I keep getting this error message that reads NullPointerException. It started showing up when I added two for loops into the code. This is the entire script and bellow I have added a note at the part I added when the error showed up.




import traer.physics.*;

ParticleSystem physics;
Particle[][] particles;
Spring[][] springs;
int gridSize = 20;

void setup()
{
 size(800, 800);
 smooth();
 fill(0);
 framerate(60);
       
   physics = new ParticleSystem(0, 0,20,0.5);
 
 particles = new Particle[gridSize][gridSize];
 springs = new Spring[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 + 20, 0);
     if (j > 0)
     {
      springs[i][j] = physics.makeSpring(particles[i][j - 1], particles[i][j], 1.0, 0.5, sin(i)+30 );
     }
   }
 }
 
 for (int j = 0; j < gridSize; j++)
 {
   for (int i = 1; i < gridSize; i++)
   {
      physics.makeSpring(particles[i - 1][j], particles[i][j], 1, 0.5, 20);
   }
 }
 for (int a = 0; a < gridSize; a++)
{
 float f = random(20);
 int b = int(f);
 particles[a][0].makeFixed();
 particles[0][a].makeFixed();
 particles[gridSize-1][a].makeFixed();
 particles[a][gridSize-1].makeFixed();
 //add switch loop to stop if value exceeds gridsize then try to use complex sequence to fix points
 particles[a][a].makeFixed();

}
//array of spring tension/compression store info for all springs and color_code
 //convert_type


//This is where I get the error!!!!!!!!!!



   for (int y = 0; y < gridSize/2; y++)
{
 for (int t = 0; t < gridSize/2; t++)
{

int[] compressTest = new int[gridSize];

 float stress = springs[y][t].restLength();
 int stressInt = int(stress);
 println(stressInt);
 compressTest[t] = stressInt;
 println("spring rest length is equal to " + compressTest[t]);
 float compressTest2 = springs[1][1].currentLength();
 println("spring2 actual length is equal to " + compressTest2);
}
}

// end of the part I added that caused the error.

}

void draw()
{
 physics.advanceTime(0.1);
 
 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 );
}