在 Android 应用程序中更改地图类型

我现在正在编写一个简单的应用程序，一切正常，除非我添加了 mapType 更改。

package com.example.preset19;

import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.animation.Animation;
import android.view.animation.AnimationUtils;
import android.widget.Toast;

import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.Marker;
import com.google.android.gms.maps.model.MarkerOptions;

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback {

private GoogleMap mMap;
static final LatLng MELBOURNE = new LatLng(-37.81319, 144.96298);

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_maps);
    // 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);
    mMap.setMapType(GoogleMap.MAP_TYPE_SATELLITE);

}




/**
 * Manipulates the map once available.
 * This callback is triggered when the map is ready to be used.
 * This is where we can add markers or lines, add listeners or move the camera. In this case,
 * we just add a marker near Sydney, Australia.
 * If Google Play services is not installed on the device, the user will be prompted to install
 * it inside the SupportMapFragment. This method will only be triggered once the user has
 * installed Google Play services and returned to the app.
 */
@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;



    // Add a marker in Sydney and move the camera
    LatLng sydney = new LatLng(-34, 151);
    mMap.addMarker(new MarkerOptions().position(sydney).title("Marker in Sydney"));
    mMap.moveCamera(CameraUpdateFactory.newLatLng(sydney));



    LatLng Bern = new LatLng(46, 56);
    mMap.addMarker(new MarkerOptions().position(Bern).title("Marker in Bern"));
    mMap.moveCamera(CameraUpdateFactory.newLatLng(Bern));

    Marker melbourne = mMap.addMarker(new MarkerOptions()
            .position(MELBOURNE)
            .title("Melbourne")
            .snippet("Population: 4'137'400"));



    googleMap.addMarker(new MarkerOptions()
            .position(new LatLng(10, 10))
            .title("Hello World"));


}

}

如果我想启动应用程序，它会崩溃而不显示之前的地图。Android Studio 没有检测到任何故障，但这是我的起始协议：

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.preset19, PID: 13639
              Theme: themes:{default=overlay:com.cyngn.hexo, iconPack:system, fontPkg:com.cyngn.hexo, com.android.systemui=overlay:com.cyngn.hexo, com.android.systemui.navbar=overlay:com.cyngn.hexo}
              java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.preset19/com.example.preset19.MapsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.gms.maps.GoogleMap.setMapType(int)' on a null object reference
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2450)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2510)
                  at android.app.ActivityThread.-wrap11(ActivityThread.java)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:148)
                  at android.app.ActivityThread.main(ActivityThread.java:5461)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$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 com.google.android.gms.maps.GoogleMap.setMapType(int)' on a null object reference
                  at com.example.preset19.MapsActivity.onCreate(MapsActivity.java:35)
                  at android.app.Activity.performCreate(Activity.java:6251)
                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2403)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2510) 
                  at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1363) 
                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                  at android.os.Looper.loop(Looper.java:148) 
                  at android.app.ActivityThread.main(ActivityThread.java:5461) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
W/DynamiteModule: Local module descriptor class for com.google.android.gms.googlecertificates not found.
I/DynamiteModule: Considering local module com.google.android.gms.googlecertificates:0 and remote module com.google.android.gms.googlecertificates:3
I/DynamiteModule: Selected remote version of com.google.android.gms.googlecertificates, version >= 3
W/System: ClassLoader referenced unknown path: /data/data/com.google.android.gms/app_chimera/m/00000023/n/armeabi-v7a
W/System: ClassLoader referenced unknown path: /data/data/com.google.android.gms/app_chimera/m/00000023/n/armeabi
I/Process: Sending signal. PID: 13639 SIG: 9
Application terminated.