Learning Java - Chapter 6 : Java : Supplements

The Canvas component provides a blank area on which to draw. The Canvas class extends the Component class and adds little additional functionality other than offering an empty display.

You can also paint on a Panel component, though this wastes the Container capabilities that it inherits. In many of the applet programs in this course we paint on the applet's panel. Note that while Swing has a parallel class for most of the other AWT components (e.g. JButton vs. Button) but does not include a parallel JCanvas class. Instead, Swing provides JPanel for drawing. (It can be said, though, that there is a certain logical consistency in offering a component intended just as a drawing surface rather than also a container/drawing surface combo.)

The applet shown below uses a Canvas subclass as a component on which to draw a message.

CanvasApplet.java

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

/** This program demonstrates the use of the
  * Canvas component for drawing.
**/
public class CanvasApplet extends Applet
                           implements ActionListener
{
  MyCanvas fCanvas;
  Button fButton;

  /** Create an interface with a Canvas and button.**/
  public void init () {
    // Use a borderlayout to control where
    // the components go.
    setLayout (new BorderLayout ());

    // Create an instance of myCanvas
    fCanvas = new MyCanvas ();
    add ("North", fCanvas);

    // Set the canvas to a fixed size.
    fCanvas.setSize (200,120);

    // Make a button and use this applet
    // as it's listener
    fButton = new Button ("Push");
    fButton.addActionListener (this);

    // Use a panel to fillup the rest of the
    // space not used by the canvas on top.
    Panel p = new Panel ();
    setLayout (new BorderLayout ());
    p.setBackground (Color.cyan);

    // And put the button in the middle of the panel
    p.add ("Center", fButton);
    add ("South", p);

  } //init

  /** Each button click will redraw the canvas. **/
  public void actionPerformed  (ActionEvent e) {
    fCanvas.repaint ();
  }

} // class CanvasApplet

/** Create a Canvas subclass for drawing.
  * It will keep track of the number of times
  * that it is painted and draw that number on
  * its display.
**/
class MyCanvas extends Canvas {

  int fNumCalls;

  MyCanvas () {
    fNumCalls = 0;
    setBackground (Color.yellow);
  } // ctor

  public void paint (Graphics g) {
    g.setColor (Color.black);
    fNumCalls++;
    String msg = "Number clicks = " + fNumCalls;
    // Draw at a hardwired coordinate position.
    g.drawString (msg, 40, 50);
  } //paint

} // class MyCanvas