Adding Graphics to a Canvas

[Jonathon Robertson]

Why is there no red rectangle showing up when I run this program? I am using a JFrame and a Canvas. Here is my source code:

package javagame;

public class Launcher {

public static void main(String[] args) {
Frame d = new Frame();

}
}

=============================================================

package javagame;

import java.awt.Canvas;
import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JFrame;

public class Frame {

public JFrame frame;
public Canvas canvas;
public Graphics g;

public Frame() {
createDisplay();
}

public void createDisplay() {
frame = new JFrame("Title");
frame.setVisible(true);
frame.setLocationRelativeTo(null);
frame.setSize(300, 300);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setResizable(false);

canvas = new Canvas();
canvas.setPreferredSize(new Dimension(300, 300));

frame.add(canvas);

g = canvas.getGraphics();
g.setColor(Color.RED);
g.fillRect(0, 0, 300, 300);

}

}

[Les Morgan]

It probably is, but it is doing it in less than a millisecond and then getting rewritten with a blank. Canvas is an AWT component and JFrame is a Swing component, light weight component, and as such it is recommended not to mix Heavy weight, AWT, and light weight, Swing, components. In Swing you would use a JPanel and override the paintComponents Method:

public void paintComponents(Graphics g){
  super.paintComponents(g); //calls parent paintComponents--clears the screen for you;
  // your graphics stuff goes here
}

So you would get something like this:

public Frame() { 
 createDisplay(); 
 } 

 public void createDisplay() { 
 frame = new JFrame("Title"); 
 frame.setVisible(true); 
 frame.setLocationRelativeTo(null); 
 frame.setSize(300, 300); 
 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
 frame.setResizable(false); 

 panel = new MyPanel(); 
 panel.setPreferredSize(new Dimension(300, 300)); 

 frame.add(panel); 
 frame.pack();
 frame.setVisible(true);

 } 
class MyPanel extends JPanel{
  //your MyPanel object implementation
public void paintComponents(Graphics g){
  super.paintComponents(g); //calls parent paintComponents--clears the screen for you;
  // your graphics stuff goes here
}

}
 } 

[Rob Camick]

You should override the paintComponent(...) method (without the "s") of the JPanel.

Read the section from the Swing tutorial on Custom Painting for more information and working examples.

[Jonathon Robertson]

Thanks for the reply! But, it is still not working. I am still not able to paint graphics on the screen. I only get a blank window which I believe is just my JFrame. Here is my source code:

package javagame;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Frame{

	public JFrame frame;
	public JPanel panel;
	public Graphics g;

	public Frame() { 
		 createDisplay(); 
		 } 
		 
		 public void createDisplay() { 
		 frame = new JFrame("Title!"); 
		 frame.setVisible(true); 
		 frame.setLocationRelativeTo(null); 
		 frame.setSize(300, 300); 
		 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
		 frame.setResizable(false); 
		 
		 panel = new MyPanel(); 
		 panel.setPreferredSize(new Dimension(300, 300)); 
		 
		 frame.add(panel);
		 frame.pack();
		 
		 } 
		class MyPanel extends JPanel{
		  
		public void paintComponents(Graphics g){
		  super.paintComponents(g);
		  g.setColor(Color.RED);
		  g.fillRect(0, 0, 300, 300);
		}
		 
		}
		 } 

[Jonathon Robertson]

Wait never mind, I just saw your reply about dropping the "s" off of the components. Now it is working. Could you please tell me why that works?

[Les Morgan]

Good catch... my brain did that type of thing on several posts this morning. Seems it's already ready for the Christmas holiday!

[Rob Camick]

Could you please tell me why that works?

I provided you with a link to learn the basics of custom painting.

Read the tutorial!!!