我正在创建一个到目前为止工作正常的Android应用程序。该应用程序应该显示电影和电视剧集列表。我在这个应用程序中使用viewpager。
我创建了一个布尔isMoviesSelected，现在，默认情况下，它的值为true。当用户通过菜单选择“tv”选项时，它的值将变为false。
所以，在viewpager中，我有一个if-else条件，如果isMoviesSelected设置为true，则viewpager将有5个不同的选项卡，每个选项卡都有不同的数据集（应用程序是在if-else条件之前为viewpager工作正常）。如果变量的值为false，则viewpager将有4个选项卡，每个选项卡将显示不同的数据。
此外，最初在viewpager中显示的数据如下：
switch(itemPosition) {
case 0:
return new movieFragment();
......
我甚至改变了这一点，以便它在打开之前以字符串格式向片段发送一些数据。为此，我声明Fragment fragment = new movieFragment();，现在使用此fragment添加一个包作为参数。
但是，现在应用程序在打开之前就崩溃了。另外，这次logcat错误没有指出代码的任何一行是导致错误的那一行，而是指向我的根文件夹，我不知道该做什么我是android开发的新手....请帮忙：
FATAL EXCEPTION: main
Process: com.example.android.jsontutorial, PID: 8304
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:19730)
at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:715)
at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:461)
at android.view.View.measure(View.java:19730)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
at android.support.design.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:714)
at android.support.design.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:786)
at android.view.View.measure(View.java:19730)
at android.support.v4.widget.DrawerLayout.onMeasure(DrawerLayout.java:1081)
at android.view.View.measure(View.java:19730)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
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:19730)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
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:19730)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
at android.view.View.measure(View.java:19730)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
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:19730)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
at com.android.internal.policy.DecorView.onMeasure(DecorView.java:687)
at android.view.View.measure(View.java:19730)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2271)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1358)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1607)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1246)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6301)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:871)
at android.view.Choreographer.doCallbacks(Choreographer.java:683)
at android.view.Choreographer.doFrame(Choreographer.java:619)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:857)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
编辑：这是捆绑部分
public Fragment getItem(int position) {
if (isMoviesSelected == true) {
Fragment fragment;
Bundle bundle = new Bundle();
fragment = new Movie_Fragment();
switch (position) {
case 0:
bundle.putString("movieURL", latestMovie);
fragment.setArguments(bundle);
return fragment;
default:
return null;
}
答案 0 :(得分：1)
在适配器中尝试以下代码：
public Fragment getItem(int position) {
switch (position) {
case 0:
Fragment fragment = new Movie_Fragment();
if (isMoviesSelected == true) {
Bundle bundle = new Bundle();
bundle.putString("movieURL", latestMovie);
fragment.setArguments(bundle);
}
return fragment;
default:
return null;
}
}
还会覆盖getCount()方法以返回片段数：
@Override
public int getCount() {
return 1; //as u have 1 fragment
}
答案 1 :(得分：1)
您的getCount()方法（您尚未显示）确定getItem()中遇到的案例。
如果没有命中，则代码返回null，这可能是错误。
如果isMoviesSelected设置为true，则viewpager将有5个不同的选项卡，每个选项卡都有不同的数据集（在if-else条件之前，app对于viewpager工作正常）。如果变量的值为false，则viewpager将有4个选项卡，每个选项卡将显示不同的数据
所以，你需要
@Override
public int getCount() {
return movieSelected ? 5 : 4; // 5 tabs if true, else 4
}
当您在getItem()中添加条件时，您的代码可能正在执行此操作
if (movieSelected) {
switch (position) {
case 0:
return ... // some fragment
default:
return null; // Will error if more positions than position=0
}
} else {
return null; // What if a movie is not selected? --> Error!
}
一种解决方法是将if (isMoviesSelected)检查移到单个case内以简化错误处理