在提交MapFragment
並獲取對其地圖的引用(getMap()
)後嘗試設置地圖類型時,出現空指針異常。空指針異常使用Android Google Maps API設置MapType
我認爲錯誤的原因是片段尚未初始化,因此我無法設置地圖類型。
我怎麼知道這個片段什麼時候被初始化了,我可以調用它的公共方法?是否有我可以在我的MainActivity
中實現的接口來知道片段何時加載?
此外,爲什麼我可以在mMapFragment
上調用getMap()
(如果尚未初始化)?它實際上只是GoogleMap
未正確初始化的對象嗎?
這裏是我的代碼:
public class MainActivity extends Activity {
private static final String LOG_TAG = MainActivity.class.getSimpleName();
private FragmentManager fm;
private MapFragment mMapFragment;
private GoogleMap mGoogleMap;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main_container);
fm = getFragmentManager();
mMapFragment = MapFragment.newInstance();
fm.beginTransaction().add(R.id.fragment_container, mMapFragment).commit();
mGoogleMap = mMapFragment.getMap();
mGoogleMap.setMapType(GoogleMap.MAP_TYPE_TERRAIN);
}
}
和XML爲main_container
:
<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/fragment_container"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
</FrameLayout>
而這裏的logcat的輸出:
10-09 11:17:16.457: E/AndroidRuntime(31679): FATAL EXCEPTION: main
10-09 11:17:16.457: E/AndroidRuntime(31679): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.matthewlogan.loopfinder/com.matthewlogan.loopfinder.MainActivity}: java.lang.NullPointerException
10-09 11:17:16.457: E/AndroidRuntime(31679): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2248)
10-09 11:17:16.457: E/AndroidRuntime(31679): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2298)
10-09 11:17:16.457: E/AndroidRuntime(31679): at android.app.ActivityThread.access$600(ActivityThread.java:142)
10-09 11:17:16.457: E/AndroidRuntime(31679): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1285)
10-09 11:17:16.457: E/AndroidRuntime(31679): at android.os.Handler.dispatchMessage(Handler.java:99)
10-09 11:17:16.457: E/AndroidRuntime(31679): at android.os.Looper.loop(Looper.java:137)
10-09 11:17:16.457: E/AndroidRuntime(31679): at android.app.ActivityThread.main(ActivityThread.java:5270)
10-09 11:17:16.457: E/AndroidRuntime(31679): at java.lang.reflect.Method.invokeNative(Native Method)
10-09 11:17:16.457: E/AndroidRuntime(31679): at java.lang.reflect.Method.invoke(Method.java:525)
10-09 11:17:16.457: E/AndroidRuntime(31679): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:974)
10-09 11:17:16.457: E/AndroidRuntime(31679): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:790)
10-09 11:17:16.457: E/AndroidRuntime(31679): at dalvik.system.NativeStart.main(Native Method)
10-09 11:17:16.457: E/AndroidRuntime(31679): Caused by: java.lang.NullPointerException
10-09 11:17:16.457: E/AndroidRuntime(31679): at com.matthewlogan.loopfinder.MainActivity.onCreate(MainActivity.java:36)
10-09 11:17:16.457: E/AndroidRuntime(31679): at android.app.Activity.performCreate(Activity.java:5133)
10-09 11:17:16.457: E/AndroidRuntime(31679): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1098)
10-09 11:17:16.457: E/AndroidRuntime(31679): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2212)
10-09 11:17:16.457: E/AndroidRuntime(31679): ... 11 more