Null Pointer Exception - Adding item to ArrayList

Question

I'm a bit new to java so please dumb down the answers as if I'm a four year old. =-D

I'm using slick for this code. If you need more code please just ask. I'm trying to post only what I see as relevant, but as I'm new to programming I may be wrong.

I've tracked the origin of the problem to adding a newly created object into an arrayList. Here's the code:

public ArrayList<Walls> walls;
Walls w1 = new Walls(new RPoint(100,100), brickwall, brickwall, 100);
walls.add(w1); //this is where the error will occur at.

Walls class:

package main;

import org.newdawn.slick.Image;
import org.newdawn.slick.geom.Rectangle;

public class Walls {

private RPoint p;
private Image i;
private Image i2;
private int durability;
//private Rectangle r = new Rectangle (1,1,1,1);

public Walls(RPoint a, Image b, Image c, int d){
    this.p=a;
    this.i=b;
    this.i2=c;
    this.durability=d;
//  Rectangle r = new Rectangle(Math.round(a.getX()), Math.round(a.getY()), Math.round(b.getWidth()), Math.round(b.getHeight()));

}

  public Image getImage()
  {
     return this.i;
  }

//  public Rectangle getRect()
//  {
//     return this.r;
//  }

  public Image getSecondaryImage()
  {
     return this.i2;
  }

  public int getLife()
  {
     return this.durability;
  }

  public RPoint getPoint()
  {
     return this.p;
  }

       }

Thanks for any help or for even looking.

Answer 1

The arrayList must be initialized! : )

public ArrayList<Walls> walls = new ArrayList<Walls>();

Edit:

True that the conventional way to declare such a field is to use the interface as type like so:

public List<Walls> walls = new ArrayList<Walls>();

Answer 2

You never initialize the ArrayList; you've reserved a place in memory for it, but until you initialize it its value is null.

Answer 3

public ArrayList<Walls> walls = new ArrayList<Walls>();

walls是一个引用，它必须指向一个对象（通常使用new创建），然后再调用它上面的任何方法。