java - Android Studio错误: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference

Question

我正在尝试为我的 Android Studio 移动应用程序获取两个位置之间的折线。但我收到以下错误:尝试在空对象引用上调用接口(interface)方法“int java.util.List.size()”

    --------- beginning of crash
2019-09-12 21:14:14.297 5106-5106/com.hfad.los E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.hfad.los, PID: 5106
    java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
        at com.hfad.los.PointsParser.onPostExecute(PointsParser.java:63)
        at com.hfad.los.PointsParser.onPostExecute(PointsParser.java:23)
        at android.os.AsyncTask.finish(AsyncTask.java:695)
        at android.os.AsyncTask.-wrap1(Unknown Source:0)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:712)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

这是我的 PointsParser.java

package com.hfad.los;

import android.content.Context;
import android.graphics.Color;
import android.os.AsyncTask;
import android.util.Log;

import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.PolylineOptions;
import com.hfad.los.DataParser;
import com.hfad.los.TaskLoadedCallback;

import org.json.JSONObject;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

/**
 * Created by Vishal on 10/20/2018.
 */

public class PointsParser extends AsyncTask<String, Integer, List<List<HashMap<String, String>>>> {
    TaskLoadedCallback taskCallback;
    String directionMode = "driving";

    public PointsParser(Context mContext, String directionMode) {
        this.taskCallback = (TaskLoadedCallback) mContext;
        this.directionMode = directionMode;
    }

    // Parsing the data in non-ui thread
    @Override
    protected List<List<HashMap<String, String>>> doInBackground(String... jsonData) {

        JSONObject jObject;
        List<List<HashMap<String, String>>> routes = null;

        try {
            jObject = new JSONObject(jsonData[0]);
            Log.d("mylog", jsonData[0].toString());
            DataParser parser = new DataParser();
            Log.d("mylog", parser.toString());

            // Starts parsing data
            routes = parser.parse(jObject);
            Log.d("mylog", "Executing routes");
            Log.d("mylog", routes.toString());

        } catch (Exception e) {
            Log.d("mylog", e.toString());
            e.printStackTrace();
        }
        return routes;
    }

    // Executes in UI thread, after the parsing process
    @Override
    protected void onPostExecute(List<List<HashMap<String, String>>> result) {
        ArrayList<LatLng> points;
        PolylineOptions lineOptions = null;
        // Traversing through all the routes
        for (int i = 0; i < result.size(); i++) {
            points = new ArrayList<>();
            lineOptions = new PolylineOptions();
            // Fetching i-th route
            List<HashMap<String, String>> path = result.get(i);
            // Fetching all the points in i-th route
            for (int j = 0; j < path.size(); j++) {
                HashMap<String, String> point = path.get(j);
                double lat = Double.parseDouble(point.get("lat"));
                double lng = Double.parseDouble(point.get("lng"));
                LatLng position = new LatLng(lat, lng);
                points.add(position);
            }
            // Adding all the points in the route to LineOptions
            lineOptions.addAll(points);
            if (directionMode.equalsIgnoreCase("walking")) {
                lineOptions.width(10);
                lineOptions.color(Color.MAGENTA);
            } else {
                lineOptions.width(20);
                lineOptions.color(Color.BLUE);
            }
            Log.d("mylog", "onPostExecute lineoptions decoded");
        }

        // Drawing polyline in the Google Map for the i-th route
        if (lineOptions != null) {
            //mMap.addPolyline(lineOptions);
            taskCallback.onTaskDone(lineOptions);

        } else {
            Log.d("mylog", "without Polylines drawn");
        }
    }
}

这是我调用 PointsParse.java 的 Activity

package com.hfad.los;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Button;

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.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
import com.google.android.gms.maps.model.Polyline;
import com.google.android.gms.maps.model.PolylineOptions;

public class TrackOrderCust extends AppCompatActivity implements OnMapReadyCallback, TaskLoadedCallback {

    GoogleMap map;
    Button track;
    MarkerOptions place1, place2;
    Polyline currentPolyline;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_track_order_cust);
        track=(Button) findViewById(R.id.track);

        MapFragment mapFragment=(MapFragment) getFragmentManager().findFragmentById(R.id.mapFrag);

        mapFragment.getMapAsync(this);
        place1=new MarkerOptions().position(new LatLng(2.9198401,101.780868)).title("You");
        place2=new MarkerOptions().position(new LatLng(2.9785683,101.736)).title("Vendor");

        String url=getUrl(place1.getPosition(), place2.getPosition(),"driving");
        new FetchURL(TrackOrderCust.this).execute(url,"driving");






    }

    @Override
    public void onMapReady(GoogleMap googleMap) {
        map=googleMap;
        map.addMarker(place1);
        map.addMarker(place2);


    }

    private String getUrl(LatLng origin, LatLng dest, String directionMode){
        String str_origin="origin:"+origin.latitude+","+origin.longitude;
        String str_dest="destination:"+dest.latitude+","+dest.longitude;
        String mode="mode:" + directionMode;
        String parameters = str_origin + "&" + str_dest + "&" + mode;
        String output = "json";

        String url = "https://maps.googleapis.com/maps/api/directions/" + output +"?"+parameters+"&key="+getString(R.string.google_maps_key);
        return url;
    }

    @Override
    public void onTaskDone(Object... values) {
        if(currentPolyline!=null)
            currentPolyline.remove();
        currentPolyline=map.addPolyline((PolylineOptions) values[0]);
    }
}

谁能帮帮我。谢谢

Answer 1

在 onPostExecute 方法中更改:

ArrayList<LatLng> points;

至:

ArrayList<LatLng> points = new ArrayList<>();