Paint, buffer,update

Question

What are the paint, buffer and update needed for? When Should I use them. I have the following code and am trying to paint to an applet but cannot see anything in my applet please help me understand what I should be using and why.
first file:
package Exercise4.Exercise4a;
import java.awt.*;
public class Exercise4a extends java.applet.Applet {
    Graphics b;
    public void init()
        b = this.getGraphics();
        Point a = new Point(0,0);
        Door door1 = new Door(a);
        door1.paint (b);
        Point c = new Point(20,0);
        Door door2 = new Door(c);
        door2.paint(b);
        //System.out.println(door2.x);
        //System.out.println(door2.y);
}secondfile:
public class Door extends java.applet.Applet {
/** Creates a new instance of Door */
    int x;
    int y;
    public void init()
    public Door()
    x = 0;
    y = 0;
    public Door(Point aPoint)
    this.x = (int)aPoint.getX();
    this.y = (int)aPoint.getY();
    public void paint(Graphics graphics)
    graphics.drawRect(0 + this.x ,0 + y, 20,30);
    graphics.setColor(Color.black);
    graphics.fillOval(15 + x,15 + y ,5,5);
}If i just use my door.class file my applet loads fine but if I use my Exercise4a.class to load the graphics it will not how come?
Thanks for your help,
Paul

Answer 1

Probably becuase your graphics context is not get created until after init() is done and paint() is called for the first time. Try using that getGraphics() thing to draw something, I bet you get a null pointer, if not then just remember: Applets are wierd (free hint: don't use them!)