Button.pde

Button button;
int count;

void setup() {
  size(300, 300);
  rectMode(CENTER);
  textSize(48);
  textAlign(CENTER);
  button = new Button(width/2, height/2 - 30, 100);
}

void draw() {
  background(0);
  button.update();
  fill(255);
  text(count, width/2, height/2 + 70);
}

void mousePressed() {
  if (button.overButton()) {
    count++;
  }
}

class Button {
  float x;
  float y;
  int sz;
  boolean hover;

  Button(float x_, float y_, int sz_) {
    x = x_;
    y = y_;
    sz = sz_;
  }

  boolean overButton() {
    return mouseX >= x-sz/2 && mouseX <= x+sz/2 && 
      mouseY >= y-sz/2 && mouseY <= y+sz/2;
  }

  void update() {
    stroke(255);
    strokeWeight(2);
    if (overButton()) {
      fill(100);
    } else {
      noFill();
    }
    rect(x, y, sz, sz);
  }
}