Java 第二次显示ViewPager时出现NullPointerException
我使用的是一个Java 第二次显示ViewPager时出现NullPointerException,java,android,nullpointerexception,android-viewpager,Java,Android,Nullpointerexception,Android Viewpager,我使用的是一个ViewPager片段,它有两个片段作为子片段。但是,当我用另一个片段替换ViewPager片段并用ViewPager片段替换此片段时,我的应用程序会崩溃,出现以下NullPointerException: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference at android
ViewPager
片段,它有两个片段作为子片段。但是,当我用另一个片段替换ViewPager
片段并用ViewPager
片段替换此片段时,我的应用程序会崩溃,出现以下NullPointerException
:
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
at android.support.v4.app.FragmentManagerImpl.getFragment(FragmentManager.java:667)
at android.support.v4.app.FragmentStatePagerAdapter.restoreState(FragmentStatePagerAdapter.java:211)
at android.support.v4.view.ViewPager.onRestoreInstanceState(ViewPager.java:1319)
at android.view.View.dispatchRestoreInstanceState(View.java:13756)
at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:2888)
at android.view.View.restoreHierarchyState(View.java:13734)
at android.support.v4.app.Fragment.restoreViewState(Fragment.java:468)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1094)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1248)
at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1613)
at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:517)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5294)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)
MyViewPager
片段由以下源代码组成,如果两个子片段为空,则将它们实例化。此外,ViewPagerAdapter
实现被实例化并分配给ViewPager
public class ConnectionPasswordViewPagerFragment extends Fragment
{
private ViewPager vpConnectionPassword;
private ConnectionPasswordViewPagerAdapter paConnectionPassword;
private ConnectionPasswordGeneratorMACAddress passwordGeneratorMACAddress;
private ConnectionPasswordGeneratorSerialNumber passwordGeneratorSerialNumber;
public ConnectionPasswordViewPagerFragment()
{
// Required empty public constructor
}
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
if ((this.passwordGeneratorSerialNumber == null) && (this.passwordGeneratorMACAddress == null))
{
this.passwordGeneratorMACAddress = new ConnectionPasswordGeneratorMACAddress();
this.passwordGeneratorSerialNumber = new ConnectionPasswordGeneratorSerialNumber();
}
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState)
{
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_connection_password_view_pager, container, false);
}
@Override
public void onActivityCreated(Bundle savedInstanceState)
{
super.onActivityCreated(savedInstanceState);
/*if (this.paConnectionPassword == null)
{*/
this.paConnectionPassword = new ConnectionPasswordViewPagerAdapter(this.getChildFragmentManager(), this.getActivity(), this.passwordGeneratorMACAddress, this.passwordGeneratorSerialNumber);
//}
this.vpConnectionPassword = (ViewPager) this.getActivity().findViewById(R.id.vpConnectionPassword);
this.vpConnectionPassword.setAdapter(this.paConnectionPassword);
}
}
下面显示的ViewPagerAdapter
实现在ArrayList
中保留对子片段的引用,并实现metodsgetItem()
、getCount()
以及getPageTitle()
。类ViewPagerFragment
只是扩展了对Fragment
类的支持,并提供了由子片段实现的抽象方法getPageTitleStringID()
public class ConnectionPasswordViewPagerAdapter extends FragmentStatePagerAdapter
{
private static final byte NUMBER_OF_FRAGMENTS = (byte) 2;
private ArrayList<ViewPagerFragment> childFragments;
private Activity displayActivity;
public ConnectionPasswordViewPagerAdapter(FragmentManager fragmentMgm, Activity displayAct, ConnectionPasswordGeneratorMACAddress generatorMACAddress, ConnectionPasswordGeneratorSerialNumber generatorSerialNumber)
{
super(fragmentMgm);
this.childFragments = new ArrayList<>(ConnectionPasswordViewPagerAdapter.NUMBER_OF_FRAGMENTS);
this.setDisplayActivity(displayAct);
if (generatorMACAddress != null)
{
this.childFragments.add(generatorMACAddress);
}
if (generatorSerialNumber != null)
{
this.childFragments.add(generatorSerialNumber);
}
}
public Activity getDisplayActivity()
{
return this.displayActivity;
}
public void setDisplayActivity(Activity value)
{
this.displayActivity = value;
}
@Override
public Fragment getItem(int position)
{
return this.childFragments.get(position);
}
@Override
public int getCount()
{
return ConnectionPasswordViewPagerAdapter.NUMBER_OF_FRAGMENTS;
}
@Override
public CharSequence getPageTitle(int position)
{
return this.getDisplayActivity().getString(this.childFragments.get(position).getPageTitleStringID());
}
}
公共类连接PasswordViewPageRadapter扩展了FragmentStatePagerAdapter
{
_片段的私有静态最终字节数_=(字节)2;
私有ArrayList子片段;
私人活动;
公共连接PasswordViewPageRadapter(FragmentManager fragmentMgm,Activity displayAct,ConnectionPasswordGeneratorMACAddress generatorMACAddress,ConnectionPasswordGeneratorSerialNumber generatorSerialNumber)
{
超级(碎片米高梅);
this.childFragments=新的数组列表(ConnectionPasswordViewPagerAdapter.NUMBER OF_FRAGMENTS);
此.setDisplayActivity(displayAct);
如果(generatorMACAddress!=null)
{
this.childFragments.add(generatorMACAddress);
}
if(generatorSerialNumber!=null)
{
this.childFragments.add(generatorSerialNumber);
}
}
公共活动getDisplayActivity()
{
返回此.displayActivity;
}
public void setDisplayActivity(活动值)
{
this.displayActivity=值;
}
@凌驾
公共片段getItem(int位置)
{
返回此.childFragments.get(位置);
}
@凌驾
public int getCount()
{
返回ConnectionPasswordViewPagerAdapter.NUMBER\u OF_片段;
}
@凌驾
公共字符序列getPageTitle(int位置)
{
返回this.getDisplayActivity().getString(this.childFragments.get(position.getPageTitleStringID());
}
}
谢谢你的帮助 我认为您的Viewpager在Fragment中,并且此Fragment已将setRetainInstance设置为true(我在尝试保留父片段时也出现此错误)我修复了此问题,每次调用support Fragment manager的
replace
方法时,我只需创建包含Viewpager的片段
public void onSwitchToConnectionPasswordGenerator(View clickedView)
{
this.fragConnectionPassword = new ConnectionPasswordViewPagerFragment();
this.getSupportFragmentManager().beginTransaction().replace(R.id.flFragmentContainer, this.fragConnectionPassword).commitAllowingStateLoss();
}
更好的方法是,您可以检查
FragmentManager
以获取Fragment(如果有),如果没有,则创建一个新实例,如下所示:
FragmentManager manager = getSupportFragmentManager();
Fragment fragment = manager.findFragmentByTag("YourFragmentName");
if(fragment == null)
fragment = new YourFragment();
FragmentTransaction fragmentTransaction = manager.beginTransaction();
fragmentTransaction.replace(Your_Fragmnet_Container_Layout_id, fragment, fragment.getClass().getSimpleName());
fragmentTransaction.commit;
谢谢我想你的意思是我应该调用
setRetainInstance(false)
。我应该在哪里调用这个方法?我正在使用一个朋友的帐户,但是你能看看我的问题吗?我大致了解了确切的情况。你是如何解决这个问题的?