ViewPager適配器上的空異常 - Android

Question

我想用我的適配器在Android中實現ViewPager並得到這個錯誤，我GOOGLE了一切似乎都很好，但仍然我的應用程序崩潰，任何幫助將不勝感激。

適配器：

public class MainPagerAdapter extends FragmentPagerAdapter { 

public MainPagerAdapter(FragmentManager fm) { 
    super(fm); 
} 

@Override 
public Fragment getItem(int position) { 
    switch (position){ 
     case 0: 
      ChatFragment.create(); 
     case 1: 
      EmptyFragment.create(); 
     case 2: 
      StoryFragment.create(); 
    } 
    return null; 
} 

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

}

MainActivity：

public class MainActivity extends AppCompatActivity { 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    View back = (View) findViewById(R.id.am_background_view); 
    ViewPager pager = (ViewPager) findViewById(R.id.am_view_pager); 
    MainPagerAdapter adapter = new MainPagerAdapter(getSupportFragmentManager()); 
    pager.setAdapter(adapter); 
} 

}

MainActivity.xml

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


<View 
    android:id="@+id/am_background_view" 
    android:layout_height="match_parent" 
    android:layout_width="match_parent" 
    android:background="@color/black" 
    /> 




<android.support.v4.view.ViewPager 
    android:id="@+id/am_view_pager" 
    android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    android:layout_alignParentTop="true" 
    android:layout_alignParentStart="true" /> 

ChatFragment

public class ChatFragment extends Fragment { 

public static ChatFragment create() { 
    return new ChatFragment(); 
} 


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


@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, 
         Bundle savedInstanceState) { 
    View view = inflater.inflate(R.layout.fragment_chat, container, false); 
    return view; 
} 

錯誤：

FATAL EXCEPTION: main 
                    Process: pk.fastech.snappy.snappy, PID: 6148 
                    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference 
                     at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:380) 
                     at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:375) 
                     at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:103) 
                     at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1034) 
                     at android.support.v4.view.ViewPager.populate(ViewPager.java:1182) 
                     at android.support.v4.view.ViewPager.populate(ViewPager.java:1116) 
                     at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1642) 
                     at android.view.View.measure(View.java:20158) 
                     at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:716) 
                     at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:462) 
                     at android.view.View.measure(View.java:20158) 
                     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6328) 
                     at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) 
                     at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139) 
                     at android.view.View.measure(View.java:20158) 
                     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6328) 
                     at android.support.v7.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:393) 
                     at android.view.View.measure(View.java:20158) 
                     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6328) 
                     at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) 
                     at android.view.View.measure(View.java:20158) 
                     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6328) 
                     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464) 
                     at android.widget.LinearLayout.measureVertical(LinearLayout.java:747) 
                     at android.widget.LinearLayout.onMeasure(LinearLayout.java:629) 
                     at android.view.View.measure(View.java:20158) 
                     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6328) 
                     at android.widget.FrameLayout.onMeasure(FrameLayout.java:194) 
                     at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:3158) 
                     at android.view.View.measure(View.java:20158) 
                     at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2644) 
                     at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1599) 
                     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1891) 
                     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1487) 
                     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7450) 
                     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:920) 
                     at android.view.Choreographer.doCallbacks(Choreographer.java:695) 
                     at android.view.Choreographer.doFrame(Choreographer.java:631) 
                     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:906) 
                     at android.os.Handler.handleCallback(Handler.java:739) 
                     at android.os.Handler.dispatchMessage(Handler.java:95) 
                     at android.os.Looper.loop(Looper.java:158) 
                     at android.app.ActivityThread.main(ActivityThread.java:7229) 
                     at java.lang.reflect.Method.invoke(Native Method) 
                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 

Answer 1

你getItem方法不正確，並始終返回null。

您應將其更改爲：

@Override 
public Fragment getItem(int position) { 
    switch (position){ 
     case 0: 
      return ChatFragment.create(); 
     case 1: 
      return EmptyFragment.create(); 
     case 2: 
      return StoryFragment.create(); 
    } 
    return null; 
} 

return Java中的結果返回給調用方法 - 但你沒有返回這樣的結果 - 它不「堆棧」。