android sharedpreferences in arraylist crashes at getItemCount()

Question

I have this code to retrieve and save items from arraylist with sharedpreferences. But if I start my activity it gives an error:

java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
        at com.sapps.tools.dedykapp.DayRecAdapter2.getItemCount(DayRecAdapter2.java:22)
        at android.support.v7.widget.RecyclerView.onMeasure(RecyclerView.java:1843)
        at android.view.View.measure(View.java:17448)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5465)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
        at android.support.v7.widget.CardView.onMeasure(CardView.java:198)
        at android.view.View.measure(View.java:17448)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5465)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
        at android.view.View.measure(View.java:17448)
        at android.widget.ScrollView.measureChildWithMargins(ScrollView.java:1263)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
        at android.widget.ScrollView.onMeasure(ScrollView.java:337)
        at android.view.View.measure(View.java:17448)
        at android.support.v4.widget.DrawerLayout.onMeasure(DrawerLayout.java:868)
        at android.view.View.measure(View.java:17448)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5465)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
        at android.view.View.measure(View.java:17448)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5465)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
        at android.view.View.measure(View.java:17448)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5465)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
        at android.view.View.measure(View.java:17448)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5465)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
        at android.view.View.measure(View.java:17448)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5465)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
        at android.view.View.measure(View.java:17448)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5465)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2560)
        at android.view.View.measure(View.java:17448)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2031)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1189)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1402)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1077)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5884)
        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:5312)
        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:901)

I think it is the arraylist that causes the error but I'm not sure because logcat says otherwise. Here is my adapter and sharedpreferences code:

Adapter

    public class DayRecAdapter2 extends RecyclerView.Adapter<DayViewHolder> {

    ArrayList<DayItems> mList;

    public DayRecAdapter2(ArrayList<DayItems> mList){
        this.mList = mList;
    }

    public int getItemCount(){
        return mList.size();
    }

    public DayViewHolder onCreateViewHolder(ViewGroup viewGroup, int position){
        View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.planitem, viewGroup, false);
        DayViewHolder DVH = new DayViewHolder(v);
        return DVH;
    }

    public void onBindViewHolder(DayViewHolder vHolder, int i){
        vHolder.dayText.setText(mList.get(i).todo);
    }

}

SharedPreference code in activity

//day planner monday
    //retrieve saved arraylist and add to arraylist
    SharedPreferences shared = context.getSharedPreferences("MONDAY", Context.MODE_PRIVATE);
    Gson gson = new Gson();
    ArrayList<DayItems> mon = gson.fromJson(shared.getString("mondAy", null), new TypeToken<List<DayItems>>() {
    }.getType());


    final RecyclerView planMon = (RecyclerView) findViewById(R.id.monLIST);
    planMon.setHasFixedSize(true);
    LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
    linearLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
    planMon.setLayoutManager(linearLayoutManager);
    DayRecAdapter2 dayAdapter = new DayRecAdapter2(mon);
    planMon.setAdapter(dayAdapter);

possible duplicate of [What is a Null Pointer Exception, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do-i-fix-it) — Selvin, Jun 23 '15 at 13:31
Well, if you give me an explanation why, it would be nice... — svenvdz, Jun 23 '15 at 13:33
something, somewhere in your code is null ... now you have to find out what and why ... hint: what will happend if data wasn't saved yet ... — Selvin, Jun 23 '15 at 13:34
There is no data in it, you need to add data to sharedpreference in that activity but if it crashes all the time I can't do anything. — svenvdz, Jun 23 '15 at 13:36
*I think it is the arraylist that causes the error* ... congratulation, you are right .... *but I'm not sure because logcat says otherwise.* ... well, not true, analyze the code ... oh @praveen it is one line method ... — Selvin, Jun 23 '15 at 13:38
Count the list size `public int getItemCount(){ return mList.size(); }` — svenvdz, Jun 23 '15 at 13:39
check whether your **mList** is null then return (one line is enough for Null pointer exception) — praveen, Jun 23 '15 at 13:40

score 0 · Accepted Answer · answered Jun 23 '15 at 13:43

The simple answer is: the List you provide to your DayRecAdapter2-Constructor is null and therefore when you call getItemCount() it will try to access a List-Object that does not exist and will throw a NullPointerException.
Check the Lists you provide when you call

DayRecAdapter2 someName = new DayRecAdapter2(someList);

that someList is a List that has been instanciated before.

android sharedpreferences in arraylist crashes at getItemCount()

1 Answers1