我在Android應用中的Google地圖上顯示用戶的當前位置時遇到問題。 誰能告訴我我該如何處理我的代碼

Question

這是我的MapActivity的代碼。 它僅用於跟蹤用戶位置，但僅在移動時才顯示位置，但是當我添加代碼以在應用啟動時顯示當前位置時，它開始崩潰。

    import android.*;
import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Build;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.support.v4.app.FragmentActivity;
import android.os.Bundle;
import android.support.v4.content.ContextCompat;
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.MarkerOptions;

public class MapsActivity extends FragmentActivity implements OnMapReadyCallback {

    private GoogleMap mMap;

    LocationManager locationManager;

    LocationListener locationListener;

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);

        if (requestCode == 1) {

            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {

                if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) == PackageManager.PERMISSION_GRANTED) {
                    {

                        locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListener);

                    }

                }

            }

        }

    }

    @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);
    }


    /**
     * 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;

        locationManager = (LocationManager) this.getSystemService(Context.LOCATION_SERVICE);

        locationListener = new LocationListener() {
            @Override
            public void onLocationChanged(Location location) {

                LatLng userLocation = new LatLng(location.getLatitude(), location.getLongitude());

                mMap.clear();
                mMap.addMarker(new MarkerOptions().position(userLocation).title("Your Location"));
                mMap.moveCamera(CameraUpdateFactory.newLatLng(userLocation));


            }

            @Override
            public void onStatusChanged(String s, int i, Bundle bundle) {

            }

            @Override
            public void onProviderEnabled(String s) {

            }

            @Override
            public void onProviderDisabled(String s) {

            }
        };

        if (Build.VERSION.SDK_INT < 23) {

            locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListener);

        } else {

            if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED) {

                ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION}, 1);

            } else {

                locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 0, 0, locationListener);

                Location lastKnownLocation = locationManager.getLastKnownLocation(LocationManager.GPS_PROVIDER);

                LatLng userLocation = new LatLng(lastKnownLocation.getLatitude(), lastKnownLocation.getLongitude());
                mMap.clear();

                mMap.addMarker(new MarkerOptions().position(userLocation).title("Your Location"));
                mMap.moveCamera(CameraUpdateFactory.newLatLng(userLocation));


            }


        }


    }
}

問題是，我可以在更改位置並從Emulator發送位置時獲取位置，但是當我添加else部分以顯示當前位置時，應用崩潰了。

這是啟動應用程序時遇到的錯誤。

2019-03-06 01：50：39.000 24094-24094 / com.example.userlocationdemo E / AndroidRuntime：致命例外：主進程：com.example.userlocationdemo，PID：24094 java.lang.RuntimeException：無法啟動活動ComponentInfo { com.example.userlocationdemo / com.example.userlocationdemo.MapsActivity}：嘗試在android.app.ActivityThread上的空對象引用上調用虛擬方法“ double android.location.Location.getLatitude（）”。 android.app.ActivityThread.handle上的performLaunchActivity（ActivityThread.java:2678）android.app.ActivityThread $ H.handleMessage（ActivityThread。）上的android.app.ActivityThread.-wrap12（ActivityThread.java）的android.os.Handler.dispatchMessage（Handler.java:102）的java：1490），android.app.ActivityThread.main（ActivityThread.java:6165）的android.os.Looper.loop（Looper.java:154）的com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（Zygote）上的java.lang.reflect.Method.invoke（本機方法） com.android.internal.os.ZygoteInit.main（ZygoteInit.java:778）上的Init.java:888）原因：java.lang.NullPointerException：嘗試調用虛擬方法'double android.location.Location.getLatitude（） '在com.example.userlocationdemo.MapsActivity.onCreate（MapsActivity.java:94）在android.app.Activity.performCreate（Activity.java:6687）在android.app.Instrumentation.callActivityOnCreate（Instrumentation.java）上的空對象引用上：1140），位於android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2631）

Answer 1

您正在嘗試從第94行的MapsActivity的oncreate方法中的位置的空對象獲取緯度，如日志打印中所述

com.example.userlocationdemo.MapsActivity.onCreate（MapsActivity.java:94）上的空對象引用上的android.location.Location.getLatitude（）'