Detecção

int rectX = 100;
int rectY = 300;
int rectSize = 200;

int bolaX = 600;
int bolaY = 400;
int bolaSize = 200;

void setup(){
  size(800,800);

}

void draw() {
  if (collisionRect()){
    fill(255);
    println("Estou no quadrado");
  }else{
    fill(0);

  }
  rect(rectX,rectY,rectSize,rectSize);

  if(collisionCircle()){
    fill(0);
    println("Estou no círculo");
  }else{
    fill(255);
  }
  ellipse(bolaX,bolaY,bolaSize,bolaSize);
}

boolean collisionRect(){
  if(((mouseX >= rectX) && (mouseX <= rectX + rectSize) && ((mouseY >= rectY) && (mouseY <= rectY + rectSize)))){
    return true;
  }
  return false;
}

boolean collisionCircle(){
  if(dist2D(mouseX,mouseY,bolaX,bolaY) <= bolaSize/2){
    return true;
  }
  return false;
}


int dist2D(int p1X,int p1Y,int p2X,int p2Y){
  return round(sqrt( pow( abs(p1X - p2X) , 2 ) + pow( abs(p1Y - p2Y) , 2 ) ) );
}