Question

I have trouble understanding a fundamental concept in Java 2D.
To give a specific example:
One can customize a swing component via implementing it’s own version of the method paintComponent(Graphics g)
Graphics is available to the body of the method.
Question:
What is exactly this Graphics object, I mean how it is related to the object that has the method paintComponent? Ok, I understand that you can do something like:

g.setColor(Color.GRAY);
g.fillOval(0, 0, getWidth(), getHeight());

To get a gray oval painted. What I can not understand is how is the Graphics object related to the component and the canvas. How is this drawing actually done?
Another example:

public class MyComponent extends JComponent {

     protected void paintComponent(Graphics g) {

                System.out.println("Width:"+getWidth()+", Height:"+getHeight());

            }

    public static void main(String args[]) {

                JFrame f = new JFrame("Some frame");
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                f.setSize(200, 90);
                MyComponent  component = new MyComponent  ();
                f.add(component);
                f.setVisible(true);       
          }
 }

This prints

Width:184, Height:52

What does this size mean? I have not added anything to the frame of size(200,90).

UPDATE:
I understand that I must override paint to give in the Graphics g object the hints required to do the repaint and that I do not have to create a Graphics object as one will be given by platform.
What happens after that is what I can not understand.
E.g. does Graphics represent the screen and the object is painted accordingly on screen as soon as I start calling the various g.setXXX methods?
Does it get stored in a queue and there is a 1-1 association among g and each component? So the framework uses each g of each component to paint it one at a time?
How does this work?
Any help on this is highly welcome

Thanks

Answer 1

The component does not create a static Graphics object association.

The graphics object is the wrapper for a platform handle giving access to a physical device, like the screen. It’s valid for the time when “paint” is executed only, you can’t store it and reuse it later. It is a resource managed by the “toolkit”.

The component itself is an abstraction on top of the windowing system, that gets asociated shortly with this device for getting rendered.

EDIT

You can force such an association calling “getGraphics” if you feel the need to paint out of the “paint” callback. This should be a very rare case and you ALWAYS should dispose the Graphics afterwards.