从网站获取数据，并显示到谷歌地图使用Android

Question

我正在尝试使用AsyncTask从网站加载数据，因为我不能使用主线程(UIThread)做这样的事情。

当进程完成时(调用onPostExecute)，我想在map中添加标记。

结果是"application crash"

这是我的代码。

MapsActivity.java

package com.example.hp.mapsapplication;
import android.support.v4.app.FragmentActivity;
import android.os.Bundle;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
import java.io.IOException;
public class MapsActivity extends FragmentActivity {
    private GoogleMap mMap; // Might be null if Google Play services APK is not available.
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_maps);
        new Parser(this, mMap).execute();
        setUpMapIfNeeded();
    }
    @Override
    protected void onResume() {
        super.onResume();
        setUpMapIfNeeded();
    }
    /**
     * Sets up the map if it is possible to do so (i.e., the Google Play services APK is correctly
     * installed) and the map has not already been instantiated.. This will ensure that we only ever
     * call {@link #setUpMap()} once when {@link #mMap} is not null.
     * <p/>
     * If it isn't installed {@link SupportMapFragment} (and
     * {@link com.google.android.gms.maps.MapView MapView}) will show a prompt for the user to
     * install/update the Google Play services APK on their device.
     * <p/>
     * A user can return to this FragmentActivity after following the prompt and correctly
     * installing/updating/enabling the Google Play services. Since the FragmentActivity may not
     * have been completely destroyed during this process (it is likely that it would only be
     * stopped or paused), {@link #onCreate(Bundle)} may not be called again so we should call this
     * method in {@link #onResume()} to guarantee that it will be called.
     */
    private void setUpMapIfNeeded() {
        // Do a null check to confirm that we have not already instantiated the map.
        if (mMap == null) {
            // Try to obtain the map from the SupportMapFragment.
            mMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map))
                    .getMap();
            // Check if we were successful in obtaining the map.
            if (mMap != null) {
                setUpMap();
            }
        }
    }
    /**
     * This is where we can add markers or lines, add listeners or move the camera. In this case, we
     * just add a marker near Africa.
     * <p/>
     * This should only be called once and when we are sure that {@link #mMap} is not null.
     */
    private void setUpMap() {
    }
}

Parser.java

package com.example.hp.mapsapplication;
import android.content.Context;
import android.os.AsyncTask;
import android.os.Handler;
import android.widget.Toast;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;
/**
 * Created by Hp on 26/06/2016.
 */
public class Parser extends AsyncTask<Void, Void, Void> {
    private StringBuilder yeloSourceCode = new StringBuilder();
    Context context;
    GoogleMap map;
    public Parser(Context context, GoogleMap map) {
        this.context = context;
        this.map = map;
    }
    @Override
    protected Void doInBackground(Void... voids) {
        try {
            URL yahoo = new URL("https://yelo.agglo-larochelle.fr/stations?address=&velos=true");
            URLConnection yc = yahoo.openConnection();
            BufferedReader in = new BufferedReader(new InputStreamReader(yc.getInputStream(), "UTF-8"));
            String inputLine;
            while ((inputLine = in.readLine()) != null) {
                yeloSourceCode.append(inputLine);
            }
            in.close();
        }catch(Exception e) {
        }
        return null;
    }
    @Override
    protected void onPostExecute(Void result) {
        map.addMarker(new MarkerOptions().position(new LatLng(46.140874, -1.171418)).title("Marker"));
    }
}

Answer 1

当使用mMap时，你会得到一个空指针异常，因为它没有被分配

private GoogleMap mMap; // Might be null if Google Play services APK is not available.
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_maps);
    // missing
    mMap = (GoogleMap) findViewById(R.id.?????);
    new Parser(this, mMap).execute();