I get a nullPointer exception error when I run this script. it wasn't showing up until I added the loop that sets the attractions between particles. I marked this loop in the code. Is there something wrong with the loop or do i not understand something about the attractions and how the effect time? Any help would be great.


import traer.physics.*;

ParticleSystem physics;
Particle[][] particles;
Particle mouse;
int gridSize = 10;

void setup()
{
 size(800, 800);
 smooth();
 fill(0);
 framerate(60);
       
 physics = new ParticleSystem(0, 0,5,0.5);
 
 particles = new Particle[gridSize][gridSize];
 
 float gridStepX = (float) ((width / 2) / gridSize);
 float gridStepY = (float) ((height / 2) / gridSize);
 
 for (int i = 0; i < gridSize; i++)
 {
   for (int j = 0; j < gridSize; j++)
   {
     //Spring makeSpring( Particle a, Particle b, float strength, float damping, float restLength )
     particles[i][j] = physics.makeParticle(0.2, j * gridStepX + (width / 4), i * gridStepY + 30, sin(gridStepY + 30));
     if (j > 0)
     {
       physics.makeSpring(particles[i][j - 1], particles[i][j], 8.0, 0.5, cos(gridStepX)+20);
     }
   }
 }
 
 for (int j = 0; j < gridSize; j++)
 {
   for (int i = 1; i < gridSize; i++)
   {
     physics.makeSpring(particles[i - 1][j], particles[i][j], 8.0, 0.5,sin(gridStepY + cos(gridStepX*20)));
   }
 }
 
 
 
 
 //when I use this loop I get an error on the line that reads "physics.advanceTime(0.1);"
 
 for (int j = 0; j < gridSize; j++)
 {
  for (int i = 1; i < gridSize; i++)
  {
     physics.makeAttraction( mouse, particles[i][j], 1000, 10 );
     physics.makeAttraction( mouse, particles[i - 1][j], 1000, 10 );
   }
 }
 //end of loop
 
 
 
 
 particles[0][0].makeFixed();
 particles[0][gridSize - 1].makeFixed();
 particles[9][gridSize - 1].makeFixed();
 particles[9][gridSize - 10].makeFixed();
 mouse = physics.makeParticle();
 mouse.makeFixed();
}

void draw()
{

// when compiled I get a nullPointerException on this line
  physics.advanceTime(0.1);
  //end of null pointer exception
 
  mouse.moveTo( mouseX, mouseY, 0 );

 
 if (mousePressed)
 {
   particles[0][gridSize - 1].moveTo(mouseX, mouseY, 0);
   particles[0][gridSize - 1].velocity().clear();
 }
 
 background(255);
 
 for (int i = 0; i < gridSize; i++)
 {
   beginShape( LINE_STRIP );
   curveVertex(particles[i][0].position().x(), particles[i][0].position().y());
   for (int j = 0; j < gridSize; j++)
   {
     curveVertex(particles[i][j].position().x(), particles[i][j].position().y());
   }
   curveVertex(particles[i][gridSize - 1].position().x(), particles[i][gridSize - 1].position().y());
   endShape();
 }
 for (int j = 0; j < gridSize; j++)
 {
   beginShape( LINE_STRIP );
   curveVertex(particles[0][j].position().x(), particles[0][j].position().y());
   for (int i = 0; i < gridSize; i++)
   {
     curveVertex(particles[i][j].position().x(), particles[i][j].position().y());
   }
   curveVertex(particles[gridSize - 1][j].position().x(), particles[gridSize - 1][j].position().y());
   endShape();
 }
}

void mouseReleased()
{
 particles[0][gridSize - 1].setVelocity( (mouseX - pmouseX), (mouseY - pmouseY), 0 );
}