AlertDialog.Builder Error

Question

I have following problem:

I copied this Code into my existing file.

if (mSensorX >= 5){


        AlertDialog.Builder builder1 = new AlertDialog.Builder(context);
        builder1.setMessage("Write your message here.");
        builder1.setCancelable(true);
        builder1.setPositiveButton("Yes",
                new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int id) {
                dialog.cancel();
            }
        });
        builder1.setNegativeButton("No",
                new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int id) {
                dialog.cancel();
            }
        });

        AlertDialog alert11 = builder1.create();
        alert11.show();
    }

This is the code:

package de.example.baum_projekt_5_punkt_fix;


import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.BitmapFactory.Options;
import android.graphics.Canvas;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.view.Display;
import android.view.Surface;
import android.view.View;
import android.view.WindowManager;

public class SimulationView extends View implements SensorEventListener {

private SensorManager mSensorManager;
private Sensor mAccelerometer;
private Display mDisplay;

private Bitmap mGrass;
private Bitmap mHole;
private Bitmap mBitmap;
private static final int BALL_SIZE = 32;
private static final int HOLE_SIZE = 40;

private float mXOrigin;
private float mYOrigin;

private float mHorizontalBound;
private float mVerticalBound;    

private float mSensorX;
private float mSensorY;
private float mSensorZ;
private long mSensorTimeStamp;

private Particle mBall = new Particle();
private Context context;
private int onlyonetime;

public SimulationView(Context context) {
    super(context);

    WindowManager mWindowManager = (WindowManager)                                              
    context.getSystemService(Context.WINDOW_SERVICE);
    mDisplay = mWindowManager.getDefaultDisplay();

    mSensorManager = (SensorManager) context.getSystemService(Context.SENSOR_SERVICE);
    mAccelerometer = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);

    Bitmap ball = BitmapFactory.decodeResource(getResources(), R.drawable.ball);
    mBitmap = Bitmap.createScaledBitmap(ball, BALL_SIZE, BALL_SIZE, true);

    Bitmap hole = BitmapFactory.decodeResource(getResources(), R.drawable.hole);
    mHole = Bitmap.createScaledBitmap(hole, HOLE_SIZE, HOLE_SIZE, true);

    Options opts = new Options();
    opts.inDither = true;
    opts.inPreferredConfig = Bitmap.Config.RGB_565;
    mGrass = BitmapFactory.decodeResource(getResources(), R.drawable.wald, opts);        
}

public void startSimulation() {
    mSensorManager.registerListener(this, mAccelerometer, SensorManager.SENSOR_DELAY_UI);
}

public void stopSimulation() {
    mSensorManager.unregisterListener(this);
}

@Override
public void onSensorChanged(SensorEvent event) {
    if (event.sensor.getType() != Sensor.TYPE_ACCELEROMETER)
        return;

    switch (mDisplay.getRotation()) {
    case Surface.ROTATION_0:
        mSensorX = event.values[0];
        mSensorY = event.values[1];
        break;
    case Surface.ROTATION_90:
        mSensorX = -event.values[1];
        mSensorY = event.values[0];
        break;
    case Surface.ROTATION_180:
        mSensorX = -event.values[0];
        mSensorY = -event.values[1];
        break;
    case Surface.ROTATION_270:
        mSensorX = event.values[1];
        mSensorY = -event.values[0];
        break;
    }
    mSensorZ = event.values[2];
    mSensorTimeStamp = event.timestamp;


    if (mSensorX >= 5){


        AlertDialog.Builder builder1 = new AlertDialog.Builder(context);
        builder1.setMessage("Write your message here.");
        builder1.setCancelable(true);
        builder1.setPositiveButton("Yes",
                new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int id) {
                dialog.cancel();
            }
        });
        builder1.setNegativeButton("No",
                new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int id) {
                dialog.cancel();
            }
        });

        AlertDialog alert11 = builder1.create();
        alert11.show();
    }

}

 @Override
public void onAccuracyChanged(Sensor sensor, int accuracy) {

}

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    mXOrigin = w * 0.5f;
    mYOrigin = h * 0.5f;

    mHorizontalBound = (w - BALL_SIZE) * 0.5f;
    mVerticalBound = (h - BALL_SIZE) * 0.5f;
}

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    canvas.drawBitmap(mGrass, 0, 0, null);
    canvas.drawBitmap(mHole, 100+mXOrigin - HOLE_SIZE/2,  100+ mYOrigin - HOLE_SIZE/2, null);

    mBall.updatePosition(mSensorX, mSensorY, mSensorZ, mSensorTimeStamp);
    mBall.resolveCollisionWithBounds(mHorizontalBound, mVerticalBound);

    canvas.drawBitmap(mBitmap, 100+(mXOrigin - BALL_SIZE/2) + mBall.mPosX, 100+(mYOrigin - BALL_SIZE/2) - mBall.mPosY, null);

    invalidate();
 }  

}

And the MainActivity:

package de.example.baum_projekt_5_punkt_fix;

import android.app.Activity;
import android.media.MediaPlayer;
import android.os.Bundle;


public class MainActivity extends Activity {

private static final String TAG = "appsrox.example.accelerometer.MainActivity";
private MediaPlayer mp;
int media_length;

//private WakeLock mWakeLock;
private SimulationView mSimulationView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mp = MediaPlayer.create(this, R.raw.bgmusic);
    //PowerManager mPowerManager = (PowerManager) getSystemService(POWER_SERVICE);
    //mWakeLock = mPowerManager.newWakeLock(PowerManager.SCREEN_BRIGHT_WAKE_LOCK, TAG);
    mp.seekTo(0);
    mp.start();
    mSimulationView = new SimulationView(this);
    setContentView(mSimulationView);
}

@Override
protected void onResume() {
    super.onResume();
    //mWakeLock.acquire();
    mSimulationView.startSimulation();
    mp.seekTo(0);
    mp.start();

}

@Override
protected void onPause() {
    super.onPause();
    mSimulationView.stopSimulation();
    media_length = mp.getCurrentPosition();
    mp.pause();
    //mWakeLock.release();
}   

@Override
protected void onDestroy() {

    mp.release();

    super.onDestroy();
    //mWakeLock.release();
 }  
}

The App crash and i get this LogCat:

02-06 21:50:06.515: E/AndroidRuntime(3314): FATAL EXCEPTION: main
02-06 21:50:06.515: E/AndroidRuntime(3314): Process:   de.example.baum_projekt_5_punkt_fix, PID: 3314
02-06 21:50:06.515: E/AndroidRuntime(3314): java.lang.NullPointerException
02-06 21:50:06.515: E/AndroidRuntime(3314):     at     android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:143)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at android.app.AlertDialog$Builder.<init>(AlertDialog.java:360)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at de.example.baum_projekt_5_punkt_fix.SimulationView.onSensorChanged(SimulationView.java:106)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at android.hardware.SystemSensorManager$SensorEventQueue.dispatchSensorEvent(SystemSensorManager.java:474)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at android.os.MessageQueue.nativePollOnce(Native Method)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at android.os.MessageQueue.next(MessageQueue.java:138)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at android.os.Looper.loop(Looper.java:123)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at android.app.ActivityThread.main(ActivityThread.java:5356)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at java.lang.reflect.Method.invokeNative(Native Method)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at java.lang.reflect.Method.invoke(Method.java:515)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
02-06 21:50:06.515: E/AndroidRuntime(3314):     at dalvik.system.NativeStart.main(Native Method)

Have anyone an idea? I think there is a problem with "context" but i dont know how to fix it.

yours faithfully

Answer 1

You're not initializing your local 'context' variable. Since you initialize the builder with the local context, the builder instance is null and can't be used properly. In the constructor, add:

this.context = context;

That should take care of it.