程序停止响应并在更改设备方向时发出NullPointerException。

的java：

public class CheatActivity extends Activity { 

     public static final String EXTRA_ANSWER_IS_TRUE = 
       "com.bignerdranch.android.geoquiz.answer_is_true"; 
     private boolean mAnswerIsTrue; 
     private TextView mAnswerTextView; 
     private Button mShowAnswer; 
     public static final String EXTRA_ANSWER_SHOWN="om.bignerdranch.android.geoquiz.answer_shown"; 
     public static final String KEY_CHEATER = "cheater"; 
     private boolean mAnswerShown; 

     private void setAnswerShowResult(boolean isAnswerShown){ 
      Intent data= new Intent(); 
      data.putExtra(EXTRA_ANSWER_SHOWN, isAnswerShown); 
      setResult(RESULT_OK,data); 
      mAnswerShown = isAnswerShown; 
     } 
     @Override 
     protected void onCreate(Bundle savedInstanceState) { 
      // TODO Auto-generated method stub 
      super.onCreate(savedInstanceState); 
      setContentView(R.layout.activiy_cheat); 
      if (savedInstanceState == null) { 
        setAnswerShowResult(false); 
       } 
      if (savedInstanceState != null){ 
       setAnswerShowResult(savedInstanceState.getBoolean(KEY_CHEATER, false)); 
       if(mAnswerIsTrue){ 
        mAnswerTextView.setText(R.string.true_button); 
       } else{ 
        mAnswerTextView.setText(R.string.false_button); 
       } 
       } 
      mAnswerIsTrue = getIntent() 
        .getBooleanExtra(EXTRA_ANSWER_IS_TRUE,false); 
      mAnswerTextView= (TextView) findViewById(R.id.answerTextView); 
      mShowAnswer=(Button) findViewById(R.id.showAnswerButton); 
      //setAnswerShowResult(false); 
      mShowAnswer.setOnClickListener(new View.OnClickListener() { 

       @Override 
       public void onClick(View v) { 
        // TODO Auto-generated method stub 
        if(mAnswerIsTrue){ 
         mAnswerTextView.setText(R.string.true_button); 
        }else{ 
         mAnswerTextView.setText(R.string.false_button); 
        } 
        setAnswerShowResult(true); 
       } 
      }); 

     } 

     public void onSaveInstanceState(Bundle savedInstanceState){ 

super.onSaveInstanceState(savedInstanceState);   
savedInstanceState.putBoolean(KEY_CHEATER, mAnswerShown); 

      } 

    }

logcat的：

10-28 23:51:20.769: E/AndroidRuntime(25185): FATAL EXCEPTION: main 
10-28 23:51:20.769: E/AndroidRuntime(25185): Process: com.bignerdranch.android.geoquiz, PID: 25185 
10-28 23:51:20.769: E/AndroidRuntime(25185): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bignerdranch.android.geoquiz/com.bignerdranch.android.geoquiz.CheatActivity}: java.lang.NullPointerException 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2198) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3805) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.app.ActivityThread.access$900(ActivityThread.java:139) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1216) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.os.Handler.dispatchMessage(Handler.java:102) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.os.Looper.loop(Looper.java:136) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.app.ActivityThread.main(ActivityThread.java:5086) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at java.lang.reflect.Method.invokeNative(Native Method) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at java.lang.reflect.Method.invoke(Method.java:515) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at dalvik.system.NativeStart.main(Native Method) 
10-28 23:51:20.769: E/AndroidRuntime(25185): Caused by: java.lang.NullPointerException 
10-28 23:51:20.769: E/AndroidRuntime(25185): at com.bignerdranch.android.geoquiz.CheatActivity.onCreate(CheatActivity.java:42) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.app.Activity.performCreate(Activity.java:5248) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110) 
10-28 23:51:20.769: E/AndroidRuntime(25185): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2162) 
10-28 23:51:20.769: E/AndroidRuntime(25185): ... 12 more

来源

2015-10-28 hemkar

您在mAnswerTextView调用setText()你一个值赋给mAnswerTextView之前。将savedInstanceState工作移动到onCreate()的末尾，然后调用findViewById()来检索您的小部件。

来源

2015-10-28 18:35:49 CommonsWare

在第42行，您正在尝试拨打setText对mAnswerTextView，但它尚未初始化。（因为当你改变设备的方向时，onCreate被再次调用）。

要解决此问题，您只需在致电setText之前初始化TextView即可。（只要它仍然是setContentView后，在此之前调用它会导致另一个错误）将以下行

mAnswerTextView= (TextView) findViewById(R.id.answerTextView);

你上面检查其中是否savedInstanceState为null

if (savedInstanceState != null){ 
....

这将确保当活动是第一次创建，并且每当它被重新创建时，您将能够拨打setTextTextView

来源

2015-10-28 18:35:49

谢谢man.My bad.it现在工作正常。但是为什么我在创建活动时第一次没有收到错误？为什么只有在旋转后我得到了nullpointerException？ – hemkar

因为第一次'savedInstanceState'不为空，所以那里的代码没有被调用。调用'setText'的'onCreate'中的其他实例是在初始化'TextView'之后，所以这些工作正常。 –

应用程序停止响应并在更改设备方向时发出NullPointerException。

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