在null对象引用上的java.lang.Class java.lang.Object.getClass()'

我尝试将viewPager与tablayout一起使用,但是当我使用视图寻呼机设置适配器时,我收到错误"java.lang.NullPointerException:尝试调用虚方法'java.lang.Class java.lang.Object.getClass()on空对象引用"我不明白为什么会显示此错误.

view pager set Adapter:

viewPager= (ViewPager) findViewById(R.id.viewPager_id);
  viewPager.setAdapter(newViewPagerAdapterUseThisClass(getSupportFragmentManager(),14));

适配器clss:

class ViewPagerAdapterUseThisClass extends FragmentPagerAdapter {
    int countTabItem;
     ViewPagerAdapterUseThisClass(FragmentManager fm,int contTabItem) {
        super(fm);

        this.countTabItem=contTabItem;
    }

    @Override
    public Fragment getItem(int position) {

        if(position==0){
            EnthusamTab enthusamTab=new EnthusamTab();
            return enthusamTab;
        }

        return null;
    }

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

查看寻呼机xml文件:

错误日志:

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:1248)
                                                                                  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:19861)
                                                                                  at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:715)
                                                                                  at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:461)
                                                                                  at android.view.View.measure(View.java:19861)
                                                                                  at android.support.v4.widget.DrawerLayout.onMeasure(DrawerLayout.java:1081)
                                                                                  at android.view.View.measure(View.java:19861)
                                                                                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6083)
                                                                                  at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
                                                                                  at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
                                                                                  at android.view.View.measure(View.java:19861)
                                                                                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6083)
                                                                                  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
                                                                                  at android.widget.LinearLayout.measureVertical(LinearLayout.java:758)
                                                                                  at android.widget.LinearLayout.onMeasure(LinearLayout.java:640)
                                                                                  at android.view.View.measure(View.java:19861)
                                                                                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6083)
                                                                                  at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
                                                                                  at android.view.View.measure(View.java:19861)
                                                                                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6083)
                                                                                  at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
                                                                                  at android.widget.LinearLayout.measureVertical(LinearLayout.java:758)
                                                                                  at android.widget.LinearLayout.onMeasure(LinearLayout.java:640)
                                                                                  at android.view.View.measure(View.java:19861)
                                                                                  at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6083)
                                                                                  at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
                                                                                  at com.android.internal.policy.DecorView.onMeasure(DecorView.java:689)
                                                                                  at android.view.View.measure(View.java:19861)
                                                                                  at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2275)
                                                                                  at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1366)
                                                                                  at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1619)
                                                                                  at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1254)
                                                                                  at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6343)
                                                                                  at android.view.Choreographer$CallbackRecord.run(Choreographer.java:874)
                                                                                  at android.view.Choreographer.doCallbacks(Choreographer.java:686)
                                                                                  at android.view.Choreographer.doFrame(Choreographer.java:621)
                                                                                  at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:860)
                                                                                  at android.os.Handler.handleCallback(Handler.java:751)
                                                                                  at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                  at android.os.Looper.loop(Looper.java:154)
                                                                                  at android.app.ActivityThread.main(ActivityThread.java:6126)
                                                                                  at java.lang.reflect.Method.invoke(Native Method)
                                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
                                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Devrim.. 48

当您null以getItem(int position)方法返回时会发生此异常PagerAdapter.

您必须在getCount()方法中设置返回值(n)并为所有(0 .. n-1)位置返回Fragmentat getItem(int position)方法.

例如,如果从getCount()方法返回3 ,则必须Fragment在getItem(int position)方法处为位置0,1和2 返回非空值.如果为位置0,1或2返回null,您将得到:

java.lang.NullPointerException:尝试在空对象引用上调用虚方法'java.lang.Class java.lang.Object.getClass()'

private static class MyPagerAdapter extends FragmentPagerAdapter {
    ...

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

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
                return new XFragment();
            case 1:
                return new YFragment();
            case 2:
                return new ZFragment();
            default:
                return null; // Problem occurs at this condition!
        }
    }
}

BYISHIMO Aud.. 10

当您返回的片段少于指定的计数时,通常会出现此错误.例如,返回count = 3,并在第二个位置返回null.请注意,count是一个基于零的索引.

当您null以getItem(int position)方法返回时会发生此异常PagerAdapter.

您必须在getCount()方法中设置返回值(n)并为所有(0 .. n-1)位置返回Fragmentat getItem(int position)方法.

例如,如果从getCount()方法返回3 ,则必须Fragment在getItem(int position)方法处为位置0,1和2 返回非空值.如果为位置0,1或2返回null,您将得到:

java.lang.NullPointerException:尝试在空对象引用上调用虚方法'java.lang.Class java.lang.Object.getClass()'

private static class MyPagerAdapter extends FragmentPagerAdapter {
    ...

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

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
                return new XFragment();
            case 1:
                return new YFragment();
            case 2:
                return new ZFragment();
            default:
                return null; // Problem occurs at this condition!
        }
    }
}

当您返回的片段少于指定的计数时,通常会出现此错误.例如,返回count = 3,并在第二个位置返回null.请注意,count是一个基于零的索引.