How to Set JPanel's Width and Height?

I'm developing Snake game using Java. Board's (where all action takes it place) width and height should be fixed (640 pixels * 480 pixels).

Structure:

• Main class that extends from JFrame class,
• Board class that extends from JPanel class;

In Main class I have something like...

setSize( 1024, 768 );

...and in Board class I have...

setSize( BOARDS_WIDTH, BOARDS_HEIGHT );

Problem is that main and only windows seems to be like 1024 * 768, but panel inside it - not like 640 * 480. I have setBackground() as well and background is filled more than 640 * 480.

Is there any way to make Board's width and height like 640 * 480, but main windows width and height - as much as Board's width and height are correct?

Right now it looks like this...

Edit:

All works... almost.

Main class:

add( new Board(), BorderLayout.CENTER );
pack();
setResizable( false );
setLocationRelativeTo( null );
setVisible( true );
setDefaultCloseOperation( EXIT_ON_CLOSE );
setTitle( "Snake (by daGrevis)" );

Board class:

setPreferredSize( new Dimension( 640, 480 ) );

Gray background is 642 * 482! Like there would be border that wraps all by 1 pixel!

please, something went xxx*x, and that's not true at all, check that

JButton Size - java.awt.Dimension[width=400,height=40]
JPanel Size - java.awt.Dimension[width=640,height=480]
JFrame Size - java.awt.Dimension[width=646,height=505]

code (basic stuff from Trail: Creating a GUI With JFC/Swing , and yet I still satisfied that that would be outdated )

EDIT: forget setDefaultCloseOperation()

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class FrameSize {

    private JFrame frm = new JFrame();
    private JPanel pnl = new JPanel();
    private JButton btn = new JButton("Get ScreenSize for JComponents");

    public FrameSize() {
        btn.setPreferredSize(new Dimension(400, 40));
        btn.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                System.out.println("JButton Size - " + btn.getSize());
                System.out.println("JPanel Size - " + pnl.getSize());
                System.out.println("JFrame Size - " + frm.getSize());
            }
        });
        pnl.setPreferredSize(new Dimension(640, 480));
        pnl.add(btn, BorderLayout.SOUTH);
        frm.add(pnl, BorderLayout.CENTER);
        frm.setLocation(150, 100);
        frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // EDIT
        frm.setResizable(false);
        frm.pack();
        frm.setVisible(true);
    }

    public static void main(String[] args) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                FrameSize fS = new FrameSize();
            }
        });
    }
}