arraylist 中的 android 共享首選項在 getItemCount() 處崩潰

Question

我有這段代碼可以使用sharedpreferences從arraylist檢索和保存項目。 但是如果我開始我的活動，它會給出一個錯誤：

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)

我認為是arraylist導致了錯誤，但我不確定，因為logcat另有說明。 這是我的adapter和sharedpreferences代碼：

適配器

    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);
    }

}

活動中的 SharedPreference 代碼

//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);

Answer 1

簡單的答案是：您提供給 DayRecAdapter2-Constructor 的 List 為空，因此當您調用 getItemCount() 時，它將嘗試訪問不存在的 List-Object 並拋出 NullPointerException。
檢查您在致電時提供的清單

DayRecAdapter2 someName = new DayRecAdapter2(someList);

someList 是一個之前已經實例化的 List。