通过构造函数|ListView传递数组适配器

Question

我正试图使用从LocationsFragment.class到DeviceAdmin.class的适配器将两个字符串(比如mylatitude和mylongitude(添加到ListView。我正在学习构造函数方法。但每当我在DeviceAdmin.class中调用adapter.add(mylatitude + " " + mylongitude);时，它似乎都会返回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行为：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, 0, 0, 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：-它将返回null，因为ArrayAdapter没有初始化，只有在片段打开时才初始化，即onCreateView((

但我找不到一个解决方案来让它发挥作用。此外，如果有人有更好的想法可以更容易地做到这一点，请告诉。

Answer 1

它不起作用，因为您正在DeviceAdmin类上创建一个新的LocationFramgent。

您需要做的是访问实际的LocationFragment。

为了正确回答你，我需要knwo:

碎片是从哪里创建的？

在哪里调用getLoc方法？