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 （）对空对象引用“我不明白为什么会显示此错误。
查看寻呼机设置适配器：
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：
<android.support.v4.view.ViewPager
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_below="@id/tab_layout_id"
android:id="@+id/viewPager_id"/>
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)
答案 0 :(得分：42)
当您使用null getItem(int position)方法返回PagerAdapter时会发生此异常。
您必须在getCount()方法设置返回值（n），并在Fragment方法中为所有（0 .. n-1）位置返回getItem(int position)。
例如，如果从getCount()方法返回3，则必须在Fragment方法中为位置0,1和2返回非空getItem(int position)。如果为位置0,1或2返回null，则会得到：
java.lang.NullPointerException：尝试调用虚方法 '对象上的'java.lang.Class java.lang.Object.getClass（）' 参考
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!
}
}
}
答案 1 :(得分：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。
答案 2 :(得分：3)
返回所有计数。试试这个：
Override
public Fragment getItem(int position) {
EnthusamTab enthusamTab=new EnthusamTab();
return enthusamTab;
}
答案 3 :(得分：2)
根据我看到你在这个方法中返回null：
@Override
public Fragment getItem(int position) {
if(position==0){
EnthusamTab enthusamTab=new EnthusamTab();
return enthusamTab;
}
return null;
}
答案 4 :(得分：0)
public Fragment getItem(int position) {
if (position == 0) {
Tab1 tab1 = new Tab1();
return tab1;
}
if (position == 2) {
Tab2 tab2 = new Tab2();
return tab2;
} else {
Tab3 tab3 = new Tab3();
return tab3;
}
}
答案 5 :(得分：0)
尝试在getCount中对所有片段进行计数：例如1..5
@Override
public int getCount() {
return 5; //her
}
答案 6 :(得分：0)
仅当您尝试使用方法getCount()返回的值大于在选项卡中的初始值时，才会发生此类异常。
更改您的返回值。
答案 7 :(得分：0)
我收到此错误，是因为我忘记设置break命令。首先检查您的中断运算符。
现在我有了以下代码：
@Override
public Fragment getItem(int position) {
switch (position) {
case 0:
return new FollowersFragment();
case 1:
return new PostsFragment();
case 2:
return new ReactsFragment();
case 3:
return new StoriesFragment();
case 4:
return new FollowersFragment();
case 5:
return new FollowersFragment();
case 6:
return new FollowersFragment();
}
return null;
}
答案 8 :(得分：0)
只需使用下面给出的代码。它避免了空错误并可以平稳运行
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