NullPointerException异常使用ViewPager

我想在我的应用程序选项卡，并为此我使用ViewPager，但我得到这个logcat的：NullPointerException异常使用ViewPager

04-17 22:03:31.291 10545-10545/de.gestureanywhere E/AndroidRuntime﹕ FATAL EXCEPTION: main 
Process: de.gestureanywhere, PID: 10545 
java.lang.NullPointerException 
     at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:416) 
     at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:411) 
     at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:99) 
     at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:837) 
     at android.support.v4.view.ViewPager.populate(ViewPager.java:987) 
     at android.support.v4.view.ViewPager.populate(ViewPager.java:919) 
     at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1441) 
     at android.view.View.measure(View.java:16514) 
     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125) 
     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404) 
     at android.widget.LinearLayout.measureVertical(LinearLayout.java:695) 
     at android.widget.LinearLayout.onMeasure(LinearLayout.java:588) 
     at android.view.View.measure(View.java:16514) 
     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125) 
     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) 
     at android.view.View.measure(View.java:16514) 
     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125) 
     at android.support.v7.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:453) 
     at android.view.View.measure(View.java:16514) 
     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125) 
     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) 
     at android.view.View.measure(View.java:16514) 
     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125) 
     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404) 
     at android.widget.LinearLayout.measureVertical(LinearLayout.java:695) 
     at android.widget.LinearLayout.onMeasure(LinearLayout.java:588) 
     at android.view.View.measure(View.java:16514) 
     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125) 
     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310) 
     at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2556) 
     at android.view.View.measure(View.java:16514) 
     at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1912) 
     at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1109) 
     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1291) 
     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:996) 
     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5600) 
     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761) 
     at android.view.Choreographer.doCallbacks(Choreographer.java:574) 
     at android.view.Choreographer.doFrame(Choreographer.java:544) 
     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747) 
     at android.os.Handler.handleCallback(Handler.java:733) 
     at android.os.Handler.dispatchMessage(Handler.java:95) 
     at android.os.Looper.loop(Looper.java:136) 
     at android.app.ActivityThread.main(ActivityThread.java:5034) 
     at java.lang.reflect.Method.invokeNative(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java:515) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:611) 
     at dalvik.system.NativeStart.main(Native Method)

这里是我的代码：

public class Vorschlag extends ActionBarActivity implements 
     ActionBar.TabListener { 

    private ViewPager viewPager; 
    private TabsAdapter mAdapter; 
    private ActionBar actionBar; 

    // Tab titles 
    private String[] tabs = { "Neuer Vorschlag", "Vorschläge" }; 

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



     setContentView(R.layout.vorschlag_layout); 

     ActionBar actionBar = getSupportActionBar(); 

     actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_LIST); 

     actionBar.setDisplayShowTitleEnabled(true); 

     mAdapter = new TabsAdapter(getSupportFragmentManager()); 

     // Initilization 
     viewPager = (ViewPager) findViewById(R.id.pager); 

     // actionBar = getApplicationContext().getS 


     viewPager.setAdapter(mAdapter); 



     // Adding Tabs 
     for (String tab_name : tabs) { 
      actionBar.addTab(actionBar.newTab().setText(tab_name) 
        .setTabListener(this)); 
     } 
    } 





    @Override 
    public void onTabSelected(ActionBar.Tab tab, android.support.v4.app.FragmentTransaction fragmentTransaction) { 

    } 

    @Override 
    public void onTabUnselected(ActionBar.Tab tab, android.support.v4.app.FragmentTransaction fragmentTransaction) { 

    } 

    @Override 
    public void onTabReselected(ActionBar.Tab tab, android.support.v4.app.FragmentTransaction fragmentTransaction) { 

    } 
}

这是布局文件：

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    android:orientation="vertical" android:layout_width="match_parent" 
    android:layout_height="match_parent"> 

    <android.support.v4.view.ViewPager 
     xmlns:android="http://schemas.android.com/apk/res/android" 
     android:id="@+id/pager" 
     android:layout_width="match_parent" 
     android:layout_height="match_parent"> 
    </android.support.v4.view.ViewPager> 



</LinearLayout>

这里是我的适配器类：

import android.support.v4.app.FragmentManager; 
import android.support.v4.app.FragmentPagerAdapter; 


/** 
* Created by Viktor on 17.04.2015. 
*/ 
public class TabsAdapter extends FragmentPagerAdapter { 

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

    @Override 
    public android.support.v4.app.Fragment getItem(int index) { 

     switch (index) { 
      case 0: 
       // Top Rated fragment activity 
       // return new TopRatedFragment(); 
       break; 
      case 1: 
       // Games fragment activity 
       // return new GamesFragment(); 
       new VorhandeneVorschlaege(); 
       break; 

     } 

     return null; 
    } 

    @Override 
    public int getCount() { 
     // get item count - equal to number of tabs 
     return 2; 
    } 

}

的标签，现在显示的感谢。但是我遇到了这个问题，只有在getCount()返回0时才显示选项卡。当我返回选项卡数量时，应用程序正在终止。下面是代码：

公共类TabsAdapter扩展FragmentPagerAdapter {

public TabsAdapter(FragmentManager fm) { 

    super(fm); 
    Log.d("hier128", "hier128"); 
} 

@Override 
public Fragment getItem(int i) { 

    Log.d("hier126", "hier126"); 

    Fragment fragment = null; 


    if (i == 0) { 
     Log.d("hier130", "hier130"); 
     fragment = new VorhandeneVorschlaege(); 
    } 


    return fragment; 

} 



@Override 
public int getCount() { 
    // get item count - equal to number of tabs 
    Log.d("hier129", "hier129"); 
    return 0; 
} 

}

这里是与标签类：

public class Vorschlag extends ActionBarActivity implements 
    android.support.v7.app.ActionBar.TabListener { 

private ViewPager viewPager; 
private TabsAdapter mAdapter; 
//private ActionBar actionBar; 

// Tab titles 
private String[] tabs = { "Neuer Vorschlag", "Vorschläge" }; 

private android.support.v7.app.ActionBar actionBar; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 

    super.onCreate(savedInstanceState); 



    setContentView(R.layout.vorschlag_layout); 
    viewPager = (ViewPager) findViewById(R.id.pager); 

    FragmentManager fragmnetManager = getSupportFragmentManager(); 
    actionBar = getSupportActionBar(); 
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); 

    viewPager.setAdapter(new TabsAdapter(fragmnetManager)); 
    viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() { 

     @Override 
     public void onPageSelected(int arg0) { 
      // TODO Auto-generated method stub 
      actionBar.setSelectedNavigationItem(arg0); 
     } 

     @Override 
     public void onPageScrolled(int arg0, float arg1, int arg2) { 
      // TODO Auto-generated method stub 

     } 

     @Override 
     public void onPageScrollStateChanged(int arg0) { 
      // TODO Auto-generated method stub 

     } 
    }); 


    android.support.v7.app.ActionBar.Tab tab1 = actionBar.newTab(); 
    tab1.setText("TAB1"); 
    tab1.setTabListener(this); 

    android.support.v7.app.ActionBar.Tab tab2 = actionBar.newTab(); 
    tab2.setText("TAB2"); 
    tab2.setTabListener(this); 



    actionBar.addTab(tab1); 
    actionBar.addTab(tab2); 


} 

@Override 
protected void onSaveInstanceState(Bundle outState) { 
    // TODO Auto-generated method stub 
    super.onSaveInstanceState(outState); 
} 

@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_gesture_any_where, menu); 
    return true; 

} 


@Override 
public void onTabReselected(android.support.v7.app.ActionBar.Tab arg0, 
          FragmentTransaction arg1) { 
    // TODO Auto-generated method stub 

} 

@Override 
public void onTabSelected(android.support.v7.app.ActionBar.Tab tab, 
          FragmentTransaction arg1) { 
    // TODO Auto-generated method stub 
    viewPager.setCurrentItem(tab.getPosition()); 
} 

@Override 
public void onTabUnselected(android.support.v7.app.ActionBar.Tab arg0, 
          FragmentTransaction arg1) { 
    // TODO Auto-generated method stub 

} 
}

是什么原因呢？

感谢

来源

2015-04-17 user3603935

您在TabsAdapter返回null片段。 –

哦，是的，我还没有意识到这一点。但我还有什么应该回到我的情况？ – user3603935

检查我的答案。 –

import android.annotation.TargetApi; 
import android.app.ActionBar; 
import android.os.Build; 
import android.os.Bundle; 
import android.support.v4.app.Fragment; 
import android.support.v4.app.FragmentManager; 
import android.support.v4.app.FragmentPagerAdapter; 
import android.support.v4.app.FragmentTransaction; 
import android.support.v4.view.ViewPager; 
import android.support.v7.app.ActionBarActivity; 
import android.view.Menu; 

@TargetApi(Build.VERSION_CODES.HONEYCOMB) 
public class MainActivity extends ActionBarActivity implements android.support.v7.app.ActionBar.TabListener { 
    ViewPager viewPager; 
    private android.support.v7.app.ActionBar actionBar; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     viewPager = (ViewPager) findViewById(R.id.pager); 
     FragmentManager fragmnetManager = getSupportFragmentManager(); 
     viewPager.setAdapter(new MyAdapter(fragmnetManager)); 
     viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() { 

      @Override 
      public void onPageSelected(int arg0) { 
       // TODO Auto-generated method stub 
       actionBar.setSelectedNavigationItem(arg0); 
      } 

      @Override 
      public void onPageScrolled(int arg0, float arg1, int arg2) { 
       // TODO Auto-generated method stub 

      } 

      @Override 
      public void onPageScrollStateChanged(int arg0) { 
       // TODO Auto-generated method stub 

      } 
     }); 
     actionBar = getSupportActionBar(); 
     actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); 

     android.support.v7.app.ActionBar.Tab tab1 = actionBar.newTab(); 
     tab1.setText("TAB1"); 
     tab1.setTabListener(this); 

     android.support.v7.app.ActionBar.Tab tab2 = actionBar.newTab(); 
     tab2.setText("TAB2"); 
     tab2.setTabListener(this); 

     android.support.v7.app.ActionBar.Tab tab3 = actionBar.newTab(); 
     tab3.setText("TAB3"); 
     tab3.setTabListener(this); 

     actionBar.addTab(tab1); 
     actionBar.addTab(tab2); 
     actionBar.addTab(tab3); 

    } 

    @Override 
    protected void onSaveInstanceState(Bundle outState) { 
     // TODO Auto-generated method stub 
     super.onSaveInstanceState(outState); 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 

     // Inflate the menu; this adds items to the action bar if it is present. 

     getMenuInflater().inflate(R.menu.main, menu); 
     return true; 

    } 


    @Override 
    public void onTabReselected(android.support.v7.app.ActionBar.Tab arg0, 
      FragmentTransaction arg1) { 
     // TODO Auto-generated method stub 

    } 

    @Override 
    public void onTabSelected(android.support.v7.app.ActionBar.Tab tab, 
      FragmentTransaction arg1) { 
     // TODO Auto-generated method stub 
     viewPager.setCurrentItem(tab.getPosition()); 
    } 

    @Override 
    public void onTabUnselected(android.support.v7.app.ActionBar.Tab arg0, 
      FragmentTransaction arg1) { 
     // TODO Auto-generated method stub 

    } 

} 

class MyAdapter extends FragmentPagerAdapter { 
    public MyAdapter(FragmentManager fm) { 
     super(fm); 

    } 

    @Override 
    public Fragment getItem(int i) { 
     // TODO Auto-generated method stub 
     Fragment fragment = null; 
     if (i == 0) { 
      fragment = new FragmentA(); 
     } 
     if (i == 1) { 
      fragment = new FragmentB(); 
     } 
     if (i == 2) { 
      fragment = new FragmentC(); 
     } 
     return fragment; 
    } 

    @Override 
    public int getCount() { 
     // TODO Auto-generated method stub 
     return 3; 
    } 
}

来源

2015-04-17 20:43:02

谢谢但仍然相同的logcat错误 – user3603935

把这段代码放在你的活动actionBar.setNavigationMode（ActionBar.NAVIGATION_MODE_TABS）; –

我改变了它但仍然一样。 – user3603935

您在适配器的getItem返回null，正在检查的指标，如果它是0比你初始化片段。如果指数大于0比返回null等，其在运行时会导致空指针异常，因为默认的Android适配器初始化左/右页，如果有两个以上的片段

来源

2015-04-24 11:45:20

NullPointerException异常使用ViewPager

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