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up vote 1 down vote favorite

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.

share|improve this question
    
public ArrayList<Walls> walls = new ArrayList<Walls>(); –  user1516873 Jun 18 '13 at 8:18

3 Answers 3

up vote 4 down vote accepted

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>();
share|improve this answer
2  
Even better, perhaps - public List<Walls> walls = new ArrayList<Walls>(); –  vikingsteve Jun 18 '13 at 8:18
    
I'm pretty freaking stupid. I even spent over 30 minutes looking and changing values around in my Wall class... Haha –  Shawn Jun 18 '13 at 8:19
    
Things like this only have to be understood once, don't worry you'll get a hold of it pretty soon I'm sure. –  Peter Jaloveczki Jun 18 '13 at 8:20
up vote 0 down vote

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

share|improve this answer
up vote 0 down vote 
public ArrayList<Walls> walls = new ArrayList<Walls>();

walls is a reference that must point to an object (usually created using new) before invoking any methods on it.

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