October 3rd, 2022

• You use paintComponent to define the code that paints in a panel.
• You use repaint when underlying instance variables change. Don’t call paintComponent, because repaint takes into account concurrency.
• If you want to pass data into classes, pass a pointer of the data object to the drawing object.

Graphics2D

More sophisticated drawing system than Graphics.

public void paintComponent(Graphics2D g2) {
	Shape s = new Rectangle2D(...); // They all have 2D at the end.
	g2.draw(s);

	// kinda pseudocode
	Shape p = new Path2D();
	p.moveTo();
	p.lineTo();
	g2.draw(p);
}

Listening for clicks in your canvas

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

public class Canvas extends JPanel implements MouseListener {
    Canvas () {
	    addMouseListener (this);
    }

    // This is the draw callback
    public void paintComponent (Graphics g) {
	super.paintComponent(g);
	g.drawLine (50, 50, 100, 50);
    }

    // Listener callback
    public void mousePressed (MouseEvent event) {
        System.out.println ("Mouse down at " + event.getPoint().x + ", " + event.getPoint().y);
    }

    // MouseListener defines all of these, so we must supply them
    public void mouseReleased (MouseEvent event) {}
    public void mouseClicked (MouseEvent event) {}
    public void mouseEntered (MouseEvent event) {}
    public void mouseExited (MouseEvent event) {}
}

MVC

We’re gonna use Model View Controller architecture.

• A data object is the model
• Canvas/JPanel is view
• Button is controller