Question

I came across this code. I thought I alleviate this confusion before I run into any hiccups along the way in programming. I am having trouble understanding whether the paint or the actionPerformed method gets executed first in Board class. I hope my java comments are correctly stated.

The thing is, I took introductory Java in the summer and graphics was only introduced towards the end of the course. The class used ImageIcon and we never touched the drawImage method and Image abstract class. I also do not understand the paint method at all. This code is more involved than the Java graphics lecture I had. Based on the Java API, the paint method originated from the JComponent class which is JPanel’s superclass.

So what is this parameter Graphics g that the paint method takes in all about and how should I think about it? How does the paint method know which object of a graphics class to paint. I looked at the Java API and it says Graphics is an abstract class. How can g be an object if its data type is abstract? I am saying g is an object because the code is calling the drawImage method on the object g.

On a side note, does repaint method mean erase the content in the JPanel and redraw the entire component like rendering?

public class Board extends JPanel implements ActionListener{

private Image apple;
private int apple_x;
private int apple_y;

// over-riding the paint method from the JComponent Class
public void paint(Graphics g){
                // recursively call the paint method
                super.paint(g);

                g.drawImage(apple, apple_x, apple_y, this); 
    }

 // does this method gets called first or the top one?
 public void actionPerformed(ActionEvent e) {



        repaint();
    }

}

Answer 1

Drawing in Java (and basically all current windowing systems) follows the Hollywood principle:

You don’t call me; I call you.

I.e. you can tell the system that a certain area will need to be redrawn (repaint()). But you’ll have to wait until the system calls you to do the drawing. In Java, the system will call the paint() method and will pass you a Graphics instance to use for drawing.

So the order of event is:

1. actionPerformed()
2. paint()

Graphics is often called a graphics context. It’s an object used for drawing. Depending on the system and the current requirements, the drawing might go directly on the screen or into an offscreen buffer that is later copied to the screen. The Graphics instance takes care of the details.