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Sunday, July 20, 2008

Controlling Java's Graphics: paintComponent() and repaint()

Why do we put all our graphical drawing code into a paintComponent() method? It seems odd, since it would seem we should be able to simply stick some simple graphics commands into our main() method in a Java application and just get the drawing done. Where does paintComponent come from? If we never call it in our code, how does it get executed?

It seems like magic, doesn't it? You just sort of set it up and somehow it happens. Hopefully when and how we want it to. It seems like a vaguely disquieting form of magic, perhaps.

When Java does graphics, suddenly you're not in the strict control of the program that you are with command line applications. With graphics, your program is now interacting with the other parts of your computer's operating system. It's not just running in its own little corner of the machine. Now it's got a window out on that shared resource, the display, and the window has to interact properly with everything else that's on the display.

Think of it this way, if you are just driving around on private property all by yourself you don't have to worry about traffic. Off on your own stretch of road, with an unused parking lot or two to tool around in, you don't have to worry about sharing. You can cross the center line, pull on and off the road without signalling, you don't even need a license so long as you don't go on a public road.

But once you head for the public roads and freeway, you've got to share. The rules matter. It gets more complicated than buzzing around in a go-cart or whatever on private property. And things happen that you don't have any direct control over, like red lights and merging traffic.

When you move your program on to the display in Java, you don't have to do everything on your own as you would have to in a car. Java's graphics system takes care of a lot for you. Unfortunately this means your code starts looking like magic. Methods get called without you doing the calling. But it's OK. All you need to know is what to expect.

The paintComponent() method redraws your graphics area. In our examples so far this has been a JPanel. There are two ways paintComponent() can get called. One is automatic, the other is up to you.

The automatic way is what we've used so far. The computer's window manager automatically calls for your program to redraw its graphics whenever a window it owns appears to need redrawing. The window manager can decide this for any of a number of reasons. Such as when the window initially appears on screen. Or if the window has been covered up in part or in whole by another window on screen, and the covered part has just been uncovered. Maybe the window had been minimized, and has just been restored.

When these sort of things happen, the Java Runtime Environment calls the paint() method for your container. In our examples so far, our container has been a JFrame. It will be a "heavyweight" component (see the JFrames article for a brief discussion on this.) Once the container redraws itself, it'll pass the command down to its children, that is, the lightweight components that live inside it. A lightweight component from the Swing package uses the paintComponent() method to redraw itself. So that's what gets called. Shazam! It's like magic!

This means you want to set up paintComponent() to draw things the way you want them when it gets called. If you're doing drawings that don't change, like the examples we've done to date, then you don't need to look up any information. If you're doing anything animated or that otherwise changes over time, you need to have paintComponent() be able to look things up at the time of drawing to properly reflect conditions in the drawing.

Now, let's say you've changed something in your program and you want to redraw to show the change. You probably don't want to wait until the window manager thinks your window needs redrawing. If nobody is changing things on the screen otherwise, it may not redraw until way too late!

Does that mean you put a call to paintComponent() in your program? No! You would be taking a chance of breaking something if you did. You want to start from the top, like the Java Runtime Environment did. In your case, you want to call repaint(). This will take things from the top. It also handles some possibilities that jumping in and calling paintComponent() on your own will not.

Like, let's say both you and the window manager decide to redraw at the same time. If you bypass repaint() you'll be slowing down your program by making it repaint twice when it really only needs to redraw once. When you call repaint(), Java catches this for you and saves you some computer time by only redrawing once. It'll also catch redraw requests made while the redrawing is going on and deal with them appropriately. Your paintComponent() method will get called in its own turn when you use repaint().

So, if you have new information you want depicted on screen, use repaint().

Put all your drawing inside paintComponent() and let the system take care of knowing when to call it otherwise. Give paintComponent() the ability to get the information it needs to draw things the way you want them now. If you're getting ahead of what I've written here, and aren't sure how to get through the Object Oriented barriers, look up "inner classes" for a start. It gives you a way to get the information where you need it when you need it.