通过构造函数传递数组适配器列表显示

问题描述

我正在尝试使用适配器从我的LocationsFragment.class到我的DeviceAdmin.class，向我的mylatitude添加两个字符串，分别为mylongitude和ListView。 我正在通过构造方法。 但是，每当我在adapter.add(mylatitude + " " + mylongitude);

中调用DeviceAdmin.class时，它似乎都返回null。

  java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ArrayAdapter.add(java.lang.Object)' on a null object reference
        at com.ayush.trixter.ui.captures.DeviceAdmin$1.onLocationChanged(DeviceAdmin.java:103)

第103行是：adapter.add("Latitude: " + mylat + "Longitude: " + mylon);

DeviceAdmin.class

import android.Manifest;
import android.annotation.SuppressLint;
import android.app.admin.DeviceAdminReceiver;
import android.app.admin.DevicePolicyManager;
import android.content.Context;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.graphics.Paint;
import android.graphics.SurfaceTexture;
import android.hardware.Camera;
import android.hardware.camera2.CameraAccessException;
import android.hardware.camera2.CameraCaptureSession;
import android.hardware.camera2.CameraCharacteristics;
import android.hardware.camera2.CameraManager;
import android.location.Criteria;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.Build;
import android.os.Looper;
import android.os.UserHandle;
import android.util.Log;
import android.util.SparseIntArray;
import android.view.ContextThemeWrapper;
import android.view.Surface;
import android.view.View;
import android.widget.Adapter;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.core.content.PermissionChecker;

import com.ayush.trixter.R;
import com.ayush.trixter.ui.locations.LocationFragment;
import com.google.android.gms.location.LocationCallback;
import com.google.android.gms.location.LocationRequest;
import com.google.android.gms.location.LocationServices;
import com.google.android.material.snackbar.Snackbar;
import com.google.firebase.database.FirebaseDatabase;

import java.util.ArrayList;

public class DeviceAdmin extends DeviceAdminReceiver
{
    private static final SparseIntArray ORIENTATIONS = new SparseIntArray();
    public CameraManager cam;
    LocationManager locationManager;
    final String TAG = "[Trixter]";
    DevicePolicyManager mgr;
    FirebaseDatabase db;
    public boolean mTracking = false;
    public ArrayList<String> listItems=new ArrayList<>();
    LocationFragment lf = new LocationFragment();
    ArrayAdapter<String> adapter = lf.adapter;



    @Override
    public void onPasswordFailed(@NonNull Context ctx,@NonNull Intent intent,@NonNull UserHandle user) {
        super.onPasswordFailed(ctx,intent,user);
        cam = (CameraManager) ctx.getSystemService(Context.CAMERA_SERVICE);
        mgr = (DevicePolicyManager) ctx.getSystemService(Context.DEVICE_POLICY_SERVICE);

        Log.d(TAG,"PasswordFailed");
        int pwdfailcount = mgr.getCurrentFailedPasswordAttempts();

        if(pwdfailcount==3)
        {
            takeSnapShots();
            getloc(ctx);

        }
    }

    public void getloc(Context ctx)
    {
        locationManager = (LocationManager) ctx.getSystemService(Context.LOCATION_SERVICE);
        Criteria criteria = new Criteria();
        String provider =  locationManager.getBestProvider(criteria,true);
        boolean gps_enabled = false;
        boolean network_enabled = false;

        try
        { gps_enabled =   locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER); }
        catch (Exception e) {}
        try { network_enabled = locationManager.isProviderEnabled(LocationManager.NETWORK_PROVIDER);} catch (Exception e) {}

        if(PermissionChecker.checkSelfPermission(ctx,Manifest.permission.ACCESS_COARSE_LOCATION) == PermissionChecker.PERMISSION_GRANTED && PermissionChecker.checkSelfPermission(ctx,Manifest.permission.ACCESS_FINE_LOCATION) == PermissionChecker.PERMISSION_GRANTED) {
            if(gps_enabled == true && network_enabled ==true) {
                locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER,new LocationListener() {
                    @Override
                    public void onLocationChanged(@NonNull Location loc) {
                        double mylat = loc.getLatitude();
                        double mylon = loc.getLongitude();
                        double myacc = loc.getAccuracy();
                        double time = loc.getTime();
                        double myalt = loc.getAltitude();
                        Log.d("[TrixterLocation]","The Latitude:- " + mylat + "\nThe Longitude:- " + mylon + "\nThe Altitude:- " + myalt + "\n The Accuracy" + myacc + "\nThe Time:- " + time);
                        adapter.add("Latitude: " + mylat + "Longitude: " + mylon);
                    }
                });
            }
            else
            {
                Toast.makeText(ctx,"Location is not enabled",Toast.LENGTH_SHORT).show();
                Log.d("[Trixter] ","Error: Location or Internet Not Enabled");
            }
        }

    }



    private void takeSnapShots()
    {

    }





}

LocationsFragment.class

package com.ayush.trixter.ui.locations;

import android.content.Context;
import android.location.LocationManager;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.ListFragment;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;

import com.ayush.trixter.R;
import com.ayush.trixter.ui.captures.DeviceAdmin;

import java.util.ArrayList;

public class LocationFragment extends ListFragment {

    ListView loclst;
    LocationManager locationManager;
    public ArrayList<String> listItems=new ArrayList<>();
    public ArrayAdapter<String> adapter = (ArrayAdapter<String>) getListAdapter();

    public LocationFragment()
    {

    }
    public View onCreateView(@NonNull LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {
        View root = inflater.inflate(R.layout.fragment_locations,container,false);
            loclst = root.findViewById(android.R.id.list);
            Context context = getActivity();
            locationManager = (LocationManager) getActivity().getSystemService(Context.LOCATION_SERVICE);
            adapter = new ArrayAdapter<String>(getActivity(),android.R.layout.simple_list_item_2,listItems);
            loclst.setAdapter(adapter);

          return root;
    }

}

据我说： 由于未初始化ArrayAdapter且仅在片段打开时才初始化，即onCreateView（）

，它将返回null

但是我找不到解决方案来使其正常工作。 另外，如果有人有更好的主意更轻松地执行此操作，请告知。

解决方法

它将无法正常工作，因为您正在DeviceAdmin类上创建新的LocationFramgent。

您需要做的是访问实际的LocationFragment。

要正确回答您，我需要知道：

片段是从哪里创建的？

您在哪里调用getLoc方法？