Following code wont work as i want it to??

Question

Im writing a program which involves drawing several different shapes on a JComponent like JPanel. Im calling paint method of the JPanel casting in an Graphics2D object.
What i want is to be able to draw shapes on the JPanel and after some time i be able to add in more shapes to the JPanel.
This is what ive got so far:
import java.awt.*;
import javax.swing.*;
public class DrawingPanel extends JPanel {
       private Graphics2D g2;
       //constructor
       public DrawingPanel() {
            setBackground(Color.RED);
        * Draws a circle & line on board.
       public void drawMoreShape(){
            //Draws line
            g2.drawLine(30, 10, 24, 57);
            Ellipse2D.Double circle = new Ellipse2D.Double(20,60,30,30);
            g2.setColor(Color.BLUE);
            g2.fill(circle); //Draws a circle
            //Repaints the board...
            repaint();
        * @see javax.swing.JComponent#paint(java.awt.Graphics)
       public void paint(Graphics g) {
              super.paint(g);
              g2 = (Graphics2D) g;
              g2.setColor(Color.BLACK);
              g2.drawLine(10, 20, 14, 47);
                                    g2.drawString("Hello", 20,20);
}                    I want to call the drawMoreShape method during later stages (by another class) so i can add in more shape after i've already drawn initial stuff (the stuff in paint).
This JPanel will ofcourse be embedded inside a JFrame class which has all common swing features such as clicking a button("Add more shapes") which calls that method to add more shapes.
But thats advanced and all i want now is to get the basic of drawing on JPanel & adding more drawings after ive initially drawn something involving the use of Graphics2D object.
The only reason i can think so far is that g2 is null but i've initiated it in paint method.
So can someone tell me whats wrong?
Edited by: Theresonly1 on Jul 31, 2008 9:15 AM

Answer 1

All drawing in Swing should be done in paintComponent function.This is not exactly 100% true. It is okay - in fact good code style - to pass the graphics object from paintComponent to sub-methods, so that you can compartmentalize your painting. Also, if you create an image buffer, you can draw to that from anywhere and just call repaint when your done.
But basically this is right. I was just trying to get @OP up and running as quickly as possible
If you're going to put everything in paintCOmponent, you need to have a boolean flag, like paint2ndPart. Then switch that on when you want to paint the second part.
There is no meaning to drawing inside drawMoreShape and then calling repaint, since repaint will invoke a paint cycle which will delete it.
Read the tutorial: [Performing Custom Painting|http://java.sun.com/docs/books/tutorial/uiswing/painting/]
And this article: [Painting in AWT and Swing|http://java.sun.com/products/jfc/tsc/articles/painting/]