JAVA: How to use paint() method to update the previously drawn object?

How to use paint() such that when repaint() is called the previously drawn object should not get deleted when drawing the new object. That is the drawing area must get UPDATED only and not REDRAWN.

In my code when one of many button is clicked, some aplhabet get displayed. I want to have functionality su开发者_Python百科ch that when other buttons are clicked the previously drawn alhabets must be present.Take is as if a string of alphabets getting created as the buttons are clicked. Im using Java Swing for coding.

Some piece of my code:

if(source == btnAlpha[i] )  
bollyDraw.repaint(); //bollydraw is an object of a JPanel extended class having implemented the paintComponent(Graphics g) method

In the paint() method:

if (word[i] == key) {
g.drawChars(word, i, 1, x, y);
}

In a project I worked on I used a List to store the objects that were to be drawn as a member of a class. Then as the user interacted with my UI I added/removed items from this list. The JPanel that rendered the items painted the items that were in this list. It's helps separate the UI logic from the paint logic as you can determine what goes into the paint list when an event is fired rather than in the paint method ( which should be as clean as possible ). However this will force you to repaint everything on every paint call.

In conjunction with this Kim's RepaintManager is a great way to limit what gets repainted. It is region based so there is some complexity in determining what region of the screen has changed. If you have the time it is better to use something like this otherwise it could be difficult to add this functionality down the road.

Your app must be prepared to re-paint anything it has painted. From your description, I'm afraid that means you have to keep track of everything you've painted.

The Swing behavior is partially dictated by the underlying window manager and graphical system, which at any time may chose to paint over an area where your application is present. You can override update() to control repaints initiated by your own app, and might be able to improve performance by implementing your own RepaintManager.

This tutorial explains Swing painting in more detail: http://java.sun.com/products/jfc/tsc/articles/painting/

Custom Painting Approaches shows a couple of ways to do this depending on your exact requirement.