Android：這個無用的logcat錯誤是什麼意思？

Question

我正在創建一個工作正常的android應用程序，直到現在。該應用程序應該顯示電影和電視劇列表。我在這個應用程序中使用viewpager。

我已經創建了布爾isMoviesSelected，現在，默認情況下，它的值爲true。當用戶通過菜單選擇'tv'選項時，它的值將變爲false。

因此，在viewpager中，我有一個if-else條件，如果isMoviesSelected設置爲true，viewpager將有5個不同的選項卡，每個選項卡都有一組不同的數據（該應用程序工作正常在if-else條件之前的viewpager）。如果變量的值爲false，viewpager將有4個選項卡，並且每個選項卡將顯示不同的數據。

此外，要顯示在viewpager數據原本是這樣的：

switch(itemPosition) { 
    case 0: 
     return new movieFragment(); 
    ...... 

我改變即使這樣，這樣它會打開它之前以字符串格式發送一些數據給片段。爲此，我宣佈Fragment fragment = new movieFragment();，現在正在使用此fragment添加一個包作爲參數。

但是，現在應用程序在打開之前崩潰了。此外，這次logcat錯誤沒有指出任何代碼行導致錯誤，但指向我的根文件夾，我不知道該怎麼做我是新的android開發....請幫助：

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) 

編輯：這裏是束部

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; 
      } 

Answer 1

你getCount()方法（你沒有示出）確定被打到在getItem()的情況。

如果沒有命中，你的代碼返回null，這可能是錯誤。

如果isMoviesSelected設置爲true，viewpager將有5個不同的選項卡，每個選項卡都有一組不同的數據（該應用程序在if-else條件之前對viewpager正常工作）。如果變量的值是假的，viewpager將有4個選項卡，每個人都會顯示不同的數據

那麼，你就需要

@Override 
public int getCount() { 
    return movieSelected ? 5 : 4; // 5 tabs if true, else 4 
} 

當您在getItem()增加的條件下，你有一些代碼可能是這樣

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! 
} 

一個解決將是移動if (isMoviesSelected)檢查單case內，以簡化你的錯誤處理

Answer 2

嘗試在適配器下面的代碼：

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; 
    } 
} 

而且覆蓋getCount()方法返回的frag數換貨：

@Override 
public int getCount() { 
    return 1; //as u have 1 fragment 
}