如何解決Android中的NullPointerException錯誤？

下面的代碼可以在Eclipse的模擬器中順利運行，但在我的Android手機和平板電腦上運行時會出現問題。如何解決Android中的NullPointerException錯誤？

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); 
}

我在「mp.start（）」行中得到一個「NullPointException」。我一直在研究這個問題一段時間，但我真的不知道發生了什麼，我在Android智能手機和平板電腦（薑餅和ICS）上都試過，但應用程序部隊關閉了。我需要你們的幫助...謝謝...

logcat的

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

林格清單

<?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>

來源

2012-11-09 user1782267

你試過調試嗎？ – rekire

如果你可以分享日誌，那將會很棒。我想這個問題不會在那裏，如果你在一個處理程序中調用mp.start有一些小的延遲 – almuneef

我覺得mp = new MediaPlayer（）;是不必要的; MediaPlayer.create（RingerActivity.this，R.raw.alarm）;無論如何返回媒體播放器對象。 –