GUI Example (HFJ)

[Mikpo Siru]

Hi,

The code from page 371 makes a JFrame, inside of which is a button and a custom widget. The custom widget fills the background black, and also draws an oval whose colour changes randomly when the button is pressed.

package test;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Main implements ActionListener{

    JFrame frame;

    public static void main(String[] args){
        Main gui = new Main();
        gui.go();
    }

    public void go(){
        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JButton button = new JButton("Change colors");
        button.addActionListener(this);

        MyDrawPanel drawPanel = new MyDrawPanel();
        frame.getContentPane().add(BorderLayout.SOUTH, button);
        frame.getContentPane().add(BorderLayout.CENTER, drawPanel);
        frame.setSize(300, 300);
        frame.setVisible(true);
    }

    public void actionPerformed(ActionEvent e){
        frame.repaint();
    }
}

class MyDrawPanel extends JPanel{

    public void paintComponent(Graphics g){

        g.fillRect(0, 0, this.getWidth(), this.getHeight());    //<-----------------

        int red = (int) (Math.random() * 255);
        int green = (int) (Math.random() * 255);
        int blue = (int) (Math.random() * 255);

        Color randomColor = new Color(red, green, blue);
        g.setColor(randomColor);
        g.fillOval(70, 70, 100, 100);
    }
}

What I don't understand is how the custom widgets' width and height attributes are being updated with the JFrames size - even as I resize the frame, becuase I haven't assigned anything to the attributes in code.

Thanks

MCU

[Rob Camick]

Because you added your panel to the CENTER of a BorderLayout. The panel is automatically resized to occupy all the spaces available to the panel.