drawing a pie chart

[dewitrydan]

Hey there, I am following a tutorial on how to draw a pie chart using JPanels and stuff.
I have spent ages trying to do this, and my my code does not work. All I get is a window with a mostly black background.
Can someone please tell me what I can do to my code to get it working and display a pie chart? thanks

import java.awt.*;

import javax.swing.*;

public class pie_window extends JFrame {
	public static void main(String[] args) {
		pie_window p = new pie_window();

	}

	public pie_window() {
		pie_shape z = new pie_shape();
		super.add(z);
		super.setVisible(true);
		this.pack();
	}

}

class pie_shape extends JPanel {
	Color color;
	segment[] parts_of_pie;
	int initial_position_of_first_arc;
	int extension_of_arc;
	float total_portion;

	public pie_shape() {
		segment[] parts_of_pie = { new segment(0.5f, Color.RED),
				new segment(0.3f, Color.BLUE) };

		for (segment segments : parts_of_pie) {
			total_portion += segments.portion;
		}

	}

	protected void paintComponent(Graphics g) {
		super.paintComponent(g);
		for (segment segments : parts_of_pie) {
			g.setColor(segments.color);
			g.drawArc(
					this.getWidth() / 2,
					this.getHeight() / 2,
					this.getWidth() / 2,
					this.getHeight() / 2,
					initial_position_of_first_arc + extension_of_arc,
					(initial_position_of_first_arc + extension_of_arc + (int) (((segments.portion * 360) / total_portion))));
			extension_of_arc += (segments.portion * 360) / total_portion;
		}

	}

}

class segment {
	Float portion;
	Color color;

	public segment(Float in_portion, Color in_color) {
		portion = in_portion;
		color = in_color;
	}

}

[Fubarable]

You may be ignoring the most important result of running your code -- you also get a NullPointerException because of variable shadowing, and this is causing most of your problems. You are re-declaring a field in a constructor shadowing the class field and not initializing the class field. Solution: don't re-declare the variable.

[dewitrydan]

Thanks for the reply
I am still really stuck though
The exception raised was:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at pie_shape.paintComponent(pie_window.java:41)

So I changed the line it was pointing at (line 41) from:

for (segment segments : parts_of_pie)

to

for (int i = 0; i <= parts_of_pie.length; i++)

But it still raises the same exception at the same place.
I am really stuck for what to do :(

[Fubarable]

Your change is cosmetic only and will do nothing (as you're finding out) because it doesn't fix the primary problem -- an object is null, and you're trying to use it. Since the error occurs on this line:

for (segment segments : parts_of_pie)

then the null object is being used on that line, and that can only mean one variable, parts_of_pie, is the null one. So you have to look into why this class field is null, and in fact I mentioned the reason above (though perhaps I wasn't clear) -- you are shadowing this variable.

class pie_shape extends JPanel {
	Color color;
	[color="red"][b]segment[] parts_of_pie; // #1 [/b][/color]
	int initial_position_of_first_arc;
	int extension_of_arc;
	float total_portion;

	public pie_shape() {
		[color="red"][b]segment[] parts_of_pie = { new segment(0.5f, Color.RED),
				new segment(0.3f, Color.BLUE) }; // #2 [/b][/color]

		for (segment segments : parts_of_pie) {
			total_portion += segments.portion;

On line // #1 above you declare the parts_of_pie class field, but then in the constructor, on line //#2 (actually it's 2 lines here) you re-declare this variable in the pie_shape constructor. So you construct the object only in the constructor, but since the shadowed variable was declared in the constructor it is only visible in the constructor.

Solution: don't re-declare this variable in the constructor.

There are other problems in your code though, but you must solve this one first to move on.

[dewitrydan]

Okay thankyou
I think I have solved that particular problem:

I have changed my code from:

class pie_shape extends JPanel {
	Color color;
	segment[] parts_of_pie; // #1 
	int initial_position_of_first_arc;
	int extension_of_arc;
	float total_portion;

	public pie_shape() {
		segment[] parts_of_pie = { new segment(0.5f, Color.RED),
				new segment(0.3f, Color.BLUE) }; // #2 

		for (segment segments : parts_of_pie) {
			total_portion += segments.portion;

to:

	segment[] parts_of_pie;
	int initial_position_of_first_arc;
	int extension_of_arc;
	float total_portion;

	public pie_shape() {
		this.parts_of_pie = new segment[]{ new segment(0.5f, Color.RED),
				new segment(0.3f, Color.BLUE) };

But a new exception is being thrown in this method:

protected void paintComponent(Graphics g) {
		super.paintComponent(g);

		for (int i = 0; i <= parts_of_pie.length; i++) {
			g.setColor(parts_of_pie[i].color);

method

Exception thrown:

Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: 3
	at pie_shape.paintComponent(pie_window.java:45)

But I can't see why it's telling me I'm trying to access elements in the array that don't exist, because I added them to the array in the pie_shape constructor.
More help please?
your time is very much appreciated!

[Fubarable]

But a new exception is being thrown in this method:

protected void paintComponent(Graphics g) {
		super.paintComponent(g);

		for (int i = 0; i <= parts_of_pie.length; i++) {
			g.setColor(parts_of_pie[i].color);

method

Exception thrown:

Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: 3
	at pie_shape.paintComponent(pie_window.java:45)

But I can't see why it's telling me I'm trying to access elements in the array that don't exist, because I added them to the array in the pie_shape constructor.

simple error and you'll kick yourself for it...
i goes from 0 to what? How many items are there in the array? when i is the maximum value is there an array item that corresponds to it?

[dewitrydan]

Thank you.
I have now fixed that problem.
I have changed my paintComponent to:

protected void paintComponent(Graphics g) {
		super.paintComponent(g);
		// extension_of_arc = 0;

		for (int i = 0; i <= parts_of_pie.length - 1; i++) {
			System.out.println(parts_of_pie.length);

			 System.out.println("extension_of_arc = " + extension_of_arc);

			g.setColor(parts_of_pie[i].color);

			

			g.fillArc(0, 0, 200, 200, extension_of_arc,
					(int) (extension_of_arc + (parts_of_pie[i].portion * 360)
							/ total_portion));
			extension_of_arc += (extension_of_arc + (parts_of_pie[i].portion * 360)
					/ total_portion);

		}

But all I get is a black circle, but it's meant to be a pie chart with differnent coloured segments :S