问题：无法从MainActiviy＆gt;按钮点击开始活动MapActivity。
P / s：我按照这个地图教程并更改了类名并将所有元素复制到我的项目工作区。 This Tutorial
按钮监听器
public void onClick(View v) {
Intent intent = new Intent(getApplicationContext(),
MapActivity.class);
startActivity(intent);}
logcat的：
07-31 02:22:58.267: E/AndroidRuntime(20784): FATAL EXCEPTION: main
07-31 02:22:58.267: E/AndroidRuntime(20784): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.fa.financeassistant/com.fa.financeassistant.MapActivity}: java.lang.NullPointerException
07-31 02:22:58.267: E/AndroidRuntime(20784): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2308)
07-31 02:22:58.267: E/AndroidRuntime(20784): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2358)
07-31 02:22:58.267: E/AndroidRuntime(20784): at android.app.ActivityThread.access$600(ActivityThread.java:153)
07-31 02:22:58.267: E/AndroidRuntime(20784): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1247)
07-31 02:22:58.267: E/AndroidRuntime(20784): at android.os.Handler.dispatchMessage(Handler.java:99)
07-31 02:22:58.267: E/AndroidRuntime(20784): at android.os.Looper.loop(Looper.java:137)
07-31 02:22:58.267: E/AndroidRuntime(20784): at android.app.ActivityThread.main(ActivityThread.java:5227)
07-31 02:22:58.267: E/AndroidRuntime(20784): at java.lang.reflect.Method.invokeNative(Native Method)
07-31 02:22:58.267: E/AndroidRuntime(20784): at java.lang.reflect.Method.invoke(Method.java:511)
07-31 02:22:58.267: E/AndroidRuntime(20784): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
07-31 02:22:58.267: E/AndroidRuntime(20784): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
07-31 02:22:58.267: E/AndroidRuntime(20784): at dalvik.system.NativeStart.main(Native Method)
07-31 02:22:58.267: E/AndroidRuntime(20784): Caused by: java.lang.NullPointerException
07-31 02:22:58.267: E/AndroidRuntime(20784): at com.fa.financeassistant.MapActivity.onCreate(MapActivity.java:46)
07-31 02:22:58.267: E/AndroidRuntime(20784): at android.app.Activity.performCreate(Activity.java:5104)
07-31 02:22:58.267: E/AndroidRuntime(20784): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
07-31 02:22:58.267: E/AndroidRuntime(20784): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2262)
07-31 02:22:58.267: E/AndroidRuntime(20784): ... 11 more
我的MapActivity类
package com.fa.financeassistant;
import java.util.ArrayList;
import android.R.array;
import android.app.ActionBar;
import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Context;
import android.location.Criteria;
import android.location.Location;
import android.location.LocationListener;
import android.location.LocationManager;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.widget.ArrayAdapter;
import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.MapFragment;
import com.google.android.gms.maps.model.BitmapDescriptorFactory;
import com.google.android.gms.maps.model.CameraPosition;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
public class MapActivity extends Activity {
private final String TAG = getClass().getSimpleName();
private GoogleMap mMap;
private String[] places;
private LocationManager locationManager;
private Location loc;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.mapactivity);
initCompo();
places = getResources().getStringArray(R.array.places);
currentLocation();
final ActionBar actionBar = getActionBar();
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_LIST);
actionBar.setListNavigationCallbacks(ArrayAdapter.createFromResource(
this, R.array.places, android.R.layout.simple_list_item_1),
new ActionBar.OnNavigationListener() {
@Override
public boolean onNavigationItemSelected(int itemPosition,
long itemId) {
Log.e(TAG,
places[itemPosition].toLowerCase().replace("-",
"_"));
if (loc != null) {
mMap.clear();
new GetPlaces(MapActivity.this,
places[itemPosition].toLowerCase().replace(
"-", "_").replace(" ", "_")).execute();
}
return true;
}
});
}
private class GetPlaces extends AsyncTask<Void, Void, ArrayList<Place>> {
private ProgressDialog dialog;
private Context context;
private String places;
public GetPlaces(Context context, String places) {
this.context = context;
this.places = places;
}
@Override
protected void onPostExecute(ArrayList<Place> result) {
super.onPostExecute(result);
if (dialog.isShowing()) {
dialog.dismiss();
}
for (int i = 0; i < result.size(); i++) {
mMap.addMarker(new MarkerOptions()
.title(result.get(i).getName())
.position(
new LatLng(result.get(i).getLatitude(), result
.get(i).getLongitude()))
.icon(BitmapDescriptorFactory
.fromResource(R.drawable.pin))
.snippet(result.get(i).getVicinity()));
}
CameraPosition cameraPosition = new CameraPosition.Builder()
.target(new LatLng(result.get(0).getLatitude(), result
.get(0).getLongitude())) // Sets the center of the map to
// Mountain View
.zoom(14) // Sets the zoom
.tilt(30) // Sets the tilt of the camera to 30 degrees
.build(); // Creates a CameraPosition from the builder
mMap.animateCamera(CameraUpdateFactory
.newCameraPosition(cameraPosition));
}
@Override
protected void onPreExecute() {
super.onPreExecute();
dialog = new ProgressDialog(context);
dialog.setCancelable(false);
dialog.setMessage("Loading");
dialog.isIndeterminate();
dialog.show();
}
@Override
protected ArrayList<Place> doInBackground(Void... arg0) {
PlacesService service = new PlacesService(
"AIzaSyBquLw9vJPDHAZO8kzKw6ft0bPxsgBmblg");
ArrayList<Place> findPlaces = service.findPlaces(loc.getLatitude(), // 28.632808
loc.getLongitude(), places); // 77.218276
for (int i = 0; i < findPlaces.size(); i++) {
Place placeDetail = findPlaces.get(i);
Log.e(TAG, "places : " + placeDetail.getName());
}
return findPlaces;
}
}
private void initCompo() {
mMap = ((MapFragment) getFragmentManager().findFragmentById(R.id.map))
.getMap();
}
private void currentLocation() {
locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
String provider = locationManager
.getBestProvider(new Criteria(), false);
Location location = locationManager.getLastKnownLocation(provider);
if (location == null) {
locationManager.requestLocationUpdates(provider, 0, 0, listener);
} else {
loc = location;
new GetPlaces(MapActivity.this, places[0].toLowerCase().replace(
"-", "_")).execute();
Log.e(TAG, "location : " + location);
}
}
private LocationListener listener = new LocationListener() {
@Override
public void onStatusChanged(String provider, int status, Bundle extras) {
}
@Override
public void onProviderEnabled(String provider) {
}
@Override
public void onProviderDisabled(String provider) {
}
@Override
public void onLocationChanged(Location location) {
Log.e(TAG, "location update : " + location);
loc = location;
locationManager.removeUpdates(listener);
}
};
}
解决方案：除了下面给出的答案。我通过删除这一行来应用快速修复，并像魅力一样工作！感谢所有帮助过我的人。祝你有愉快的一天：）
android:theme="@style/Theme.NoTitle"
答案 0 :(得分：1)
第46行是;
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_LIST);
这里唯一可以为null的是actionBar，因此getActionBar()返回null，原因是这样的;
getActionBar() returns null - 标题栏不可见。
getActionBar returns null - 您必须使用getWindow().requestFeature(Window.FEATURE_ACTION_BAR);请求操作栏，您可能认为自己的信息栏是操作栏。
等
你应该能够通过提出同样问题的其他SO帖子找到答案。