Java: Example - Circles

// File   : CircleMain.java - Applet/main program
// Purpose: Illustrate the following:
//          * A program that is both an applet and an application.
//          * How to do graphics by subclassing JComponent.
//          * Implementing a mouse listener.
// Tag: <applet code="CircleMain.class" width="200" height="200"></applet>
// Author : Fred Swartz - 2006-11-30 - Placed in public domain.

import java.awt.*;
import javax.swing.*;

///////////////////////////////////////////////////////////// CircleMain
public class CircleMain extends JApplet {
    
    //====================================================== constructor
    public CircleMain() {
        //... Set layout and add our new component.
        setLayout(new BorderLayout());
        add(new CirclePanel(200), BorderLayout.CENTER); 
    }
    
    //============================================================= main
    public static void main(String[] args) {
        JFrame window = new JFrame("Circles");
        window.setContentPane(new CircleMain());
        window.pack();
        window.setLocationRelativeTo(null);         // Center window.
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setResizable(false);
        window.setVisible(true);
    }
}

// File   : CirclePanel.java - Draws random circles where mouse clicked.
// Purpose: Show how to define a component to display graphics and
//          use a mouse listener.
//
// Issues : What's a little strange is that this component simply records
//          the generated circles in an ArrayList in itself.
//          What would normally happen is that the user interaction would
//          change a "model", the logic of the program.  The model would
//          be a separate object that represented the logical structure
//          that the user is manipulating.  The closest that this small
//          brain-dead application comes to a model is the ArrayList of
//          circles.
// Author : Fred Swartz - 2006-11-30 - Placed in the public domain.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;

//////////////////////////////////////////////////////////////// CirclePanel
public class CirclePanel extends JComponent implements MouseListener {
    //=============================================================== fields
    private int               _size;     // Height/width of component.
    private ArrayList<Circle> _circles = new ArrayList<Circle>();

    //========================================================== constructor
    public CirclePanel(int size) {
        _size = size;
        setPreferredSize(new Dimension(_size, _size));
        addMouseListener(this);  // This component listens to mouse events.
    }

    //======================================================== paintComponent
    @Override public void paintComponent(Graphics g) {
        //... Draw background.
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, _size, _size);
        
        //... Draw each of the circles.
        for (Circle c : _circles) {
            c.draw(g);
        }
    }
    
    //====================================================== mouse listeners
    //... Ignore all mouse events except mouseClicked.
    public void mousePressed(MouseEvent e)  {}
    public void mouseReleased(MouseEvent e) {}
    public void mouseEntered(MouseEvent e)  {}
    public void mouseExited(MouseEvent e)   {}
    
    public void mouseClicked(MouseEvent e) {
        //... Generate random color and size circle at mouse click point.
        Color randColor = new Color((int)(256*Math.random()), 
                                    (int)(256*Math.random()), 
                                    (int)(256*Math.random()));
        int randRadius = (int) (_size/4 * Math.random());
        Circle c = new Circle(e.getPoint(), randRadius, randColor);
        
        //... Save this circle for painting later.
        _circles.add(c);
        
        //... Cause screen to be repainted.
        repaint();
    }
}

// File   : Circle.java - Stores and draws a circle.
// Purpose: Shows a simple class that saves and draws a circle.
// Author : Fred Swartz - 2006-11-30 - Placed in public domain.

import java.awt.*;

///////////////////////////////////////////////////////////////// Circle
class Circle {
    //=========================================================== fields
    int   _x;           // x coord of bounding rect upper left corner.
    int   _y;           // y coord of bounding rect upper left corner.
    int   _diameter;    // Height and width of bounding rectangle.
    Color _color;
    
    //====================================================== constructor
    Circle(Point center, int radius, Color color) {
        //... Change user oriented parameters into more useful values.
        _x        = center.x - radius;
        _y        = center.y - radius;
        _diameter = 2 * radius;
        _color    = color;
    }
    
    //============================================================= draw
    void draw(Graphics g) {
        //... Should we save and restore the previous color?
        g.setColor(_color);
        g.fillOval(_x, _y, _diameter, _diameter);
    }
}