Question:
My applet draws lines. I click at the place where I want my line to start and then I click at the end of it. It draws a line from one point to the other. How do I use repaint() so that when I draw the second line my first line does not disappear?

Answer:
Calling repaint() calls update(g) which first erases your graphics context, then calls paint(g). Try redefining update so it doesn’t erase the graphics context:

public void update(Graphics g) {           paint(g);    }