所以我已经下载了这个教程，但每当我在模拟器上运行时，logcat都会显示以下错误......
09-20 04:33:26.790: E/AndroidRuntime(1900): FATAL EXCEPTION: main
09-20 04:33:26.790: E/AndroidRuntime(1900): java.lang.RuntimeException: Unable to start activity ComponentInfo{in.wptrafficanalyzer.locationwaypointmapv2/in.wptrafficanalyzer.locationwaypointmapv2.MainActivity}: java.lang.NullPointerException
09-20 04:33:26.790: E/AndroidRuntime(1900): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
09-20 04:33:26.790: E/AndroidRuntime(1900): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
09-20 04:33:26.790: E/AndroidRuntime(1900): at android.app.ActivityThread.access$600(ActivityThread.java:141)
09-20 04:33:26.790: E/AndroidRuntime(1900): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
09-20 04:33:26.790: E/AndroidRuntime(1900): at android.os.Handler.dispatchMessage(Handler.java:99)
09-20 04:33:26.790: E/AndroidRuntime(1900): at android.os.Looper.loop(Looper.java:137)
09-20 04:33:26.790: E/AndroidRuntime(1900): at android.app.ActivityThread.main(ActivityThread.java:5103)
09-20 04:33:26.790: E/AndroidRuntime(1900): at java.lang.reflect.Method.invokeNative(Native Method)
09-20 04:33:26.790: E/AndroidRuntime(1900): at java.lang.reflect.Method.invoke(Method.java:525)
09-20 04:33:26.790: E/AndroidRuntime(1900): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
09-20 04:33:26.790: E/AndroidRuntime(1900): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-20 04:33:26.790: E/AndroidRuntime(1900): at dalvik.system.NativeStart.main(Native Method)
09-20 04:33:26.790: E/AndroidRuntime(1900): Caused by: java.lang.NullPointerException
09-20 04:33:26.790: E/AndroidRuntime(1900): at in.wptrafficanalyzer.locationwaypointmapv2.MainActivity.onCreate(MainActivity.java:58)
09-20 04:33:26.790: E/AndroidRuntime(1900): at android.app.Activity.performCreate(Activity.java:5133)
09-20 04:33:26.790: E/AndroidRuntime(1900): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
09-20 04:33:26.790: E/AndroidRuntime(1900): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
我对android编程很新。所以我似乎无法弄清楚这一点。
我猜错误是在MainActivity文件中的某处，因为他的nullpointer错误似乎来自那里......
如有任何帮助，将不胜感激......
我已经将谷歌播放服务库包括在内了...所以没有问题...请帮忙......
这是代码：
package in.wptrafficanalyzer.locationwaypointmapv2;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import org.json.JSONObject;
import android.graphics.Color;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.util.Log;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.GoogleMap.OnMapClickListener;
import com.google.android.gms.maps.GoogleMap.OnMapLongClickListener;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.BitmapDescriptorFactory;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
import com.google.android.gms.maps.model.PolylineOptions;
public class MainActivity extends FragmentActivity {
GoogleMap map;
ArrayList<LatLng> markerPoints;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Initializing
markerPoints = new ArrayList<LatLng>();
// Getting reference to SupportMapFragment of the activity_main
SupportMapFragment fm = (SupportMapFragment)getSupportFragmentManager().findFragmentById(R.id.map);
// Getting reference to Button
Button btnDraw = (Button)findViewById(R.id.btn_draw);
// Getting Map for the SupportMapFragment
map = fm.getMap();
// Enable MyLocation Button in the Map
map.setMyLocationEnabled(true);
// Setting onclick event listener for the map
map.setOnMapClickListener(new OnMapClickListener() {
@Override
public void onMapClick(LatLng point) {
// Already 10 locations with 8 waypoints and 1 start location and 1 end location.
// Upto 8 waypoints are allowed in a query for non-business users
if(markerPoints.size()>=10){
return;
}
// Adding new item to the ArrayList
markerPoints.add(point);
// Creating MarkerOptions
MarkerOptions options = new MarkerOptions();
// Setting the position of the marker
options.position(point);
/**
* For the start location, the color of marker is GREEN and
* for the end location, the color of marker is RED and
* for the rest of markers, the color is AZURE
*/
if(markerPoints.size()==1){
options.icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_GREEN));
}else if(markerPoints.size()==2){
options.icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_RED));
}else{
options.icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_AZURE));
}
// Add new marker to the Google Map Android API V2
map.addMarker(options);
}
});
// The map will be cleared on long click
map.setOnMapLongClickListener(new OnMapLongClickListener() {
@Override
public void onMapLongClick(LatLng point) {
// Removes all the points from Google Map
map.clear();
// Removes all the points in the ArrayList
markerPoints.clear();
}
});
// Click event handler for Button btn_draw
btnDraw.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// Checks, whether start and end locations are captured
if(markerPoints.size() >= 2){
LatLng origin = markerPoints.get(0);
LatLng dest = markerPoints.get(1);
// Getting URL to the Google Directions API
String url = getDirectionsUrl(origin, dest);
DownloadTask downloadTask = new DownloadTask();
// Start downloading json data from Google Directions API
downloadTask.execute(url);
}
}
});
}
private String getDirectionsUrl(LatLng origin,LatLng dest){
// Origin of route
String str_origin = "origin="+origin.latitude+","+origin.longitude;
// Destination of route
String str_dest = "destination="+dest.latitude+","+dest.longitude;
// Sensor enabled
String sensor = "sensor=false";
// Waypoints
String waypoints = "";
for(int i=2;i<markerPoints.size();i++){
LatLng point = (LatLng) markerPoints.get(i);
if(i==2)
waypoints = "waypoints=";
waypoints += point.latitude + "," + point.longitude + "|";
}
// Building the parameters to the web service
String parameters = str_origin+"&"+str_dest+"&"+sensor+"&"+waypoints;
// Output format
String output = "json";
// Building the url to the web service
String url = "https://maps.googleapis.com/maps/api/directions/"+output+"?"+parameters;
return url;
}
/** A method to download json data from url */
private String downloadUrl(String strUrl) throws IOException{
String data = "";
InputStream iStream = null;
HttpURLConnection urlConnection = null;
try{
URL url = new URL(strUrl);
// Creating an http connection to communicate with url
urlConnection = (HttpURLConnection) url.openConnection();
// Connecting to url
urlConnection.connect();
// Reading data from url
iStream = urlConnection.getInputStream();
BufferedReader br = new BufferedReader(new InputStreamReader(iStream));
StringBuffer sb = new StringBuffer();
String line = "";
while( ( line = br.readLine()) != null){
sb.append(line);
}
data = sb.toString();
br.close();
}catch(Exception e){
Log.d("Exception while downloading url", e.toString());
}finally{
iStream.close();
urlConnection.disconnect();
}
return data;
}
// Fetches data from url passed
private class DownloadTask extends AsyncTask<String, Void, String>{
// Downloading data in non-ui thread
@Override
protected String doInBackground(String... url) {
// For storing data from web service
String data = "";
try{
// Fetching the data from web service
data = downloadUrl(url[0]);
}catch(Exception e){
Log.d("Background Task",e.toString());
}
return data;
}
// Executes in UI thread, after the execution of
// doInBackground()
@Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
ParserTask parserTask = new ParserTask();
// Invokes the thread for parsing the JSON data
parserTask.execute(result);
}
}
/** A class to parse the Google Places in JSON format */
private class ParserTask extends AsyncTask<String, Integer, List<List<HashMap<String,String>>> >{
// 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]);
DirectionsJSONParser parser = new DirectionsJSONParser();
// Starts parsing data
routes = parser.parse(jObject);
}catch(Exception e){
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 = null;
PolylineOptions lineOptions = null;
// Traversing through all the routes
for(int i=0;i<result.size();i++){
points = new ArrayList<LatLng>();
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);
lineOptions.width(2);
lineOptions.color(Color.RED);
}
// Drawing polyline in the Google Map for the i-th route
map.addPolyline(lineOptions);
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}