Java AWT: Setting background of Canvas changes the background for the whole frame

Question

Why is it that changing the background color for the java.awt.Canvas, changes the background for the whole frame?

I set up a Frame object as follows:

public class Gui extends Frame {

    public Gui() {

        setSize(800, 600);
        setLocation(0, 0);

        Canvas cd=new Canvas();
        cd.setSize(500, 300);
        cd.setBackground(Color.BLUE);
        add(cd);


        addWindowListener(new WindowAdapter() {

            @Override
            public void windowClosing(WindowEvent arg0) {
                System.exit(0);
            }

        });
    }

    @Override
    public void paint(Graphics g) {
        paintComponents(g);
    }

    public static void main(String[] args) {

        Gui g=new Gui();
        g.setVisible(true);

    }

}

The above code sets the frame size to be 800x600, then adds a significantly smaller canvas to it - 500x300, and finally sets the background color to be Color.BLUE, but instead of getting a 500x300 blue rectangle inside a bigger, 800x600 window-frame (with default grey color), the result is a 800x600 frame with blue background:

The docs says:

public void setBackground(Color c)

Sets the background color of this component.

The background color affects each component differently and the parts of the component that are affected by the background color may differ between operating systems.

Could that be the issue (I'm running this on Ubuntu)?
Or am I missing something else here?

Answer 1

The default layout of a Frame is BorderLayout (though a lot of the example code using the AWT based Frame class was written in a time when the default layout was FlowLayout). Without layout constraints, a component added to a BorderLayout will end up in the CENTER constraint which stretches a component so it fits to the available height and width.

Instead we might use a GridBagLayout. When we add a single component to a GridBagLayout without any constraint, it will be centered and the size will be respected. E.G. (See further comments in code.)

import java.awt.*;
import java.awt.event.*;

public class Gui extends Frame {

    public Gui() {
        setSize(400, 200);
        setLocationByPlatform(true);
        setLayout(new GridBagLayout());

        Canvas cd=new Canvas();
        // TODO: Override rather than set & make it preferred rather than actual
        cd.setSize(300, 100); 
        cd.setBackground(Color.BLUE);
        add(cd);

        addWindowListener(new WindowAdapter() {

            @Override
            public void windowClosing(WindowEvent arg0) {
                System.exit(0);
            }
        });
    }

    @Override
    public void paint(Graphics g) {
        //paintComponents(g); // wrong method!
        super.paint(g); // right method, but does nothing different to original!
    }

    public static void main(String[] args) {
        // TODO: AWT/Swing based GUIs should be started on the EDT
        Gui g=new Gui();
        g.setVisible(true);
    }
}