标记未显示在Google地图中

Question

我在Android STudio中实现了Google地图中有关标记的代码，但它们并没有出现。我按照本教程https://www.sitepoint.com/adding-maps-to-an-android-app-with-google-play-services/进行了操作，但事后我做了教程所说的所有事情，我没有得到任何标记。

package com.example.user.appsightseeing;

导入android.Manifest; import android.content.pm.PackageManager;

import android.os.Bundle;
import android.support.v4.app.ActivityCompat;
import android.support.v4.app.FragmentActivity;
import android.widget.Toast;

import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.MapFragment;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.CameraPosition;
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;

    private static final LatLng PERTH = new LatLng(41.3114444,19.8338172);
    private static final LatLng SYDNEY = new LatLng(41.3183397,19.8189917);
    private static final LatLng BRISBANE = new LatLng(41.3279672,19.8167301);

    private Marker mPerth;
    private Marker mSydney;
    private Marker mBrisbane;


    @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);
        MapFragment fragment_home = (MapFragment) getFragmentManager().findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);
    }


    /**
     * 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

        mMap.setMapType(GoogleMap.MAP_TYPE_SATELLITE);

        CameraPosition googlePlex = CameraPosition.builder()

                .target(new LatLng(41.3114444,19.8338172))
                .zoom(20)
                .bearing(0)
                .tilt(45)
                .build();
        mPerth = mMap.addMarker(new MarkerOptions()
                .position(PERTH)
                .title("Perth"));
        mPerth.setTag(0);

        mSydney = mMap.addMarker(new MarkerOptions()
                .position(SYDNEY)
                .title("Sydney"));
        mSydney.setTag(0);

        mBrisbane = mMap.addMarker(new MarkerOptions()
                .position(BRISBANE)
                .title("Brisbane"));
        mBrisbane.setTag(0);
        mMap.moveCamera(CameraUpdateFactory.newCameraPosition(googlePlex));
        // Set a listener for marker click.
        mMap.setOnMarkerClickListener((GoogleMap.OnMarkerClickListener) this);
    }

        public boolean onMarkerClick (final Marker marker) {

            // Retrieve the data from the marker.
            Integer clickCount = (Integer) marker.getTag();

            // Check if a click count was set, then display the click count.
            if (clickCount != null) {
                clickCount = clickCount + 1;
                marker.setTag(clickCount);
                Toast.makeText(this,
                        marker.getTitle() +
                                " has been clicked " + clickCount + " times.",
                        Toast.LENGTH_SHORT).show();
            }


            if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
                // TODO: Consider calling
                //    ActivityCompat#requestPermissions
                // here to request the missing permissions, and then overriding
                //   public void onRequestPermissionsResult(int requestCode, String[] permissions,
                //                                          int[] grantResults)
                // to handle the case where the user grants the permission. See the documentation
                // for ActivityCompat#requestPermissions for more details.

            }
            mMap.setMyLocationEnabled(true);
            return true;
        }}