Swing graphics problem

[Rob Brew]

Hi All.

Copying from the oriley head first java book, which i recommend. The following won't compile:

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

public class SimpleGui1B implements ActionListener {
	JButton button;

	public static void main (String args[]) {
		SimpleGui1B gui = new SimpleGui1B();
		gui.go();
	}

	public void go () {
		JFrame frame = new JFrame();
		button = new JButton ("click me");

		button.addActionListener(this);

		frame.getContentPane().add(button);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setSize(300,300);
		frame.setVisible(true);
	}
	public void actionPerformed(ActionEvent event)
	{
		button.setText("I've been clicked");
	}
}

class MyDrawPanel extends JPanel {
	public void paintComponent(Graphics g)
	{
		g.setColor(Color.orange);
		g.fillRect(20,50,100,100);
	}
}

Error message:

C:\Users\rob\Documents\SimpleGui1B.java:30: cannot find symbol
symbol : class Graphics
location: class MyDrawPanel
public void paintComponent(Graphics g)
^
C:\Users\rob\Documents\SimpleGui1B.java:32: cannot find symbol
symbol : variable Color
location: class MyDrawPanel
g.setColor(Color.orange);
^
2 errors

Tool completed with exit code 1

any ideas what's wrong?

Thanks,
Rob.

[Dawid Skrzypczynski]

You have to import two class: Graphics and Color.

paste it...

import javax.swing.*;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.*;

[Rob Brew]

Thanks, should have realised the imports.

What the book doesn't say is how to get Grapics "g" to display. Any ideas?

[Paul Clapham]

No, that's not right. You don't get a Graphics to display. A Graphics object is an internal object which can be used by classes which need to display data, such as JPanel.

Looking at that code, I see you have declared a class which extends JPanel, but you haven't used any instances of that class anywhere. That means your GUI isn't going to contain any MyDrawPanel objects.

[Dawid Skrzypczynski]

Paul Clapham wrote:No, that's not right. You don't get a Graphics to display. A Graphics object is an internal object which can be used by classes which need to display data, such as JPanel.

Looking at that code, I see you have declared a class which extends JPanel, but you haven't used any instances of that class anywhere. That means your GUI isn't going to contain any MyDrawPanel objects.

Eclipse shows errors if you don't import these classes...

[Paul Clapham]

Dawid Skrzypczynski wrote:

Paul Clapham wrote:No, that's not right. You don't get a Graphics to display. A Graphics object is an internal object which can be used by classes which need to display data, such as JPanel.

Looking at that code, I see you have declared a class which extends JPanel, but you haven't used any instances of that class anywhere. That means your GUI isn't going to contain any MyDrawPanel objects.

Eclipse shows errors if you don't import these classes...

That's true. But was your statement supposed to be about what I posted in some way?

[Rob Brew]

Yes Paul. How do i display the graphics object?

Thanks.

[Paul Clapham]

Paul Clapham wrote:No, that's not right. You don't get a Graphics to display. A Graphics object is an internal object which can be used by classes which need to display data, such as JPanel.

Looking at that code, I see you have declared a class which extends JPanel, but you haven't used any instances of that class anywhere. That means your GUI isn't going to contain any MyDrawPanel objects.

[Rob Brew]

Ok.

So how do i display the rectangle defined in the PaintComponent method? I'm a bit lost with swing at the moment.

[Jesper de Jong]

You'd have to create an instance of class MyDrawPanel and add it to the frame. Right now MyDrawPanel is an unused class in your code. Just declaring the class by itself doesn't automatically show it in your JFrame. Right now you're only adding a button to the JFrame (in line 18).

[Rob Spoor]

And you should always start by calling super.paintComponent(g) when overriding paintComponent.

[Campbell Ritchie]

The reason for calling super.paintComponent is that you always start with a blank canvas after any changes.

I think this thread would sit better on the GUIs forum: moving.

[Darryl Burke]

It's a common misconception the a Grphics object represents some kind of image. No. Think of a Graphics as a paintbrush, which paints to a surface -- commonly a screen element aka Component, or an image like BufferedImage, or, when printing with the Print API, to the paper in the printer.

Review the Graphics methods with this in mind and it'll be a lot clearer.

[Rob Brew]

Solved.

Thanks loads guys

[Darryl Burke]

Time for a tutorial: Lesson: Performing Custom Painting

[Rob Brew]

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

class SimpleGui2A extends JPanel {


	public void paintCompnonent(Graphics g) {

		super.paintComponent(g);
		g.fillRect(0,0,this.getWidth(),this.getHeight());

		int red = (int) (Math.random() * 255);
		int green = (int) (Math.random() * 255);
		int blue = (int) (Math.random() * 255);

		Color randomColor = new Color(red, green, blue);
		g.setColor(randomColor);
		g.fillOval(70,70,50,50);
	}

	public static void  main(String args[])
	{
		SimpleGui2A gui = new SimpleGui2A();
		JFrame frame = new JFrame("circle");
		frame.getContentPane().add(gui);

		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setSize(200,200);
		frame.setVisible(true);
		frame.repaint();
	}
}

This displays an empty pane, what is missing guys?

[Darryl Burke]

Check your spelling.

In general, when you intend to override a method, add the @Override annotation above the method.

@Override
public void paintCompnonent(Graphics g) {   
:
:

What do you get?

[Campbell Ritchie]

. . . and shouldn't that method have protected access?