創建OnClickListener時的NullPointerException（Android）

Question

這是一個非常簡單的android應用程序。我剛學，現在寫爲Android的代碼，而且我不確定爲什麼行：add.setOnClickListener（新View.OnClickListener（）拋出一個NullPointerException

public class StartingPoint extends ActionBarActivity{ 

int counter; 
Button add, sub; 
TextView display; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_starting_point); 

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

    counter = 0; 
    add = (Button) findViewById(R.id.bAdd); 
    sub = (Button) findViewById(R.id.bSub); 
    display = (TextView) findViewById(R.id.tvDisplay); 

    add.setOnClickListener(new View.OnClickListener() 
    { 

     @Override 
     public void onClick(View v) 
     { 
      counter++; 
      display.setText("Your total is " + counter); 
     } 
    }); 

    sub.setOnClickListener(new View.OnClickListener() 
    { 

     @Override 
     public void onClick(View v) 
     { 
      counter--; 
      display.setText("Your total is " + counter); 
     } 
    }); 

} 

下面是XML代碼：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:paddingBottom="@dimen/activity_vertical_margin" 
android:paddingLeft="@dimen/activity_horizontal_margin" 
android:paddingRight="@dimen/activity_horizontal_margin" 
android:paddingTop="@dimen/activity_vertical_margin" 
tools:context="com.thenewboston.jaredh.StartingPoint$PlaceholderFragment" > 

<TextView 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_centerHorizontal="true" 
    android:text="@string/total" 
    android:textSize="40sp" 
    android:id="@+id/tvDisplay" 
    /> 

<Button 
    android:layout_width="250dp" 
    android:layout_height="wrap_content" 
    android:layout_alignLeft="@+id/tvDisplay" 
    android:layout_below="@+id/tvDisplay" 
    android:layout_gravity="center" 
    android:text="@string/addButton" 
    android:textSize="20sp" 
    android:id="@+id/bAdd" /> 

<Button 
    android:layout_width="250dp" 
    android:layout_height="wrap_content" 
    android:layout_alignLeft="@+id/bAdd" 
    android:layout_below="@+id/bAdd" 
    android:layout_gravity="center" 
    android:text="@string/subButton" 
    android:textSize="20sp" 
    android:id="@+id/bSub"/> 

Logcat Error: 
     03-29 17:44:24.930: D/AndroidRuntime(25367): Shutting down VM 
03-29 17:44:24.930: W/dalvikvm(25367): threadid=1: thread exiting with uncaught exception (group=0x41831898) 
03-29 17:44:24.940: E/AndroidRuntime(25367): FATAL EXCEPTION: main 
03-29 17:44:24.940: E/AndroidRuntime(25367): java.lang.RuntimeException: Unable to start activity ComponentInf{com.thenewboston.jaredh/com.thenewboston.jaredh.StartingPoint}: java.lang.NullPointerException 
03-29 17:44:24.940: E/AndroidRuntime(25367): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2359) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at android.app.ActivityThread.access$700(ActivityThread.java:165) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1326) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at android.os.Handler.dispatchMessage(Handler.java:99) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at android.os.Looper.loop(Looper.java:137) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at android.app.ActivityThread.main(ActivityThread.java:5455) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at java.lang.reflect.Method.invokeNative(Native Method) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at java.lang.reflect.Method.invoke(Method.java:525) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at dalvik.system.NativeStart.main(Native Method) 
03-29 17:44:24.940: E/AndroidRuntime(25367): Caused by: java.lang.NullPointerException 
03-29 17:44:24.940: E/AndroidRuntime(25367): at com.thenewboston.jaredh.StartingPoint.onCreate(StartingPoint.java:39) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at android.app.Activity.performCreate(Activity.java:5372) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1104) 
03-29 17:44:24.940: E/AndroidRuntime(25367): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267) 
03-29 17:44:24.940: E/AndroidRuntime(25367): ... 11 more 

正如你可以看到這是一個非常簡單的應用程序只需加1時，無論是按鈕被點擊的文本字段。任何幫助，可以大大apprecitated。只是不清楚爲什麼NullPointerExcepti正在導致。

Answer 1

我猜你所提交的XML（不說真的是哪個文件，儘管XML文本甚至不是有效的）是你在開頭添加的PlaceholderFragment的佈局。但是，該片段不會立即添加，只是預定由FragmentManager添加。因此，當您立即嘗試查找其UI組件時，該活動無法執行此操作，並且add引用設置爲空findViewById。