How to set Width of a JPanel?

Question

I am overwirting paintComponent(Graphics g) method to
draw some g2D in a JPanel.
I want the drawing processing shown on screen (not flashed out after drawn because it is VERY SLOW).
So that I CAN NOT use setsize() or setBound() because to change JPanle.width because this is NOT what I wanted(the graphics is shown AFTER is drawn).
Everytime I call paintComponent(), the width of JPanel is changed.
My question is : How can change JPanel.width without calling method like setsize() or setBound() .
Thanks a lot!!

Answer 1

My code too large, I just show some idea here:
class caller (){
myPanel p;
Graphics2D g2 = (Graphics2D)p.getGraphics();
//I nee to increase the p.width before call my_paint
//I CAN NOT use setsize() or setBound()
//because I want show the drawing process
//on screen( and that is fast)
p.my_paint(g2); // how to set p.width ??
public class myPanel extends JPanel
public void my_paint(Graphics2D g2)
//Do my drawing here ,
// I need to draw thousands of Line2D.
public void paintComponent(Graphics g){
Graphics2D g2 = (Graphics2D)g;
my_paint(g2);