Simple JPanel problem

Question

In the below code I try to make a JPanel thats 300 * 300 pixels big where I can draw some shapes. Just to test the program so far I tryed adding a red line. When I run the program (compile is ok) I only get a small rectangular area with a white background but without the red line, the area that supposed to be the JPanel is way too small. What am I doing wrong ?
import javax.swing.*;
import java.awt.*;
class test extends JFrame
     public Sheet sheet = new Sheet();
     public static void main(String[] arg)
          new test();     
     public test()
          Toolkit kit = getToolkit();
          Dimension screen = kit.getScreenSize();
          setBounds(screen.width/3, screen.height/3, 400, 400);
          getContentPane().setLayout(new BorderLayout());
          JPanel main = new JPanel();     
          getContentPane().add(main, BorderLayout.CENTER);
          main.add (sheet);
          setVisible (true);          
     public class Sheet extends JPanel
          public Sheet ()
               setSize(300,300);
               setBackground(Color.white);
               setVisible (true);     
          public void paintComponent(Graphics g)
               super.paintComponent(g);
               g.setColor(Color.red);     
               g.drawLine(1,1,300,300);
}

Answer 1

Of course not.
As a default, a JPanel has a FlowLayout. In a flow layout, all components are added left to right, top to bottom in the center of the container. The flow layout ignores your size settings as well. As of these facts, a FlowLayout mostly isn't very useful.
A BorderLayout stretches out your panel (when you add it to the center). The JPanel (LayoutManager) constructor takes a layout manager (such as BorderLayout), that's why I created main that way.
When you want to have your panel exactly some size you can try BoxLayout, null or GridBagLayout.
I hope this information will help you on your way.
Greetings,
Levi