的Android - NullPointerException異常 - 無法啓動活動ComponentInfo

Question

你好我得到的這行代碼一個NullPointerException：

findViewById(R.id.mapScreen).setDrawingCacheEnabled(true); 

，當我嘗試運行在模擬器這一活動。

我認爲這可能是我的XML的問題，但一切看起來都是正確的。下面是Java代碼：

@Override 
protected void onCreate(Bundle load) 
{ 
    super.onCreate(load); 

    //CODE FROM RIDETRACKING.JAVA 
    FrameLayout frame = new FrameLayout(this); 
    frame.setId(CONTENT_VIEW_ID); 
    setContentView(frame, new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT)); 
    //END OF CODE FROM RIDETRACKING.JAVA 

    //setContentView(R.layout.map); 
    findViewById(R.id.mapScreen).setDrawingCacheEnabled(true); 
    mUnits = new UnitsI18n(this); 
    mLoggerServiceManager = new GPSLoggerServiceManager(this); 

}

這是我的XML代碼：

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:orientation="vertical" 
android:id="@+id/mapScreen"> 
<com.google.android.maps.MapView 
    xmlns:android="http://schemas.android.com/apk/res/android" 
    android:id="@+id/myMapView" 
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent" 
    android:enabled="true" 
    android:clickable="true" 
    android:apiKey="0S37nJcRwodWjzWn7yJwFRc9qr5gvaD2bdvOpEQ" 
    android:visibility="visible" /> 
</RelativeLayout> 

這裏是我的logcat輸出：

03-20 16:14:05.294: E/AndroidRuntime(2424): FATAL EXCEPTION: main 
03-20 16:14:05.294: E/AndroidRuntime(2424): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android.gpstracker/com.polaris.epicriders.Rides.RideTracking}: java.lang.NullPointerException 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at android.app.ActivityThread.access$600(ActivityThread.java:141) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at android.os.Handler.dispatchMessage(Handler.java:99) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at android.os.Looper.loop(Looper.java:137) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at android.app.ActivityThread.main(ActivityThread.java:5039) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at java.lang.reflect.Method.invokeNative(Native Method) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at java.lang.reflect.Method.invoke(Method.java:511) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at dalvik.system.NativeStart.main(Native Method) 
03-20 16:14:05.294: E/AndroidRuntime(2424): Caused by: java.lang.NullPointerException 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at com.android.riders.Rides.RideTracking.onCreate(RideTracking.java:181) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at android.app.Activity.performCreate(Activity.java:5104) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144) 
03-20 16:14:05.294: E/AndroidRuntime(2424):  ... 11 more 

Answer 1

設置活動的內容，而不是的FrameLayout。

setContentView(R.layout.layout_main); 

因爲您沒有設置活動佈局而您在這裏試圖從佈局中獲取視圖，所以您將得到空值。

findViewById(R.id.mapScreen).setDrawingCacheEnabled(true);