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A strange mistake when adding a listener to RecyclerView, the application crashes with these errors:

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.support.v7.widget.RecyclerView$ViewHolder.shouldIgnore()' on a null object reference at android.support.v7.widget.RecyclerView.findMinMaxChildLayoutPositions(RecyclerView.java:2799) at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:2586) at android.support.v7.widget.RecyclerView.onLayout(RecyclerView.java:2971) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.support.v4.widget.SwipeRefreshLayout.onLayout(SwipeRefreshLayout.java:562) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1076) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579) at android.widget.FrameLayout.onLayout(FrameLayout.java:514) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1076) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.support.v4.widget.DrawerLayout.onLayout(DrawerLayout.java:907) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579) at android.widget.FrameLayout.onLayout(FrameLayout.java:514) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703) at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557) at android.widget.LinearLayout.onLayout(LinearLayout.java:1466) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579) at android.widget.FrameLayout.onLayout(FrameLayout.java:514) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703) at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557) at android.widget.LinearLayout.onLayout(LinearLayout.java:1466) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.widget.FrameLayout.layoutChildren(FrameLayout.java:579) at android.widget.FrameLayout.onLayout(FrameLayout.java:514) at android.view.View.layout(View.java:15671) at android.view.ViewGroup.layout(ViewGroup.java:5038) at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2114) at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1843) at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1061) at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5885) at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767) at android.view.Choreographer.doCallbacks(Choreographer.java:580) at android.view.Choreographer.doFrame(Choreographer.java:550) at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753) 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:5254) 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:903) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

The code goes inside, even doing something, but the application still crashes.

  mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() { @Override public void onScrolled(RecyclerView recyclerView, int dx, int dy) { super.onScrolled(recyclerView, dx, dy); setToolbarAlpha(dy); } @Override public void onScrollStateChanged(RecyclerView recyclerView, int newState) { super.onScrollStateChanged(recyclerView, newState); } });

UPD error climbs somewhere in this block:

  private void setToolbarAlpha(int current) { int desiredWidth = View.MeasureSpec.makeMeasureSpec(mRecyclerView.getWidth(), View.MeasureSpec.EXACTLY); View listItem = mLayoutManager.findViewByPosition(0); listItem.setLayoutParams(new RecyclerView.LayoutParams(0, 0)); listItem.measure(desiredWidth, View.MeasureSpec.UNSPECIFIED); int firstItemHeigth = listItem.getMeasuredHeight(); if (firstItemHeigth > 0) { float d = (float) Math.abs(current) / (float) firstItemHeigth; Drawable drawable = new ColorDrawable(getResources().getColor(R.color.primary_dark)); if (d < 1) { drawable.setAlpha((int) (d * 255)); } else { drawable.setAlpha(255); } ((AppCompatActivity) getActivity()).getSupportActionBar().setBackgroundDrawable(drawable); } }

Reported as a duplicate by members of Pavel Mayorov , aleksandr barakin , cheops , Grundy , zRrr 20 May '16 at 15:51 .

A similar question was asked earlier and an answer has already been received. If the answers provided are not exhaustive, please ask a new question .

    1 answer 1

    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.support.v7.widget.RecyclerView $ ViewHolder.shouldIgnore ()' on a null object reference

    The key phrase in the error text is NullPointerException.
    Habrahabr: How to understand NullPointerException
    Stackoverflow: What is a Null Pointer Exception?