Java Android开发中启动活动（NullPointerException）时出错

问题：从MainActivity>MapActivity单击按钮启动活动失败

p/s:I遵循此地图教程，更改类名并将所有元素复制到我的项目工作区

按钮侦听器

public void onClick(View v) {
                Intent intent = new Intent(getApplicationContext(),
                        MapActivity.class); 
                startActivity(intent);}

Logcat:

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

我的地图活动类

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);
        }
    };

}

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，原因是什么

-标题栏不可见

-必须使用

getWindow（）.requestFeature（Window.FEATURE\u action\u bar）请求操作栏，您可能认为您的信息栏是一个动作栏

等等

您应该能够通过许多其他问相同问题的SO帖子找到您的答案。
确保
getActionBar（）
不返回
null
。或者考虑到它可以返回null。如果我计算正确，它是
actionBar.setNavigationMode（actionBar.NAVIGATION\u MODE\u LIST）-在此行上放置断点。actionBar无效吗？谢谢各位。现在我在阅读评论后解决了这个问题。谢谢你的快速回复！它起作用了！很抱歉再次在此网站上提出愚蠢的问题：P（我对这个开发非常陌生，我不知道NULL会导致这个问题。）将来，如果你得到一个NULL指针错误，找到它发生的代码行，并选择该语句中可能为NULL的所有变量，然后，中断此行或添加
System.out
，并检查哪些变量为空。一旦你发现了空变量，追踪它的创建，然后研究为什么它被创建为空。这将帮助您解决大多数空指针错误。
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_LIST);