How to solve NullPointerException error in Android?

Question

The below code runs smoothly in the emulator from Eclipse, but there are problems when I run it on my Android phone and tablet.

public class RingerActivity extends Activity{
/** Called when the activity is first created. */
Button press;
boolean tone = true;
MediaPlayer mp;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    mp = new MediaPlayer();
    mp = MediaPlayer.create(RingerActivity.this, R.raw.alarm);
    //Error occurs at here
    mp.start();

    Handler h = new Handler();
    Runnable stopPlaybackRun = new Runnable() {
        public void run(){
            mp.stop();
            mp.release();
        }    
    };
    h.postDelayed(stopPlaybackRun, 20 * 1000);

    AudioManager manager = (AudioManager)getSystemService(AUDIO_SERVICE);
    if(manager!=null){
        manager.setRingerMode(AudioManager.RINGER_MODE_NORMAL);
        int maxVolume = manager.getStreamMaxVolume(AudioManager.STREAM_MUSIC);
        manager.setStreamVolume(AudioManager.STREAM_MUSIC, maxVolume, 0);
    }
}

@Override
public boolean onKeyDown(int keyCode, KeyEvent event){

    if (keyCode == KeyEvent.KEYCODE_VOLUME_UP){
        Toast.makeText(this, "Volume Up", Toast.LENGTH_LONG).show();
        return true;
    }

    if (keyCode == KeyEvent.KEYCODE_VOLUME_DOWN){
        Toast.makeText(this, "Volume Down", Toast.LENGTH_LONG).show();
        return true;
    }

    return super.onKeyDown(keyCode, event);
}

I get a "NullPointException" in line "mp.start()". I have been working on this problem for a while, but I really do not know what is happening, I tried this on both my Android smartphone and tablet (Gingerbread and ICS), but the app force closes. I need help from you guys... thanks...

Logcat

11-09 15:41:09.909: E/AndroidRuntime(11975): FATAL EXCEPTION: main
11-09 15:41:09.909: E/AndroidRuntime(11975): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.testing.ringer/com.testing.ringer.RingerActivity}: java.lang.NullPointerException
11-09 15:41:09.909: E/AndroidRuntime(11975):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1872)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1893)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at android.app.ActivityThread.access$1500(ActivityThread.java:135)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1054)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at android.os.Handler.dispatchMessage(Handler.java:99)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at android.os.Looper.loop(Looper.java:150)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at android.app.ActivityThread.main(ActivityThread.java:4385)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at java.lang.reflect.Method.invokeNative(Native Method)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at java.lang.reflect.Method.invoke(Method.java:507)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:849)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at dalvik.system.NativeStart.main(Native Method)
11-09 15:41:09.909: E/AndroidRuntime(11975): Caused by: java.lang.NullPointerException
11-09 15:41:09.909: E/AndroidRuntime(11975):    at com.testing.ringer.RingerActivity.onCreate(RingerActivity.java:23)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1072)
11-09 15:41:09.909: E/AndroidRuntime(11975):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1836)
11-09 15:41:09.909: E/AndroidRuntime(11975):    ... 11 more

Ringer Manifest

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.testing.ringer"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="10" />

    <uses-permission android:name="android.permission.RECORD_AUDIO"/>

    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name" >
        <activity
            android:name=".RingerActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

Answer 1

NullPointerException means that mp is null. This means the MediaPlayer.create() failed. Does R.raw.alarm exist?

Answer 2

NullPointerException, cause when there is no instance or array to use, the best to trace is to put breakpoint and debug your code.

Seem your code is perfect, just make sure whether .mp3 is exist or not? and you perfectly kept inside raw folder.

At last try to uninstall your existing and then clean your project and run. good luck.