Tentativo di richiamare il metodo virtuale 'void android.support.v4.view.ViewPager.setAdapter (android.support.v4.view.PagerAdapter)' su un oggetto nullo di riferimento

Question

Sto cercando di implementare il layout della scheda scorrevole utilizzando materiale Android design. Ma mi dà NullPointerException. Ecco il mio codice finora:

MainActivity.java

public class MainActivity extends ActionBarActivity { 
    private Toolbar toolbar; 
    private ViewPager pager; 
    private SlidingTabLayout nLayout; 


    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 

     setContentView(R.layout.activity_main_appbar); 
     toolbar = (Toolbar) findViewById(R.id.app_bar); 
     setSupportActionBar(toolbar); 
     getSupportActionBar().setDisplayShowHomeEnabled(true); 

     NavigationDrawerFragment drawerFragment = (NavigationDrawerFragment) 
       getSupportFragmentManager().findFragmentById(R.id.navigation_drawer_fragment); 
     drawerFragment.setUp(R.id.navigation_drawer_fragment, (DrawerLayout) findViewById(R.id.drawer_layout), toolbar); 
     pager = (ViewPager) findViewById(R.id.pager); 
     pager.setAdapter(new MyPagerAdapter(getSupportFragmentManager())); 

     nLayout = (SlidingTabLayout) findViewById(R.id.tabs); 
     nLayout.setViewPager(pager); 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.menu_main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 

     //noinspection SimplifiableIfStatement 
     if (id == R.id.action_settings) { 
      return true; 
     } 
     if (id == R.id.navigate) { 
      startActivity(new Intent(this, SubActivity.class)); 

     } 

     return super.onOptionsItemSelected(item); 
    } 

    class MyPagerAdapter extends FragmentPagerAdapter { 

     String tabs[]; 

     public MyPagerAdapter(FragmentManager fm) { 
      super(fm); 
      tabs = getResources().getStringArray(R.array.tabs); 
     } 

     @Override 
     public Fragment getItem(int position) { 
      MyFragment myFragment = MyFragment.getInstance(position); 
      return myFragment; 
     } 

     @Override 
     public CharSequence getPageTitle(int position) { 
      return tabs[position]; 
     } 

     @Override 
     public int getCount() { 
      return 3; 
     } 
    } 

    public static class MyFragment extends Fragment { 
     private TextView textView; 

     public static MyFragment getInstance(int position) { 
      MyFragment myFragment = new MyFragment(); 
      Bundle args = new Bundle(); 
      args.putInt("position", position); 
      myFragment.setArguments(args); 
      return myFragment; 
     } 

     @Nullable 
     @Override 
     public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
      View layout = inflater.inflate(R.layout.fragment_my, container, false); 
      textView = (TextView) layout.findViewById(R.id.position); 
      Bundle bundle = getArguments(); 
      if (bundle != null) { 
       textView.setText("The page selected is " + bundle.getInt("position")); 
      } 
      return layout; 
     } 
    } 


} 

activity_main.xml

<LinearLayout 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:orientation="vertical" 
    tools:context="carsaleapp.shanaka.com.carsaleapp.MainActivity"> 

    <include 
     android:id="@+id/app_bar" 
     layout="@layout/app_bar" /> 

    <app.shanaka.tabs.SlidingTabLayout 
     android:id="@+id/tabs" 
     android:layout_width="match_parent" 
     android:layout_height="wrap_content" /> 


    <android.support.v4.view.ViewPager 
     android:id="@+id/pager" 
     android:layout_width="match_parent" 
     android:layout_height="0dp" 
     android:layout_weight="1" /> 


</LinearLayout> 

Questo è l'errore mi sono dopo corro la mia domanda:

java.lang.RuntimeException: Unable to start activity ComponentInfo{carsaleapp.shanaka.com.carsaleapp/carsaleapp.shanaka.com.carsaleapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298) 
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
      at android.app.ActivityThread.access$800(ActivityThread.java:144) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) 
      at android.os.Handler.dispatchMessage(Handler.java:102) 
      at android.os.Looper.loop(Looper.java:135) 
      at android.app.ActivityThread.main(ActivityThread.java:5221) 
      at java.lang.reflect.Method.invoke(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:372) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 
    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference 
      at carsaleapp.shanaka.com.carsaleapp.MainActivity.onCreate(MainActivity.java:42) 
      at android.app.Activity.performCreate(Activity.java:5933) 
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105) 
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251) 
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) 
            at android.app.ActivityThread.access$800(ActivityThread.java:144) 
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) 
            at android.os.Handler.dispatchMessage(Handler.java:102) 
            at android.os.Looper.loop(Looper.java:135) 
            at android.app.ActivityThread.main(ActivityThread.java:5221) 
            at java.lang.reflect.Method.invoke(Native Method) 
            at java.lang.reflect.Method.invoke(Method.java:372) 
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 

Il Continuo a dire che l'oggetto pager è nullo. Ma per me l'ho impostato correttamente.

pager = (ViewPager) findViewById(R.id.pager); 

Answer 1

avete la vostra vista cercapersone in activity_main.xml mentre il layout per l'attività è setContentView(R.layout.activity_main_appbar). Non hai il pager di visualizzazione in activity_main_appbar.xml.

findViewById cerca una vista nella gerarchia della vista corrente. Immagino che dovrebbe essere setContentView(R.layout.activity_main); ma controlla quali viste hai nel layout incluso app_bar.xml