java 2D and swing

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.

Answer 2

I understand your problem as I struggled with it for some time when I was learning Java graphics. It's not just Java 2D graphics - it is part of AWT.

When you create a JFrame or some other top-level object, it does a lot of work "behind the scenes" - part of which is creating a Graphics object. (There is not explicit notification of this, though if you stepped through the code with a debugger you may see classes which create the Graphics).

You then create components which you add, or register with, the top-level object. These all have to implement a call-back method, including

paint(Graphics g);

You will then @Override these methods so that when the component is rendered it uses YOUR paint method.

Do not try to save the Graphic or create a new one. Think of it as the framework taking the responsibility off you.

The size of components is often taken out of your hands. If you use a layout manager then it may decide to resize your component.

If you are coming from a procedural imperative background you may well have problems (I came from FORTRAN). My advice would be to try a number of tutorials and - at some stage - enlightenment will start to come.

The general documentation for Java graphics is poor. There are many concepts which are opaque (see How does Java Graphics.drawImage() work and what is the role of ImageObserver ). The early implementation was rushed through and had many bugs. Even now it is often unclear whether and in what order you should call methods such as setPack() and setVisible().

This doesn't mean you shouldn't use it! Just that the learning curve is a bit longer than IMO it should be.

MORE: Also YOU don't decide when something is painted, the framework does. paint(g) really means "The framweork is repainting its components. What to you want this component to provide at this stage".

Maybe providePaintingInstructionsWhenRequiredForComponentGraphics(Graphics g) would be a useful name.

Similarly repaint() does not repaint at your orders, but when the system thinks it should. I haven't found it useful.

If you (say) resize a component interactively every slight change will normally trigger a paint(g). try putting a LOG.debug() in the paint code and seeing when it gets called.

Answer 3

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

You added your component to the frame and set the size of the frame to be (200, 90). The default layout manager for the content pane of the frame is the BorderLayout, which means the component you added gets all the available space. The frame needs some space for the title bar and borders, so your component gets the remaining space.

Answer 4

Think of a Graphics like a piece of paper which you draw on to show how the Component looks like at that moment. After you've drawn it, the framework toolkit will trim off the edges and show what you've drawn to display the component. Moreover, the next time you draw the component, you'll be drawing on a different piece of paper, so don't keep the old Graphics around.