Drawing in Java using Canvas

Question

I want to draw in Java's Canvas but can't get it work because I don't know what I'm doing. Here's my simple code:

import javax.swing.JFrame;
import java.awt.Canvas;
import java.awt.Graphics;
import java.awt.Color;

public class Program
{
    public static void main(String[] args)
    {
        JFrame frmMain = new JFrame();
        frmMain.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frmMain.setSize(400, 400);

        Canvas cnvs = new Canvas();
        cnvs.setSize(400, 400);

        frmMain.add(cnvs);
        frmMain.setVisible(true);

        Graphics g = cnvs.getGraphics();
        g.setColor(new Color(255, 0, 0));
        g.drawString("Hello", 200, 200);
    }
}

Nothing appears on the window.

Am I wrong to think that Canvas is a paper and Graphics is my Pencil? Is that how it works?

Answer 1

Suggestions:

• Don't use Canvas as you shouldn't mix AWT with Swing components unnecessarily.
• Instead use a JPanel or JComponent.
• Don't get your Graphics object by calling getGraphics() on a component as the Graphics object obtained will be transient.
• Draw in the JPanel's paintComponent() method.
• All this is well explained in several tutorials that are easily found. Why not read them first before trying to guess at this stuff?

---

Key tutorial links:

• Basic Tutorial: Lesson: Performing Custom Painting
• More advanced information: Painting in AWT and Swing

Answer 2

You've got to override your Canvas's paint(Graphics g) method and perform your drawing there. See the paint() documentation.

As it states, the default operation is to clear the canvas, so your call to the canvas' graphics object doesn't perform as you would expect.