Collision with boundaries

Hello, do you know how to do that when a ball collisions with a specific boundary, make a sound? This is the code

//import pbox2d.*; //import org.jbox2d.common.*; //import org.jbox2d.dynamics.joints.*; //import org.jbox2d.collision.shapes.*; //import org.jbox2d.collision.shapes.Shape; //import org.jbox2d.common.*; //import org.jbox2d.dynamics.*;

//PBox2D box2d; ArrayList balls; ArrayList boundaries;

boundaries.add(new Boundary(108.32, 402.37, 33.72, 5, 320)); boundaries.add(new Boundary(184, 461, 36.27, 5, 319));

void setup() { size(1280, 720); smooth(); //box2d = new PBox2D(this); //box2d.createWorld(); //box2d.setGravity(0, -20); balls = new ArrayList(); }

void keyPressed() { if (key=='z') { balls.add(new Ball(63, 0, 8)); } }

void draw() { background(0); //image (fondo, 0, 0); //box2d.step(); for (Ball b : balls) { b.draw(); } }

class Ball { float px, py, r, vx, vy, ax, ay; Ball(float ix, float iy, float ir) { px=ix; py=iy; r=ir; vx=0; vy=0; ax=0; ay=1; } void draw() { simulate(); render(); } void simulate() { vx+=ax; vy+=ay; px+=vx; py+=vy; } void render(){ fill(255,0,0); ellipse(px,py,r,r); } }