java - 方法 getItemCount 中出现空指针异常

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我正在使用带有卡片 View 和回收器 View 的列表。我已经为列表添加了适配器并添加了 getItemCount 方法。我想为我的列表添加搜索功能。所以我也添加了一个过滤器。但是当我在搜索 View 中输入时，我在 return ItemList.size(); 上遇到异常这条线..我错过了什么？？

适配器

public class ItemAdapter  extends RecyclerView.Adapter<ListViewHolder> implements Filterable{

    public ItemsFilter itemFilter;
    ArrayList<Item> mStringFilterList;
    private ArrayList<Item> ItemList;
    private Context context;
    private MyItemClickListener mItemClickListener;
    private MyItemLongClickListener mItemLongClickListener;

        public ItemAdapter(Context context,ArrayList<Item> ItemList) {
           this.context= context;
            this.ItemList = ItemList;

        }

        @Override
        public ListViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
            View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.card_layout, null);
            ListViewHolder mh = new ListViewHolder(v);

            return mh;
        }

        @Override
        public void onBindViewHolder(final ListViewHolder ListViewHolder, int i) {
            Item Item = ItemList.get(i);
            ListViewHolder.textName.setText(Item.getItem_name());
            ListViewHolder.textDesc.setText(Item.getItem_desc());
            ListViewHolder.textQty.setText(Item.getItem_qty());
            ListViewHolder.textName.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {

                }
            });
        }

        @Override
        public int getItemCount() {
            return  ItemList.size();
        }
    public class ItemsFilter extends Filter {

            @Override
            protected FilterResults performFiltering(CharSequence constraint) {
                FilterResults results = new FilterResults();

                if (constraint != null && constraint.length() > 0) {
                    ArrayList<Item> filterList = new ArrayList<Item>();
                    for (int i = 0; i < mStringFilterList.size(); i++) {
                        if ((mStringFilterList.get(i).getItem_name())
                                .contains(constraint.toString())) {

                            Item item = new Item(mStringFilterList.get(i)
                                    .getItem_name(), mStringFilterList.get(i)
                                    .getItem_desc(), mStringFilterList.get(i)
                                    .getItem_qty());

                            filterList.add(item);
                        }
                    }
                    results.count = filterList.size();
                    results.values = filterList;
                } else {
                    results.count = mStringFilterList.size();
                    results.values = mStringFilterList;
                }
                return results;

            }

            @Override
            public void publishResults(CharSequence constraint,
                                       Filter.FilterResults results) {
                ItemList = (ArrayList<Item>) results.values;
                notifyDataSetChanged();
            }


        }
    @Override
    public Filter getFilter() {
        // return a filter that filters data based on a constraint

        if (itemFilter == null) {
            itemFilter = new ItemsFilter();
        }
        return itemFilter;
    }
}

fragment

public class ViewStock extends Fragment  implements SearchView.OnQueryTextListener {
    final MainActivity act = (MainActivity) this.getActivity();


    public ViewStock() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View rootview = inflater.inflate(R.layout.fragment_view_stock, container, false);
        setHasOptionsMenu(true);
        act.array_data = new ArrayList<Item>();
        //   act.listview.setTextFilterEnabled(true);
       // act. = new ArrayList<Item>();
        // Context mCtx = getActivity().getApplicationContext();
        RecyclerView mRecyclerView = (RecyclerView)rootview.findViewById(R.id.my_recycler_view);
        LinearLayoutManager llm = new LinearLayoutManager(getActivity());
        llm.setOrientation(LinearLayoutManager.VERTICAL);
        mRecyclerView.setLayoutManager(llm);

        //   ItemAdapter adapter = new ItemAdapter(getActivity(), R.layout.list_item, act.arrayList);
        // listview.setAdapter(adapter);
        act.db = new DBHandler(getActivity());
        ArrayList<Item> item_array_from_db = act.db.Get_items();

        for (int i = 0; i < item_array_from_db.size(); i++) {

            int idno = item_array_from_db.get(i).getID();
            String name = item_array_from_db.get(i).getItem_name();
            String desc = item_array_from_db.get(i).getItem_desc();
            String qty = item_array_from_db.get(i).getItem_qty();
            Item cnt = new Item();
            cnt.setID(idno);
            cnt.setItem_name(name);
            cnt.setItem_desc(desc);
            cnt.setItem_qty(qty);

            act.array_data.add(cnt);
        }
        act.db.close();
        act.adapter=new ItemAdapter(getActivity(),act.array_data);
        mRecyclerView.setAdapter(act.adapter);

        return rootview;
    }

    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        inflater.inflate(R.menu.menu_main, menu);

        final MenuItem item = menu.findItem(R.id.menu_search);
        final SearchView searchView = (SearchView) MenuItemCompat.getActionView(item);
        searchView.setOnQueryTextListener(this);
    }

    @Override
    public boolean onQueryTextChange(String newText) {
        if (TextUtils.isEmpty(newText)) {
            act.adapter.getFilter().filter("");
        } else {
            act.adapter.getFilter().filter(newText.toString());
        }
        return true;
    }

    @Override
    public boolean onQueryTextSubmit(String query) {
        return false;
    }
}

类

public class Item {
    public int id;
    public String item_name;
    public String item_desc;
    public String item_qty;

    public Item(){}
    public Item(int id, String item_name, String item_desc, String item_qty) {
        super();
        this.item_name = item_name;
        this.item_desc = item_desc;
        this.item_qty = item_qty;
    }

    public Item(String item_name, String item_desc, String item_qty){
        this.item_name = item_name;
        this.item_desc=item_desc;
        this.item_qty = item_qty;
    }

    public int getID(){
        return id;
    }

    public void setID(int id){
        this.id= id;
    }
    public String getItem_name(){
        return item_name;
    }

    public void setItem_name(String item_name)
    {
        this.item_name=item_name;
    }

    public String getItem_desc()
    {
        return  item_desc;
    }
    public  void setItem_desc(String item_desc)
    {
        this.item_desc=item_desc;
    }
    public String getItem_qty()
    {
        return  item_qty;
    }
    public void setItem_qty(String item_qty) {
        this.item_qty = item_qty;
    }
}

Answer 1

ItemList 在某处设置为 null。防止这种情况的一种方法是将 getItemCount 方法更改为在列表为 null 时返回 0:

int getItemCount(){
    return  ItemList == null ? 0 : ItemList.size();
}