Impossible de démarrer l'activité ComponentInfo {..}: java.lang.NullPointerException

Question

J'essaie de démarrer une nouvelle activité lorsque le bouton de sélection ButtonBtn est cliqué, et j'obtiens une exception NullPointerException, mais je ne vois pas où cela s'est produit. Je me trompe avec mes références.

erreur complète journal:

04-08 17:02:07.847: E/AndroidRuntime(5869): FATAL EXCEPTION: main 
04-08 17:02:07.847: E/AndroidRuntime(5869): java.lang.RuntimeException: Unable to start activity ComponentInfo{app.and.androidmusic/app.and.androidmusic.PlaylistActivity}: java.lang.NullPointerException 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at android.app.ActivityThread.access$600(ActivityThread.java:141) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at android.os.Handler.dispatchMessage(Handler.java:99) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at android.os.Looper.loop(Looper.java:137) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at android.app.ActivityThread.main(ActivityThread.java:5039) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at java.lang.reflect.Method.invokeNative(Native Method) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at java.lang.reflect.Method.invoke(Method.java:511) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at dalvik.system.NativeStart.main(Native Method) 
04-08 17:02:07.847: E/AndroidRuntime(5869): Caused by: java.lang.NullPointerException 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at app.and.androidmusic.PlaylistActivity.onCreate(PlaylistActivity.java:68) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at android.app.Activity.performCreate(Activity.java:5104) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080) 
04-08 17:02:07.847: E/AndroidRuntime(5869):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144) 

MainActivity.java:

public class MainActivity extends ListActivity { 
... 
protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
... 
Button btnPlaylist = (Button) findViewById(R.id.playlistBtn); 
btnPlaylist.setOnClickListener(new View.OnClickListener() {     
        @Override 
      public void onClick(View arg0) { 
      Intent in = new Intent(getApplicationContext(), PlaylistActivity.class); 
         startActivity(in); 

PlaylistActivity.java:

public class PlaylistActivity extends ListActivity{ 
... 
@Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_playlist); { 

AndroidManifest.xml:

<activity 
      android:name="app.and.androidmusic.MainActivity" 
      android:label="@string/app_name" > 
      <intent-filter> 
       <action android:name="android.intent.action.MAIN" /> 
       <category android:name="android.intent.category.LAUNCHER" /> 
      </intent-filter> 
     </activity>   
     <activity 
      android:name=".PlaylistActivity" /> 

Ok, voici donc activity_main.xml que vous lui avez demandé est assez longue:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    tools:context=".MainActivity" > 

    <app.and.androidmusic.DragNDropListView 
     android:id="@+id/android:list" 
     android:layout_width="match_parent" 
     android:layout_height="300dp" 
     android:layout_below="@+id/playlistBtn" 
     android:layout_alignParentLeft="true" > 

    </app.and.androidmusic.DragNDropListView> 

    <Button 
     android:id="@+id/playBtn" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@id/android:list" 
     android:layout_toRightOf="@+id/prevBtn" 
     android:text="@string/playBtn" /> 

    <Button 
     android:id="@+id/pauseBtn" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@id/android:list" 
     android:layout_toRightOf="@+id/playBtn" 
     android:text="@string/pauseBtn" /> 

    <Button 
     android:id="@+id/prevBtn" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@id/android:list" 
     android:text="@string/prevBtn" /> 

    <Button 
     android:id="@+id/nextBtn" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@id/android:list" 
     android:layout_toRightOf="@+id/pauseBtn" 
     android:text="@string/nextBtn" /> 

    <Button 
     android:id="@+id/songsBtn" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/songTxt" 
     android:text="@string/songsBtn" /> 

    <Button 
     android:id="@+id/playlistBtn" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/songTxt" 
     android:layout_toRightOf="@+id/songsBtn" 
     android:text="@string/playlistBtn" /> 

    <Button 
     android:id="@+id/eqBtn" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/songTxt" 
     android:layout_toRightOf="@+id/playlistBtn" 
     android:text="@string/eqBtn" /> 

    <TextView 
     android:id="@+id/songTxt" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_alignParentTop="true" 
     android:layout_centerHorizontal="true" 
     android:text="" /> 

</RelativeLayout> 

Answer 1

conjecture sauvage:

btnPlaylist est nul et il n'y a pas de bouton avec l'id playlistBtn dans activity_main.xml.

Edit:

Le problème est à l'intérieur PlaylistActivity (ligne 68). Tout objet auquel vous accédez est nul.