Canvases

If your applet will deal with graphics directly, canvases are a good idea. Once you start filling up a window with lots of interface elements and multiple panels, it is best not to draw directly onto the window surface any more. This is because your drawing will interfere with the buttons. Instead you add a canvas to the window.

A canvas is simply a rectangular area in which you can draw. (In contrast, a panel is a rectangular area into which you can place user interface components.) Making a canvas is a bit more complex than using a panel because you need to specify how to draw on the canvas. This means you must derive a new class from Canvas and then override the paint method in your extended class.

Canvases provide a basic component that catches expose events, mouse events, and other events. While the basic canvas does nothing with these events, you can extend canvas to create your own functioning canvases.

import java.awt.*; import java.awt.event.*; import java.applet.Applet; public class CanvasDemo extends Applet{ Panel p1; Button b1; public void init() { setLayout(new BorderLayout(15,15)); b1 = new Button("Test"); MyCanvas c1 = new MyCanvas(200,180); b1.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e){ System.out.println("Applet Button has been pressed");}}); add("Center", c1); add("South", b1); } }

By creating a subclass of Canvas, we can override the usual event handling methods in addition to the paint methods.

Canvases simplify the production of applets requiring unique functionality for several distinct areas.

Below we override the paint() method and provide mouse access to the canvas
Here it is running

import java.awt.*; import java.awt.event.*; public class MyCanvas extends Canvas implements MouseListener{ private int width; private int height; public MyCanvas(int w, int h) { width = w; height = h; setBounds(0,0,w,h); addMouseListener(this); } public void paint(Graphics g) { g.setColor(Color.blue); g.fillRect(0,0,width,height); } public void mousePressed(MouseEvent e){ if (( e.getX() < width) && (e.getY() < height)) System.out.println("Canvas mouse works: ("+e.getX() +","+e.getY() +")"); } public void mouseClicked(MouseEvent e){ } public void mouseEntered(MouseEvent e){ } public void mouseExited(MouseEvent e){ } public void mouseReleased(MouseEvent e){ } }