How is the position of a RecyclerView adapter related to the index of its dataset?

Question

I thought they were the same, but they're not. The following code gives an indexOutOfBounds exception when I try to access the "position" index of my dataset, in this case a list of a model I created called Task:

public class TaskAdapter extends RecyclerView.Adapter<TaskAdapter.TaskViewHolder>   {

private List<Task> taskList;
private TaskAdapter thisAdapter = this;

// cache of views to reduce number of findViewById calls
public static class TaskViewHolder extends RecyclerView.ViewHolder {
    protected TextView taskTV;
    protected ImageView closeBtn;

    public TaskViewHolder(View v) {
        super(v);
        taskTV = (TextView)v.findViewById(R.id.taskDesc);
        closeBtn = (ImageView)v.findViewById(R.id.xImg);
    }
}


public TaskAdapter(List<Task> tasks) {
    if(tasks == null)
        throw new IllegalArgumentException("tasks cannot be null");
    taskList = tasks;
}


// onBindViewHolder binds a model to a viewholder
@Override
public void onBindViewHolder(TaskViewHolder taskViewHolder, int pos) {
    final int position = pos;
    Task currTask = taskList.get(pos);
    taskViewHolder.taskTV.setText(currTask.getDescription());

    **taskViewHolder.closeBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Log.d("TRACE", "Closing task at position " + position);
            // delete from SQLite DB
            Task taskToDel = taskList.get(position);
            taskToDel.delete();
            // updating UI
            taskList.remove(position);
            thisAdapter.notifyItemRemoved(position);
        }
    });**
}

@Override
public int getItemCount() {
    //Log.d("TRACE", taskList.size() + " tasks in DB");
    return taskList.size();
}


// inflates row to create a viewHolder
@Override
public TaskViewHolder onCreateViewHolder(ViewGroup parent, int pos) {
    View itemView = LayoutInflater.from(parent.getContext()).
                                   inflate(R.layout.list_item, parent, false);
    Task currTask = taskList.get(pos);

    //itemView.setBackgroundColor(Color.parseColor(currTask.getColor()));
    return new TaskViewHolder(itemView);
}
}

Deleting from my recyclerview gives unexpected results sometimes. Sometimes the element ahead of the one clicked is deleted, other times an indexOutOfBounds exception occurs at "taskList.get(position)".

Reading https://developer.android.com/reference/android/support/v7/widget/RecyclerView.Adapter.html and https://developer.android.com/training/material/lists-cards.html did not give me any more insight into why this was happening and how to fix it.

It looks like RecyclerView recycles the rows, but I wouldn't expect an indexoutofbounds exception using a smaller subset of numbers to index my list.

Answer 1

Like yigit said, RecyclerView works like that:

A B C D

and you add item X via

mItems.add(1, X);
notifyItemInserted(1, 1);

you get

A X B C D

Using holder.getAdapterPosition() in onClickListener() will give you the right item from dataset to be removed, not the "static" view position. Here's the doc about it onBindViewHolder