JFrame setBackground(Color c)

Question

i'm just curious about Swing so i started reading sun tut Using Swing Components /Using Top-Level Containers and at the same time reviewing a tut i built some time ago called DiveLog (also from sun).
package divelog;
import javax.swing.*;
import java.awt.*;   // Contains all of the classes for creating user
                     // interfaces and for painting graphics and images.
import java.awt.event.*; // Provides interfaces and classes for
                         // dealing with different types of events fired by AWT components
public class DiveLog {
    private JFrame dlframe;
    //private JTabbedPane tabbedPane;
    public DiveLog() {
        JFrame.setDefaultLookAndFeelDecorated(true);
        dlframe = new JFrame ("T?tulo do frame");
        dlframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        dlframe.setSize(400, 200);
        //dlframe.setBackground(Color(49, 106, 197));
        Color c = new Color(49, 106, 197);
        dlframe.setBackground(c);
        //dlframe.setForeground(c);
        //dlframe.pack();
        // Causes this Window to be sized to fit the
        // preferred size and layouts of its subcomponents.
        // If the window and/or its owner are not yet displayable,
        // both are made displayable before calculating the preferred size.
        // The Window will be validated after the preferredSize is calculated.
        dlframe.setVisible(true);
        System.out.print(dlframe.getBackground());
    public static void main(String args[]) {
        DiveLog dl = new DiveLog();
}My Q:
what's the use of
.setBackground(c); if we cant see it?

Answer 1

The content pane is always painted over the JFrame. So you would set the background color of the content pane, not the frame. So, I agree with you, there is no need for that line of code.
The only possible reason I could think of is that when using a slow computer the painting may not be done fast enough and you could get a flicker as the frame is painted and then the content pane.
JFrame frame = new JFrame();
frame.setBackground( Color.RED );
frame.getContentPane().setBackground( Color.GREEN );
frame.setSize(300, 300);
frame.setVisible(true);
Run the above code and you will notice a flicker as the frame is first painted red and then green. So to reduce flicker you could set the frame background to green as well. In most cases when using the default background color of gray you don't notice the flicker so you don't usually worry about it.