confused about paintocomponent

[degamer106]

One of the things that's been confusing me so far is why we have to override the paintcomponent() function after extending it. For example, this program draws an ellipse:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Ellipse2D;
import javax.swing.JComponent;

public class DrawEllipseComponent extends JComponent
{
	public void paintComponent(Graphics g)
	{
		Graphics2D g2 = (Graphics2D)g;
		Ellipse2D.Double ellipse = new Ellipse2D.Double(0,0,this.getWidth(),this.getHeight());
		g2.draw(ellipse);
		g2.setColor(Color.GREEN);
		g2.fill(ellipse);
	}
}

import javax.swing.JFrame;

public class DrawEllipse
{
	public static void main(String[] args)
	{
		DrawEllipseComponent ellipse = new DrawEllipseComponent();

		JFrame frame = new JFrame();
		frame.setSize(300, 400);
		frame.setTitle("An Ellipse");
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.add(ellipse);
		frame.setVisible(true);
	}
}

I can see that you create an instance of the object, create a frame, and put the object into the frame. Outside of that, I'm not quite sure I understand how the ellipse is being drawn here.

[nschessnerd]

The section of the code

public void paintComponent(Graphics g)
	{
		Graphics2D g2 = (Graphics2D)g;
		Ellipse2D.Double ellipse = new Ellipse2D.Double(0,0,this.getWidth(),this.getHeight());
		g2.draw(ellipse);
		g2.setColor(Color.GREEN);
		g2.fill(ellipse);
	}

paints the ellipse. The paintComponents() method is already defined in the Graphics class and it is called whenever the frame is loaded. We need to override it to tell it what to do, otherwise you would have a blank frame.

[degamer106]

Thank you, that cleared some confusion up for me. That helped me finish this Bar Chart program.

However, there's one thing I don't understand from here and that is the Graphics2D.

In the BarChart class, I have a method that accepts a parameter of type Graphics2D. In my BarChartComponent class, I create a Graphics2D object by casting g. What I don't quite understand from both of these classes is the need to pass g2 in as an argument for draw.

o_O

import java.awt.Graphics2D;
import java.awt.Rectangle;

public class BarChart
{
	public BarChart(int x, int y, int t, int w, String name)
	{
		xTop = x;
		yTop = y;
		top = t;
		width = w;
		title = name;
	}
	public void draw(Graphics2D g2)
	{
		Rectangle rect = new Rectangle(xTop, yTop, top, width);
		g2.draw(rect);
		g2.drawString(title, xTop + 5, yTop + 20);
	}
	private int xTop;
	private int yTop;
	private int top;
	private int width;
	private String title;
}

import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JComponent;

public class BarChartComponent extends JComponent
{
	public void paintComponent(Graphics g)
	{
		Graphics2D g2 = (Graphics2D)g;

		BarChart myBar1 = new BarChart(0, 0, 420, 30, "Golden Gate");
		myBar1.draw(g2);

		BarChart myBar2 = new BarChart(0, 40, 159, 30, "Brooklyn");
		myBar2.draw(g2);

		BarChart myBar3 = new BarChart(0, 80, 215, 30, "Delaware Memorial");
		myBar3.draw(g2);

		BarChart myBar4 = new BarChart(0, 120, 380, 30, "Mackinac");
		myBar4.draw(g2);
	}
}

import javax.swing.JFrame;

public class BarChartViewer
{
	public static void main(String[] args)
	{
		BarChartComponent myBar = new BarChartComponent();

		JFrame frame = new JFrame();
		frame.setSize(500, 200);
		frame.setTitle("BarChartViewer");
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.add(myBar);
		frame.setVisible(true);
	}
}

[nschessnerd]

BarChart uses graphics2d to draw itself, so you pass on g2 and g2 draws a barchart

[Ezzaral]

Yes, the draw() method encapsulates how to draw the chart and the Graphics2D parameter is what to draw the chart on. In that way, your method can draw onto anything that can supply a graphics context to it and is not limited to a graphics context from one particular source.