Что значит ошибка NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.List.get(int)' on a null object reference?

Question

[dimasik100200]

Что значит ошибка и как ее исправить

NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.List.get(int)' on a null object reference

Вот LogCat

ava.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.List.get(int)' on a null object reference
                                                       at com.example.asus.testapp.ContentAdapter.onBindViewHolder(ContentAdapter.java:46)
                                                       at com.example.asus.testapp.ContentAdapter.onBindViewHolder(ContentAdapter.java:12)

Logcat указывает на
строчку 12

public class ContentAdapter extends RecyclerView.Adapter<ContentAdapter.ViewHolder> {

и
строчку 46
CardFeed cardfeed = cardfeedlist.get(position);

Вот код самого ContentAdapter.

package com.example.asus.testapp;

import android.support.v7.widget.CardView;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;

import java.util.List;

public class ContentAdapter extends RecyclerView.Adapter<ContentAdapter.ViewHolder> {

    private List<CardFeed> cardfeedlist;

    public ContentAdapter(List<CardFeed> cardfeed) {
        this.cardfeedlist = cardfeed;
    }


    private static final int LENGTH = 18;

    public static class ViewHolder extends RecyclerView.ViewHolder {
        CardView cv;
        TextView cvUser;
        TextView cvLocation;
        ImageView cvPhoto;
        TextView cvContent;

        public ViewHolder(LayoutInflater inflater, ViewGroup parent) {
            super(inflater.inflate(R.layout.card_feed, parent, false));
            cvUser = (TextView) itemView.findViewById(R.id.card_user);
            cvLocation = (TextView) itemView.findViewById(R.id.card_location);
            cvPhoto = (ImageView) itemView.findViewById(R.id.card_image);
            cvContent = (TextView) itemView.findViewById(R.id.card_text);
        }
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        return new ViewHolder(LayoutInflater.from(parent.getContext()), parent);
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        CardFeed cardfeed = cardfeedlist.get(position);
        holder.cvUser.setText(cardfeed.getName());
        holder.cvLocation.setText(cardfeed.getLocation());
        holder.cvContent.setText(cardfeed.getDescription());
        holder.cvPhoto.setImageResource(cardfeed.getPhoto());
    }

    @Override
    public int getItemCount() {

        return LENGTH;
    }
}

Answer 1

[Сергей]

обычная практика с viewholder

public int getItemCount() {
        if(cardfeedlist==null) return 0;
        return cardfeedlist.size();
    }

это позволяет налету менять cardfeedlist например так

public void swapCardfeeds(List<CardFeed> cardfeedlist){
    this.cardfeedlist= cardfeedlist;
    notifyDataSetChanged();
}

конструктор по умолчанию.

Comments

[dimasik100200]

А что за метод notifyDataSetChanged();?

[Сергей]

dimasik100200, notifyDataSetChanged()
Notify any registered observers that the data set changed
Проще сообщает что данные поменялись и требуется перерисовка.

[dimasik100200]

Теперь вылезла ошибка

E/RecyclerView: No adapter attached; skipping layout

[Сергей]

dimasik100200, from gooble

[Сергей]

adapter.swapCardfeeds после этого

[dimasik100200]

Сергей, опять этаже ошибка (я все сделал по твоему совету)

public class ContentAdapter extends RecyclerView.Adapter<ContentAdapter.ViewHolder> {

    private List<CardFeed> cardfeedlist;

    public ContentAdapter(List<CardFeed> cardfeed) {
        this.cardfeedlist = cardfeed;
    }


    private static final int LENGTH = 18;

    public static class ViewHolder extends RecyclerView.ViewHolder {
        TextView cvUser;
        TextView cvLocation;
        ImageView cvPhoto;
        TextView cvContent;

        public ViewHolder(LayoutInflater inflater, ViewGroup parent) {
            super(inflater.inflate(R.layout.card_feed, parent, false));
            cvUser = (TextView) itemView.findViewById(R.id.card_user);
            cvLocation = (TextView) itemView.findViewById(R.id.card_location);
            cvPhoto = (ImageView) itemView.findViewById(R.id.card_image);
            cvContent = (TextView) itemView.findViewById(R.id.card_text);
        }
    }


    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        return new ViewHolder(LayoutInflater.from(parent.getContext()), parent);
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        CardFeed cardfeed = cardfeedlist.get(position);
        holder.cvUser.setText(cardfeed.getName());
        holder.cvLocation.setText(cardfeed.getLocation());
        holder.cvContent.setText(cardfeed.getDescription());
        holder.cvPhoto.setImageResource(cardfeed.getPhoto());
    }

    @Override
    public int getItemCount() {
        if(cardfeedlist==null) return 0;
        return cardfeedlist.size();
    }

    public void swapCardfeeds(List<CardFeed> cardfeedlist){
        this.cardfeedlist= cardfeedlist;
        notifyDataSetChanged();
    }
}