這兩個對象爲什麼是空的？

我正在編寫一個Android應用程序和兩個按鈕（「btnGPSShowLocation」和「按鈕」），我使用的是null。我已經將它們實例化了，所以我不知道它們爲什麼是null。這裏是我的代碼：這兩個對象爲什麼是空的？

public class AndroidLocationActivity extends Activity { 
Button btnGPSShowLocation; 
Button button; 


AppLocationService appLocationService; 
private final static int INTERVAL = 1000 * 60 * 2; //2 minutes 
Handler mHandler = new Handler(); 


@Override 
protected void onCreate(Bundle savedInstanceState) { 

    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    appLocationService = new AppLocationService(
      AndroidLocationActivity.this); 

    btnGPSShowLocation = (Button) findViewById(R.id.btnGPSShowLocation); 

    btnGPSShowLocation.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View arg0) { 

      Location gpsLocation = appLocationService 
        .getLocation(LocationManager.GPS_PROVIDER); 

      if (gpsLocation != null) { 
       double latitude = gpsLocation.getLatitude(); 
       double longitude = gpsLocation.getLongitude(); 
       Toast.makeText(
         getApplicationContext(), 
         "Mobile Location (GPS): \nLatitude: " + latitude 
           + "\nLongitude: " + longitude, 
         Toast.LENGTH_LONG).show(); 
      } else { 
       showSettingsAlert("GPS"); 
      } 

     } 
    }); 

    button = (Button) findViewById(R.id.button12); 

    button.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      startActivity(new Intent("locationActivity")); 
     } 
    }); 
}

這裏是XML，可能是相關的：

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:app="http://schemas.android.com/apk/res-auto" 
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" 
app:layout_behavior="@string/appbar_scrolling_view_behavior" 
tools:context="it226.myapplicationit226androidapp.AndroidLocationActivity" 
tools:showIn="@layout/activity_gps"> 

<Button 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="Show Location\n(GPS)" 
    android:id="@+id/btnGPSShowLocation" 
    android:layout_alignParentTop="true" 
    android:layout_marginTop="83dp" 
    android:layout_alignParentLeft="true" 
    android:layout_alignParentStart="true" 
    android:layout_alignParentRight="true" 
    android:layout_alignParentEnd="true" /> 

<Button 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="@string/nextgps" 
    android:id="@+id/button12" 
    android:layout_alignParentBottom="true" 
    android:layout_centerHorizontal="true" 
    android:layout_marginBottom="106dp" /> 
</RelativeLayout>

編輯：這裏是logcat的

> 04-27 23:34:30.525 18310-18310/it226.myapplicationit226androidapp 
> E/AndroidRuntime: FATAL EXCEPTION: main 
>                      Process: it226.myapplicationit226androidapp, PID: 18310 
>                      java.lang.RuntimeException: Unable to start activity 
> ComponentInfo{it226.myapplicationit226androidapp/it226.myapplicationit226androidapp.AndroidLocationActivity}: 
> java.lang.NullPointerException: Attempt to invoke virtual method 'void 
> android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' 
> on a null object reference 
>                       at 
> android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416) 
>                       at 
> android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
>                       at android.app.ActivityThread.-wrap11(ActivityThread.java) 
>                       at 
> android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
>                       at android.os.Handler.dispatchMessage(Handler.java:102) 
>                       at android.os.Looper.loop(Looper.java:148) 
>                       at android.app.ActivityThread.main(ActivityThread.java:5417) 
>                       at java.lang.reflect.Method.invoke(Native Method) 
>                       at 
> com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
>                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
>                      Caused by: java.lang.NullPointerException: Attempt to invoke virtual 
> method 'void 
> android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' 
> on a null object reference 
>                       at 
> it226.myapplicationit226androidapp.AndroidLocationActivity.onCreate(AndroidLocationActivity.java:39) 
>                       at android.app.Activity.performCreate(Activity.java:6237) 
>                       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.-wrap11(ActivityThread.java)  
>                       at 
> android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)  
>                       at android.os.Handler.dispatchMessage(Handler.java:102)  
>                       at android.os.Looper.loop(Looper.java:148)  
>                       at android.app.ActivityThread.main(ActivityThread.java:5417)  
> 
                        at java.lang.reflect.Method.invoke(Native Method)

來源

2016-04-28 thefreeman

後stracktrace！ –

後logcat顯示空對象錯誤 –

你可以確認它是'activity_main.xml'包含按鈕？ –

刪除tools:showIn="@layout/activity_gps"

此屬性在佈局的根元素上設置通過另一種佈局進行編輯。這允許您指向包含此佈局的其中一個佈局，並且在設計時這個包含的佈局將使用圍繞它的外部佈局進行渲染。

在這裏看到reference

來源

2016-04-28 04:58:52

FWIW，這不是崩潰的問題，因爲工具名稱空間只適用於設計時，而不是在發生問題的運行時。 – laalto

@laalto是的。欣賞你的話！ –

這兩個對象爲什麼是空的？

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