Java setAdapter（）引发NullPointException；viewPager和pagerAdapter don'；你不觉得是空的吗？

因此，我在使用

ViewPager

和

Navigation Drawer

的同时乱搞。我有

导航抽屉

在工作

但是，试图调用

viewPager.setAdapter（pagerAdapter）

会引发

NullPointerException

我正在检查

viewPager

和

pagerAdapter

是否为null，但是当我尝试

Log

调用它们各自的

toString（）

方法时，会发生以下情况：

03-11 12:31:54.156: D/My Debug Bitches(2893): android.support.v4.view.ViewPager{b1e75710 VFED.... ......I. 0,0-0,0 #7f090000 app:id/pager}
03-11 12:31:54.156: D/MyDebugBitches(2893): com.sweidenkopf.discretesuite.DataSetViewActivity$DataSetViewPagerAdapter@b1e7b4c8

注意

viewPager

toString（）

如何返回一些奇怪的

android.support.v4

信息？我认为这可能与这个问题有关

这是我的

viewPager

和

pagerAdapter

的初始化。我标记了导致异常的行

@Override
protected void onCreate(Bundle args) {
    super.onCreate(args);
    setContentView(R.layout.activity_data_set_view);
    super.onCreateDrawer();
    //initialize the pager adapter, giving it access to the fragment manager
    pagerAdapter = new DataSetViewPagerAdapter(super.getSupportFragmentManager());

    final ActionBar actionBar = getActionBar();

    // Specify that we will be displaying tabs in the action bar.
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    //initialize the view pager!
    viewPager = (ViewPager) findViewById(R.id.pager);
    Log.d("My Debug Bitches", viewPager.toString());
    Log.d("MyDebugBitches", pagerAdapter.toString());
    //here we give the pager its adapter
    viewPager.setAdapter(pagerAdapter);//<-- this is causing exception
}

以下是日志：

03-11 12:31:54.476: E/AndroidRuntime(2893): FATAL EXCEPTION: main
03-11 12:31:54.476: E/AndroidRuntime(2893): Process: com.sweidenkopf.discretesuite, PID: 2893
03-11 12:31:54.476: E/AndroidRuntime(2893): java.lang.NullPointerException  
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:394)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.support.v4.app.BackStackRecord.add(BackStackRecord.java:389)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:99)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:832)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.support.v4.view.ViewPager.populate(ViewPager.java:982)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.View.measure(View.java:16497)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:719)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:455)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.View.measure(View.java:16497)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.View.measure(View.java:16497)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.View.measure(View.java:16497)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2291)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.View.measure(View.java:16497)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1916)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1113)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1295)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5670)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.Choreographer.doCallbacks(Choreographer.java:574)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.Choreographer.doFrame(Choreographer.java:544)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.os.Handler.handleCallback(Handler.java:733)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.os.Handler.dispatchMessage(Handler.java:95)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.os.Looper.loop(Looper.java:136)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at android.app.ActivityThread.main(ActivityThread.java:5017)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at java.lang.reflect.Method.invokeNative(Native Method)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at java.lang.reflect.Method.invoke(Method.java:515)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
03-11 12:31:54.476: E/AndroidRuntime(2893):     at       dalvik.system.NativeStart.main(Native Method)

编辑： 我将我的

null

检查更改为：

 //initialize the view pager!
    viewPager = (ViewPager) findViewById(R.id.pager);
    if(viewPager == null){
        Log.d("My Debug Bitches", "viewPager is null");
    }
    if(pagerAdapter == null){
        Log.d("MyDebugBitches", "pagerAdapter is null");
    }

Log

调用都没有被触发，这让我觉得

viewPager

和

pagerAdapter

都不是

null

这是我的寻呼机适配器：

private class DataSetViewPagerAdapter extends FragmentPagerAdapter{

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

    @Override
    public Fragment getItem(int arg0) {
        // TODO Auto-generated method stub
        return null;
    }

    /**
     * This method must return the number of tabs, which is the length of the 
     * view_pager_tabs string array (it contains the names of all of the tabs)
     */
    @Override
    public int getCount() {
        return getResources().getStringArray(R.array.view_pager_tabs).length;
    }

    /**
     * returns the element with in the view_pager_tabs array that corresponds with
     * the given position
     */
    @Override
    public CharSequence getPageTitle(int position) {
        return getResources().getStringArray(R.array.view_pager_tabs)[position];
    }



}

---

你的问题就在这里。您的

getItem

不能返回null。在内部，

ViewPager

为您在

getItem

中返回的片段创建一些事务，您不能向事务添加空片段

为什么

super.getSupportFragmentManager（）

？你能把适配器贴上去吗？一些东西。您在哪里检查viewPager或pagerAdapter上的空值？我在你贴的代码里没看到。其次，您在toString（）方法中看到“android.support.v4”信息的原因是，这就是在xml文件中声明项的方式：

android.support.v4.view.ViewPager xmlns:android

。我的第一个怀疑是

findViewById（R.id.pager）

失败了，因此您正在为您的viewPager设置一个空值。请重新发布显示您正在检查空值的代码。@Blackbelt这是最后一次尝试使其工作；）它以前也不起作用。@blh83我没有显式地检查NULL；相反，我是在viewPager上记录toString（）调用。通常，如果调用方为NULL，此调用将导致程序崩溃。我会做一个更好的空检查，尽管保持on@blh83请参阅我的编辑。在任何一个

if语句被触发时，我都不认为该方法实际上被调用了。谢谢我只是将该方法设置为暂时返回一个空片段
private class DataSetViewPagerAdapter extends FragmentPagerAdapter{

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

    @Override
    public Fragment getItem(int arg0) {
        // TODO Auto-generated method stub
        return null;
    }

    /**
     * This method must return the number of tabs, which is the length of the 
     * view_pager_tabs string array (it contains the names of all of the tabs)
     */
    @Override
    public int getCount() {
        return getResources().getStringArray(R.array.view_pager_tabs).length;
    }

    /**
     * returns the element with in the view_pager_tabs array that corresponds with
     * the given position
     */
    @Override
    public CharSequence getPageTitle(int position) {
        return getResources().getStringArray(R.array.view_pager_tabs)[position];
    }



}

@Override
public Fragment getItem(int arg0) {
    // TODO Auto-generated method stub
    return null;
}