PCheckBox option;
PFont font;
String msg;

void setup() {
  option = new PCheckBox(false);
  option.setBounds((width - 20) / 2, 
                   (height - 20) / 2, 
                   20, 20);
  
  font = loadFont();
  msg = "";
  textFont(font);
  textAlign(CENTER);
}

void draw() {
  background(200);
  option.draw();
  fill(0);
  text(msg, width / 2, 
       option.y + option.height + 4 + font.baseline);
}

void keyPressed() {
  option.keyPressed();
}

void keyReleased() {
  option.keyReleased();
}

void libraryEvent(Object library, int event, Object data) {
  if (library == option) {
    if (option.checked) {
      msg = "Checked!";
    } else {
      msg = "Not checked anymore";
    }
  }
}