java - 应用程序停止响应并在更改设备方向时给出 NullPointerException。

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

我的应用程序工作正常，除非我旋转设备，在这种情况下它会给出 NullPointerException .为了处理屏幕旋转，我已经覆盖了 onSaveInstanceState .我检查了 logcat 也说 Caused by: java.lang.NullPointerException
10-28 23:51:20.769: E/AndroidRuntime(25185): at com.bignerdranch.android.geoquiz.CheatActivity.onCreate(CheatActivity.java:42)第 42 行的代码是 mAnswerTextView.setText(R.string.false_button) 。下面发布了 java 文件以及 logcat 详细信息。可能是什么原因？我是 Android 新手。

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:

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

Answer 1

在为 mAswerTextView 赋值之前，您正在对 mAswerTextView 调用 setText()。在调用 findViewById() 检索小部件后，将您的 savedInstanceState 工作移至 onCreate() 的末尾。