Java 恢复活动-NullPointerException

我在恢复一个活动时遇到问题，当我打开它时，我的应用程序崩溃了，我得到一个NullPointerException，这很奇怪，因为我有一个=空条件

    @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //setContentView(R.layout.activity_maps);//frag
    setContentView(R.layout.map_view);//linear
    // Obtain the SupportMapFragment and get notified when the map is ready to be used.
    SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
            .findFragmentById(R.id.map);
    mapFragment.getMapAsync(this);
}

@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;
    gotoLocation(51.508182, -0.140315, DEFAULT_ZOOM);
    //mMap.setMyLocationEnabled(true);

}
    @Override
protected void onResume(){
    super.onResume();
    MapStateManager mgr = new MapStateManager(this);
    CameraPosition position = mgr.getSavedCameraPosition();
    if(position != null){
        CameraUpdate update = CameraUpdateFactory.newCameraPosition(position);
        mMap.moveCamera(update);
    }
}

错误在线mMap.moveCamera（更新）

MapStateManager.java

公共类MapStateManager{

private static final String LONGITUDE = "longitude";
private static final String LATITUDE = "latitude";
private static final String ZOOM = "zoom";
private static final String BEARING = "bearing";
private static final String TILT = "tilt";
private static final String MAPTYPE = "MAPTYPE";

private static final String PREFS_NAME="mapCameraState";//definition for shared

private SharedPreferences mapStatePrefs;// save data on the device

public MapStateManager(Context context){
    mapStatePrefs = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);
}

public void saveMapState(GoogleMap map){
    SharedPreferences.Editor editor = mapStatePrefs.edit();
    CameraPosition position = map.getCameraPosition();
    editor.putFloat(LATITUDE, (float) position.target.latitude);
    editor.putFloat(LONGITUDE, (float) position.target.longitude);
    editor.putFloat(TILT, position.tilt);
    editor.putFloat(ZOOM, position.zoom);
    editor.putFloat(BEARING, position.bearing);

    editor.commit();
}

public CameraPosition getSavedCameraPosition(){
    double latitude = mapStatePrefs.getFloat(LATITUDE, 0);
    double longitude = mapStatePrefs.getFloat(LONGITUDE, 0);

    if (latitude==0){
        return null;
    }
    LatLng target = new LatLng(latitude, longitude);

    float zoom = mapStatePrefs.getFloat(ZOOM, 0);
    float bearing = mapStatePrefs.getFloat(BEARING, 0);
    float tilt = mapStatePrefs.getFloat(TILT, 0);

    CameraPosition position = new CameraPosition(target, zoom, tilt, bearing);
    return position;
}

---

}

您忘了检查mMap

---

mMap为空。您也不需要在oncreate中调用map，而是通过接口来实现。它可以在任何时候出现，在onResume期间不太可能出现（如never）。

您可以查看

帖子！=null

但是

mMap

仍然可以为null（这就是发生的情况）

在您的

onResume（）

方法中，您需要再次获取地图，因为它可能在您的应用程序处于非活动状态时丢失

试着这样做：

@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;

    //Maybe move this to an else on the if below? I'm not sure exactly what this does
    gotoLocation(51.508182, -0.140315, DEFAULT_ZOOM);

    //Update to the saved position.
    MapStateManager mgr = new MapStateManager(this);
    CameraPosition position = mgr.getSavedCameraPosition();
    if(position != null){
        CameraUpdate update = CameraUpdateFactory.newCameraPosition(position);
        mMap.moveCamera(update);
    }
}

@Override
protected void onResume(){
    super.onResume();

    //Get the map asynchronously again.

    // Obtain the SupportMapFragment and get notified when the map is ready to be used.
    SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map);
    mapFragment.getMapAsync(this);
}

---

哪一行抛出NullPointerException？我编辑了我的问题：错误在第mMap.moveCamera（更新）行上；你在哪里实例化mMap@noev这将停止错误，但应用程序可能不会按照他想要的方式运行。@Khalos是的，我刚刚读了他说“奇怪，因为我有一个！=null条件。”即使他实际上没有检查null。非常感谢Khalos。它是固定的，应用程序不会粉碎和保存以前的相机位置！我是android开发的新手，对此非常兴奋：）也谢谢你的解释！干杯