P.18 (Change Once Only: boolean isDown)

// This method is called whenever
public void act() {
  // if( not is Down and "g" is down )
  if( !isDown && Greenfoot.isKeyDown ("g") ) {
    setImage ("white-key-down.png");
    isDown = true;
  }

  // if( isDown and "g" is not down )
  if( isDown && !Greenfoot.isKeyDown ("g") ) {
    setImage ("white-key.png");
    isDown = false;
  }
}