Próba wywołania metody wirtualnej "void android.support.v4.view.ViewPager.setAdapter (android.support.v4.view.PagerAdapter)" na odwołanym obiekcie o wartości zerowej

Question

Próbuję zaimplementować układ przesuwanej karty za pomocą materiału Android projekt. Ale to daje mi NullPointerException. Oto mój kod do tej pory:

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> 

Jest to błąd mam po uruchomieniu aplikacji:

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) 

The ciągle mówię, że obiekt pager ma wartość NULL. Ale dla mnie ustawiłem to poprawnie.

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

Answer 1

Masz widok pagera w activity_main.xml w czasie, gdy układ dla działalności jest setContentView(R.layout.activity_main_appbar). Nie masz pagera widoku w activity_main_appbar.xml.

wyszukuje widok w bieżącej hierarchii widoków. Przypuszczam, że powinien to być setContentView(R.layout.activity_main);, ale sprawdź, jakie widoki masz w dołączonym układzie. app_bar.xml