Help - draw objects doesn't appear on JFrame

Question

i got this code from a book...
import javax.swing.*;
import java.awt.*;
class Ch5SampleGraphics {
public static void main( String[] args ) {
JFrame win;
Container contentPane;
Graphics g;
win = new JFrame("My First Rectangle");
win.setSize(300, 200);
win.setLocation(100,100);
win.setVisible(true);
contentPane = win.getContentPane();
g = contentPane.getGraphics();
g.drawRect(50,50,100,30);
the problem is when i run the app, the frame doesn't always show the rect drawn. sometimes it shows it, but usualy not. if it does show the rect, once i click on minimize, then reopen it from the taskbar, the rect will disappear again and will never show up.
can anyone help me? i've been searching for answers but to no avail. thanks 4 reading, hope someone will offer some help.

Answer 1

an intoduction to oop with java
Jesus wept. The code's about as un-object-oriented as you could possibly make it.
Looking at his sample chapter on the book's web page all I see is main() methods with linear code in and just simple number crunching with primitive types. No objects to speak of at all.
I'm sticking with my "throw the book away" advice :o)