setOnClickListeners（）崩潰我的應用程序

Question

所以我通過Eclipse ADT創建了一個新項目，我的應用程序不能運行，這裏有一些有用的信息。如果你能解決問題，我將非常感激。

protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    if (savedInstanceState == null) { 
     getSupportFragmentManager().beginTransaction() 
       .add(R.id.container, new PlaceholderFragment()) 
       .commit(); 
    } 

    final TextView answer = (TextView) findViewById(R.id.textView1); 
    Button askButton = (Button) findViewById(R.id.button1); 
    askButton.setOnClickListener(new View.OnClickListener() { 

     @Override 
     public void onClick(View v) { 
      // TODO Auto-generated method stub 
      answer.setText("veik"); 
     } 
    }); 

} 

這是MainActivity.java的唯一部分被編輯，這裏是XML文件的編輯部分：

<Button 
    android:id="@+id/button1" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_centerInParent="true" 
    android:text="Button" /> 

最後，消息的logcat的：

06-19 00:04:56.740: D/AndroidRuntime(7026): Shutting down VM 
06-19 00:04:56.740: W/dalvikvm(7026): threadid=1: thread exiting with uncaught exception (group=0x41760ba8) 
06-19 00:04:56.740: E/AndroidRuntime(7026): FATAL EXCEPTION: main 
06-19 00:04:56.740: E/AndroidRuntime(7026): Process: com.example.babyplease, PID: 7026 
06-19 00:04:56.740: E/AndroidRuntime(7026): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.babyplease/com.example.babyplease.MainActivity}: java.lang.NullPointerException 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2198) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2248) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at android.app.ActivityThread.access$800(ActivityThread.java:138) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1199) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at android.os.Handler.dispatchMessage(Handler.java:102) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at android.os.Looper.loop(Looper.java:136) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at android.app.ActivityThread.main(ActivityThread.java:5050) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at java.lang.reflect.Method.invokeNative(Native Method) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at java.lang.reflect.Method.invoke(Method.java:515) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:605) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at dalvik.system.NativeStart.main(Native Method) 
06-19 00:04:56.740: E/AndroidRuntime(7026): Caused by: java.lang.NullPointerException 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at com.example.babyplease.MainActivity.onCreate(MainActivity.java:31) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at android.app.Activity.performCreate(Activity.java:5241) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2162) 
06-19 00:04:56.740: E/AndroidRuntime(7026):  ... 11 more 
06-19 00:04:59.365: I/Process(7026): Sending signal. PID: 7026 SIG: 9 

對不起，如果有太多的文字，我只是很困惑，爲什麼它不工作。

Answer 1

在textView1中沒有在XML中定義的TextView。

您需要添加一個TextView小部件來解決該NullPointerException。

<TextView 
    android:id="@+id/textView1" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_centerInParent="true"/>