Question

我正在尝试为我的android studio移动应用获取2个位置之间的折线。但出现以下错误：试图在空对象引用上调用接口方法'int java.util.List.size（）'

async_id

这是我的PointsParser.java

    --------- 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Activity，调用PointsParse.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");
        }
    }
}

任何人都可以帮助我。谢谢

Answer 1

如果看来super.logSomething()方法中的结果参数是onPostExecute。通常，除非您知道返回的参数为NonNull，否则在从外部接收结果时请始终检查空指针。

null

这将防止启动器崩溃。现在检查为什么结果返回为if (result != null) { .... Your code .... }。

Answer 2

在您的onPostExecute方法中进行更改：

<div class="table-div">
  <table id="myTable">
    <tr>
      <th class="button"><button class="my-btn" type="button" onclick="sortTable()">
        shuffle</button></th>
      <th>Text:</th>
      <th></th>
    </tr>

    <tr>
      <td class="left">Some text 1</td>
      <td><input type="text"></td>
      <td class="right">more text.</td>
      <td class="button"><button class="my-btn" type="button">
                check</button></td>
    </tr>

    <tr>
      <td class="left">Some text 2</td>
      <td><input type="text"></td>
      <td class="right">more text.</td>
      <td class="button"><button class="my-btn" type="button">
                check</button></td>
    </tr>

    <tr>
      <td class="left">Some text 3</td>
      <td><input type="text"></td>
      <td class="right">more text.</td>
      <td class="button"><button class="my-btn" type="button">
                check</button></td>
    </tr>

    <tr>
      <td class="left">Some text 4</td>
      <td><input type="text"></td>
      <td class="right">more text.</td>
      <td class="button"><button class="my-btn" type="button">
                check</button></td>
    </tr>

    <tr>
      <td class="left">Some text 5</td>
      <td><input type="text"></td>
      <td class="right">more text.</td>
      <td class="button"><button class="my-btn" type="button">
                check</button></td>
    </tr>

    <tr>
      <td class="left">Some text 6</td>
      <td><input type="text"></td>
      <td class="right">more text.</td>
      <td class="button"><button class="my-btn" type="button">
                check</button></td>
    </tr>

    <tr>
      <td class="left">Some text 7</td>
      <td><input type="text"></td>
      <td class="right">more text.</td>
      <td class="button"><button class="my-btn" type="button">
                check</button></td>
    </tr>

收件人：

ArrayList<LatLng> points;

Answer 3

如日志中所示：

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)

问题在#63行中：

for (int i = 0; i < result.size(); i++) {

因此，您的result对象为null，并且result.size()引发异常。因此，可能的原因是在parser.parse(jObject)调用中-恰好它填充了routes对象的值。 DataParser类没有源代码，但是如果您使用https://www.androidtutorialpoint.com中的代码，则意味着您使用的"results"字符串中没有名为jsonData[0]的数组进行解析。因此，在调用后打印url：

String url=getUrl(place1.getPosition(), place2.getPosition(),"driving");

通过致电：

Log.d("mylog", "url='" + url + "'");

然后将其复制到浏览器地址行，执行并分析服务器响应：如果没有带有"results"标签的数组，则应检查key和其他请求参数。

Android Studio错误：尝试在空对象引用上调用接口方法'int java.util.List.size（）'

3 个答案: