java - 在空对象引用上调用虚拟方法 'double android.location.Location.getLatitude()' 时出错

Question

我想要做的就是让用户获得他喜欢的类型的位置列表。例如，如果输入是医院，我的应用程序将使用搜索字符串“医院”打开谷歌地图。但正如文档中所建议的那样，使用像 geo:0,0?q=hospital 这样的地理编码 uri 显示坐标 0 纬度和 0 经度附近的所有医院。因此，当我尝试使用以下代码首先获取用户坐标时。

Places Decoder.java

package com.kkze.Mappy;

import android.app.Activity;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.location.Criteria;
import android.location.Location;
import android.location.LocationManager;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.provider.Settings;
import android.text.TextUtils;
import android.util.Log;
import android.widget.Toast;

public class PlacesDecoder extends Activity {
Intent intentThatCalled;
public double latitude;
public double longitude;
public LocationManager locationManager;
public Criteria criteria;
public String bestProvider;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    intentThatCalled = getIntent();
    String voice2text = intentThatCalled.getStringExtra("v2txt");
    getLocation(voice2text);
}
public static boolean isLocationEnabled(Context context)
{
    int locationMode = 0;
    String locationProviders;
    if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT)
    {
        try
        {
            locationMode = Settings.Secure.getInt(context.getContentResolver(), Settings.Secure.LOCATION_MODE);
        } catch (Settings.SettingNotFoundException e) {
            e.printStackTrace();
        }
        return locationMode != Settings.Secure.LOCATION_MODE_OFF;
    }
    else
    {
        locationProviders = Settings.Secure.getString(context.getContentResolver(), Settings.Secure.LOCATION_PROVIDERS_ALLOWED);
        return !TextUtils.isEmpty(locationProviders);
    }
}

public void getLocation(String voice2txt) {
    locationManager = (LocationManager)  this.getSystemService(Context.LOCATION_SERVICE);
    criteria = new Criteria();
    bestProvider = String.valueOf(locationManager.getBestProvider(criteria, true)).toString();
    Location location = locationManager.getLastKnownLocation(bestProvider);
    if (isLocationEnabled(PlacesDecoder.this)) {
            Log.e("TAG", "GPS is on");
            latitude = location.getLatitude();
            longitude = location.getLongitude();
            Toast.makeText(PlacesDecoder.this, "latitude:" + latitude + " longitude:" + longitude, Toast.LENGTH_SHORT).show();
            searchNearestPlace(voice2txt);

        }
    else
    {
        AlertDialog.Builder notifyLocationServices = new AlertDialog.Builder(PlacesDecoder.this);
        notifyLocationServices.setTitle("Switch on Location Services");
        notifyLocationServices.setMessage("Location Services must be turned on to complete this action. Also please take note that if on a very weak network connection,  such as 'E' Mobile Data or 'Very weak Wifi-Connections' it may take even 15 mins to load. If on a very weak network connection as stated above, location returned to application may be null or nothing and cause the application to crash.");
        notifyLocationServices.setPositiveButton("Ok, Open Settings", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                Intent openLocationSettings = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
                PlacesDecoder.this.startActivity(openLocationSettings);
                finish();
            }
        });
        notifyLocationServices.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                finish();
            }
        });
        notifyLocationServices.show();
    }
}

public void searchNearestPlace(String v2txt) {
    Log.e("TAG", "Started");
    v2txt = v2txt.toLowerCase();
    String[] placesS = {"accounting", "airport", "aquarium", "atm", "attraction", "bakery", "bakeries", "bank", "bar", "cafe", "campground", "casino", "cemetery", "cemeteries", "church", "courthouse", "dentist", "doctor", "electrician", "embassy", "embassies", "establishment", "finance", "florist", "food", "grocery", "groceries", "supermarket", "gym", "health", "hospital", "laundry", "laundries", "lawyer", "library", "libraries", "locksmith", "lodging", "mosque", "museum", "painter", "park", "parking", "pharmacy", "pharmacies", "physiotherapist", "plumber", "police", "restaurant", "school", "spa", "stadium", "storage", "store", "synagog", "synagogue", "university", "universities", "zoo"};
    String[] placesM = {"amusement park", "animal care", "animal care", "animal hospital", "art gallery", "art galleries", "beauty salon", "bicycle store", "book store", "bowling alley", "bus station", "car dealer", "car rental", "car repair", "car wash", "city hall", "clothing store", "convenience store", "department store", "electronics store", "electronic store", "fire station", "funeral home", "furniture store", "gas station", "general contractor", "hair care", "hardware store", "hindu temple", "home good store", "homes good store", "home goods store", "homes goods store", "insurance agency", "insurance agencies", "jewelry store", "liquor store", "local government office", "meal delivery", "meal deliveries", "meal takeaway", "movie rental", "movie theater", "moving company", "moving companies", "night club", "pet store", "place of worship", "places of worship", "post office", "real estate agency", "real estate agencies", "roofing contractor", "rv park", "shoe store", "shopping mall", "subway station", "taxi stand", "train station", "travel agency", "travel agencies", "veterinary care"};
    int index;
    for (int i = 0; i <= placesM.length - 1; i++) {
        Log.e("TAG", "forM");
        if (v2txt.contains(placesM[i])) {
            Log.e("TAG", "sensedM?!");
            index = i;
            Uri gmmIntentUri = Uri.parse("geo:" + latitude + "," + longitude + "?q=" + placesM[index]);
            Intent mapIntent = new Intent(Intent.ACTION_VIEW, gmmIntentUri);
            mapIntent.setPackage("com.google.android.apps.maps");
            startActivity(mapIntent);
            finish();
        }
    }
    for (int i = 0; i <= placesS.length - 1; i++) {
        Log.e("TAG", "forS");
        if (v2txt.contains(placesS[i])) {
            Log.e("TAG", "sensedS?!");
            index = i;
            Uri gmmIntentUri = Uri.parse("geo:" + latitude + "," + longitude + "?q=" + placesS[index]);
            Intent mapIntent = new Intent(Intent.ACTION_VIEW, gmmIntentUri);
            mapIntent.setPackage("com.google.android.apps.maps");
            startActivity(mapIntent);
            finish();
        }
    }
}
}

问题是位置始终返回 null。而且我知道一个事实，只要启用位置，另一个应用程序 Jarvis 就可以轻松完成任务，如果没有，则只是要求用户这样做。但在相同条件下，我的应用程序总是崩溃。

我做了什么:

1. 日日夜夜试图解决这个问题。
2. 浏览了有关该主题的大量页面。
3. 尝试了我自己的不同代码。

它仍然发生。我是 Android 的初学者。请，请帮助我。没有办法解决我的问题吗？甚至认为其他应用程序以某种方式设法做到这一点。

这是我的 LogCat。

Process: com.kkze.Mappy, PID: 6742
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.kkze.Mappy/com.kkze.Mappy.PlacesDecoder}: java.lang.NullPointerException: Attempt to invoke virtual method 'double android.location.Location.getLatitude()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2658)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2723)
        at android.app.ActivityThread.access$900(ActivityThread.java:172)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1422)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:145)
        at android.app.ActivityThread.main(ActivityThread.java:5832)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'double android.location.Location.getLatitude()' on a null object reference
        at com.kkze.Mappy.PlacesDecoder.getLocation(PlacesDecoder.java:62)
        at com.kkze.Mappy.PlacesDecoder.onCreate(PlacesDecoder.java:32)
        at android.app.Activity.performCreate(Activity.java:6221)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2611)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2723)
at android.app.ActivityThread.access$900(ActivityThread.java:172)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1422)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:5832)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

同时检查 location!=null 需要 ACCESS_FINE_LOCATION 权限，这会扰乱我的应用，导致它始终返回 null。

Answer 1

正如 Jon Skeet 在评论中提到的，getLastKnownLocation() 方法可以而且将会返回 null。主要问题是它不会提示操作系统请求新的位置锁定，而只是检查是否有来自其他应用程序位置请求的最后一个已知位置。如果最近没有其他应用发出位置请求，则会向您返回一个空位置。

保证您实际获得位置的唯一方法是请求一个位置，这是通过调用 requestLocationUpdates() 来完成的.

传递给 onLocationChanged() 回调方法的位置不会为 null，因为回调仅发生在成功的位置锁定时。

请注意，在您的应用注册位置更新的整个过程中，它会导致额外的电池消耗，因此请务必尽快取消注册位置更新。在这里，您似乎可以在第一个位置出现后立即取消注册。

此外，您可能需要考虑在此 Activity 等待位置锁定时显示一个进度对话框，以便向用户提供应用正在等待某事的一些反馈。

这是您的代码的一般结构:

public class MainActivity extends Activity
        implements LocationListener {

    Intent intentThatCalled;
    public double latitude;
    public double longitude;
    public LocationManager locationManager;
    public Criteria criteria;
    public String bestProvider;

    String voice2text; //added

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        intentThatCalled = getIntent();
        voice2text = intentThatCalled.getStringExtra("v2txt");
        getLocation();
    }

    public static boolean isLocationEnabled(Context context)
    {
       //...............
        return true;
    }

    protected void getLocation() {
        if (isLocationEnabled(MainActivity.this)) {
            locationManager = (LocationManager)  this.getSystemService(Context.LOCATION_SERVICE);
            criteria = new Criteria();
            bestProvider = String.valueOf(locationManager.getBestProvider(criteria, true)).toString();

            //You can still do this if you like, you might get lucky:
            Location location = locationManager.getLastKnownLocation(bestProvider);
            if (location != null) {
                Log.e("TAG", "GPS is on");
                latitude = location.getLatitude();
                longitude = location.getLongitude();
                Toast.makeText(MainActivity.this, "latitude:" + latitude + " longitude:" + longitude, Toast.LENGTH_SHORT).show();
                searchNearestPlace(voice2text);
            }
            else{
                //This is what you need:
                locationManager.requestLocationUpdates(bestProvider, 1000, 0, this);
            }
        }
        else
        {
            //prompt user to enable location....
            //.................
        }
    }

    @Override
    protected void onPause() {
        super.onPause();
        locationManager.removeUpdates(this);

    }

    @Override
    public void onLocationChanged(Location location) {
        //Hey, a non null location! Sweet!

        //remove location callback:
        locationManager.removeUpdates(this);

        //open the map:
        latitude = location.getLatitude();
        longitude = location.getLongitude();
        Toast.makeText(MainActivity.this, "latitude:" + latitude + " longitude:" + longitude, Toast.LENGTH_SHORT).show();
        searchNearestPlace(voice2text);
    }

    @Override
    public void onStatusChanged(String provider, int status, Bundle extras) {

    }

    @Override
    public void onProviderEnabled(String provider) {

    }

    @Override
    public void onProviderDisabled(String provider) {

    }

    public void searchNearestPlace(String v2txt) {
        //.....
    }
}