Java 2D Graphics Question

Question

I'm attempting to make a Java Applet which will allow me to draw a graph data structure in a canvas by clicking where I want to create nodes, and clicking the nodes to connect them. The problem is I cannot get the paint() method to behave correctly. I add new nodes to the graph (and squares on the canvas) inside the mousePressed(MouseEvent e) method using,

Graphics g = this.getGraphics();

g.setColor(Color.blue);

g.fillRect(e.getX(), e.gety(), 40, 40);

Everything works fine, until I resize the window, and then all the filled rectangles vanish. I overrided the paint method to just an empty method, but the same thing still happens. I can't add the fillRect commands inside paint() because I don't know what rectangles exist until the user interacts with it using the mouse.

How can I use g.fillRect inside the mouse listener methods and make them stick?

Answer 1

You've got to override the window resize action listener and call repaint inside of it.

Answer 2

I don't know if I'm reading this correctly, but why not just store the location of the last click in a variable to be painted later, when the paint() method is called?

Answer 3

The problem is the place you're drawing to isn't persistant. At any moment, you can lose everything you've drawn to it. The paint(Graphics) method is called when this happens. You'll either need to repaint the entire picture every time this happens, or you'll need to set aside a canvas to draw to and copy the contents to your applet's Graphics as needed.

Here's how to create and draw to an image:
http://java.sun.com/docs/books/tutorial/2d/images/drawonimage.html

Then, in your paint method, use your Graphics' drawImage(...) method to display the image you've created.

Answer 4

The Graphics is temporary. When a region gets dirty, it will be repainted.

The best way is to create a BufferedImage, paint to it on mousePressed and call repaint.
When paint is called, draw the Image onto the passed Graphics Object. This way you don't need to store the Rectangles and you got a buffer which will improve performance.