我如何在回收站适配器中初始化侦听器？

Question

我需要能够通过侦听器将窃听的回收器项目的位置发送到父片段，但是每次我点击它时，它都说侦听器为空。

java.lang.NullPointerException: Attempt to invoke interface method 'void com.eataway.partner.adapters.MenuAdapter$SectionAdapterListener.onSectionSend(int)' on a null object reference
        at com.rest.partner.adapters.MenuAdapter.lambda$onBindViewHolder$0$MenuAdapter(MenuAdapter.java:82)
        at com.rest.partner.adapters.-$$Lambda$MenuAdapter$TzEQ_dT_4Yb5iXO0LwjA0DjCNEI.onClick(Unknown Source:4)

这里是相关的适配器代码。顶部是构造函数类，后跟侦听器定义和设置。然后在onBindViewHolder中触发该项目的点击动作，即该项目的整个视图都是可点击的。底部是相关viewHolder的内部类。

// Constructor class
    public MenuAdapter(Context adapterContext, ArrayList<DishItem> restaurantDishItemArrayList) {
        this.adapterContext = adapterContext;
        this.restaurantDishItemArrayList = restaurantDishItemArrayList;
    }

    // Define listener to send chosen section
    public interface SectionAdapterListener {
        void onSectionSend(int position);
    }

    // Set the listener. Must be called from the fragment
    public void setSectionAdapterListener(SectionAdapterListener sectionAdapterListener) {
        this.sectionAdapterListener = sectionAdapterListener;
    }

@Override
    public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, final int position) {
        if (adapterContext instanceof SectionAdapterListener) sectionAdapterListener = (SectionAdapterListener) adapterContext;
        if (holder instanceof SectionViewHolder) {
            ((SectionViewHolder)holder).tvHeaderName.setText(restaurantDishItemArrayList.get(position).getHeaderName());
            if (sectionAdapterListener == null ) Log.d(TAG, "listener is null");
            ((SectionViewHolder)holder).viewForeground.setOnClickListener(v -> sectionAdapterListener.onSectionSend(position));

        } else if (holder instanceof DishViewHolder) {
            ((DishViewHolder)holder).tvMenuItemName.setText(restaurantDishItemArrayList.get(position).getMenuItemName());
            ((DishViewHolder)holder).tvMenuItemDescription.setText(restaurantDishItemArrayList.get(position).getMenuItemDescription());
            }
    }

public class SectionViewHolder extends RecyclerView.ViewHolder {

        private TextView tvHeaderName;
        private RelativeLayout viewForeground;

        public SectionViewHolder(@NonNull View itemView) {
            super(itemView);

            tvHeaderName = itemView.findViewById(R.id.menu_header_name);
            viewForeground = itemView.findViewById(R.id.foreground_view);
        }
    }

Answer 1

创建接口类。然后将接口添加到您的RecyclerAdapter构造函数。

 YourInterface listener;

 public MenuAdapter(YourInterface listener, Context adapterContext, ArrayList<DishItem> restaurantDishItemArrayList) {
        this.listener = listener;
        this.adapterContext = adapterContext;
        this.restaurantDishItemArrayList = restaurantDishItemArrayList;
    }

然后参加您的活动

MenuAdapter menuAdapter = new MenuAdapter(listener, context, arraylist);

Answer 2

为了避免使用NullPointerException，请在使用侦听器的方法之前检查其是否为null，因此，如果onBindViewHolder()为sectionAdapterListener，则将null更改为返回值>

@Override
    public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, final int position) {
        if (adapterContext instanceof SectionAdapterListener) sectionAdapterListener = (SectionAdapterListener) adapterContext;
        if (holder instanceof SectionViewHolder) {
            ((SectionViewHolder)holder).tvHeaderName.setText(restaurantDishItemArrayList.get(position).getHeaderName());
            if (sectionAdapterListener == null ) {
                Log.d(TAG, "listener is null");
                return;
            }
            ((SectionViewHolder)holder).viewForeground.setOnClickListener(v -> sectionAdapterListener.onSectionSend(position));

        } else if (holder instanceof DishViewHolder) {
            ((DishViewHolder)holder).tvMenuItemName.setText(restaurantDishItemArrayList.get(position).getMenuItemName());
            ((DishViewHolder)holder).tvMenuItemDescription.setText(restaurantDishItemArrayList.get(position).getMenuItemDescription());
        }
    }
否则，您需要从创建此适配器的活动/片段中调用setSectionAdapterListener()