void setup() {  
  size(600, 600);
}

void draw() {
}

void mouseDragged() {
  // code here is called 
  // when mouse is dragged
}

void mousePressed() {
  // code here is called
  // when mouse is pressed
}

void keyPressed() {
  // code here is called
  // when any key is pressed
  
  println(key); // key is a built-in variable
  if (key == 'a') {
    // code here is called
    // when the a key is pressed
    // NOTICE THE SINGLE QUOTES!!
  }
}