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Verlet particle sytem not working

4 posts in this topic

I've a very basic particle system based on verlet intergration. It seems fine but the updateVerlet() function returns a nullpointer exception. I just can't find the problem.
Here's the source (exluding imports):

Main class,
public class ParticleSystem {
int lastFrame, width=800, height=600;
boolean simulating;
int ticker;

Vertex[] vertex = new Vertex[100];
int temp;

public void Start(){

try {
Display.setDisplayMode(new DisplayMode(width,height));
} catch (LWJGLException e) {


boolean Close = Display.isCloseRequested();


int delta=getDelta();

if(Keyboard.isKeyDown(Keyboard.KEY_ESCAPE) || Display.isCloseRequested())
Close = true;

simulating = true;

if(temp <100 && Mouse.isButtonDown(0) && ticker>=100){
vertex[temp]= new Vertex(Mouse.getX(), height-Mouse.getY());




public void logic(int delta){
for(int i=0;i<temp;i++){

public void renderer(){
for(int i=0;i<100;i++){

public int getDelta(){
long time = getTime();
int delta = (int) (time - lastFrame);
lastFrame = (int) time;

return delta;

public long getTime(){
return (Sys.getTime()*1000)/Sys.getTimerResolution();

public void initGL(){
GL11.glOrtho(0, width, height, 0, 1, -1);
public static void main(String args[]){
ParticleSystem ps = new ParticleSystem();

Vertex class,
public class Vertex {
protected Point2D.Double pos;
protected Point2D.Double oldpos;
protected Point2D.Double acceleration;
protected Point2D.Double temp;

public Vertex(double x, double y){
pos = new Point2D.Double(x,y);
oldpos = new Point2D.Double(x,y);

public void updateVerlet(int delta){
double t=((double)delta)/1000;

pos.x += (pos.x - oldpos.x) + (acceleration.x*t*t);
pos.y += (pos.y - oldpos.y) + (acceleration.y*t*t);


public void draw(){
GL11.glColor3f(1f, 0f, 0f);
GL11.glVertex2d(pos.x-1, pos.y-1);
GL11.glVertex2d(pos.x-1, pos.y+1);
GL11.glVertex2d(pos.x+1, pos.y+1);
GL11.glVertex2d(pos.x+1, pos.y-1);

Thanks in advance.

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What does the null pointer exception say. Usually it give a line indication as to what's wrong.

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Everything* in the [i]updateVerlet()[/i] function except for the first line (the [i]delta[/i] one) returns a nullpointer exception.

*I checked line by line.

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Usually you should stop at the first line the returns the error since that's probably where the error originates at.

When you get these kinds of problems there is a couple of ways to attack it. First thing to remember is that Null Pointer exceptions is reserved for objects only. So you just have to check each object that is in the line. Your first line in updateVerlet contains no object so that's why it's not throwing an exception since you can't not initialize a primitive.

Put this code at the first line of updateVerlet
if(temp == null) System.out.println("Temp is NULL");
if(pos == null) System.out.println("Pos is NULL");
if(oldpos == null) System.out.println("OldPos is NULL");
if(acceleration == null) System.out.println("Acceleration is NULL");

You probably have two uninitialized objects in the updateVerlet function.

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Thank you. I wasn't initializing [i]temp[/i] and [i]acceleration[/i]. I assumed that they had already initialized as (0,0).

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