导致Context.getPackageName（）出现NullPointerException的Java Android上下文问题>> [

Question

我想做的就是调用此方法从MainActivity中启动意图，而我不断得到：

java.lang.NullPointerException：尝试调用虚拟方法'java.lang.String android.content.Context.getPackageName（）'在null上对象参考

我假设的问题是在某个时候传递上下文，但是我看不到如何正确设置？

public class Manager extends AppCompatActivity implements Serializable {

public void startUpdatingSensors(){
        //Start sensor service
-->Here       Intent startService = new Intent(this, SensorService.class);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            startForegroundService(startService);
        } else {
            startService(startService);
        }

    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_trace_manager);
    }
}

从Main称为：

public class MainActivity extends AppCompatActivity {

    public static final String TAG = "Main";
    private Manager Manager = new TraceManager(this);



    @RequiresApi(api = Build.VERSION_CODES.O)
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        //initialise trace

        Manager.startUpdatingSensors();
        Manager.startUpdatingLocation();
        Manager.startUpdatingTrace();

        //get permissions for location
Dexter.withActivity(this)
                .withPermissions(Arrays.asList(
                        Manifest.permission.ACCESS_FINE_LOCATION,
                        Manifest.permission.ACCESS_BACKGROUND_LOCATION,
                        Manifest.permission.ACCESS_COARSE_LOCATION
                ))
                .withListener(new MultiplePermissionsListener() {
                    @Override
                    public void onPermissionsChecked(MultiplePermissionsReport multiplePermissionsReport) {

                        Manager.startUpdatingSensors();
                        Manager.startUpdatingLocation();
                        Manager.startUpdatingTrace();

                    }

                    @Override
                    public void onPermissionRationaleShouldBeShown(List<PermissionRequest> list, PermissionToken permissionToken) {

                    }
                });



    }

}

我已经尝试通过各种上下文和活动，但是我无法使其正常工作。

完整的堆栈跟踪：

 Process: com.app.android, PID: 11760
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.app.android/com.app.android.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
        at android.content.ContextWrapper.getPackageName(ContextWrapper.java:145)
        at android.content.ComponentName.<init>(ComponentName.java:131)
        at android.content.Intent.<init>(Intent.java:6513)
        at com.app.android.Manager.startUpdatingSensors(Manager.java:187)
        at com.app.android.MainActivity.onCreate(MainActivity.java:76)

在Manager中为187的行是：Intent startService = new Intent（this，SensorService.class）; 

我要做的就是从MainActivity调用此方法以启动意图，而我不断得到：java.lang.NullPointerException：尝试调用虚拟方法'java.lang.String android...。

Answer 1

更改下一行：

Intent startService = new Intent（context，SensorService.class）;