How to draw a circle in Java with a given center point?

General Discussions
1

I’m working on a telecommunication application to visualize signal strengths using java draw circle. I have the X and Y coordinates of a mobile signal transmitter and a calculated radius.

However, when using g.drawOval(X, Y, r, r), the X and Y coordinates are treated as the top-left corner instead of the center.

How can I correctly position the circle?

2

By default, g.drawOval(x, y, width, height) treats (x, y) as the top-left corner. To make (X, Y) the center, subtract half the radius from both coordinates:

i

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

public class CircleDrawer extends JPanel {
    private int centerX = 200;
    private int centerY = 200;
    private int radius = 100;

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.BLUE);
        g.drawOval(centerX - radius, centerY - radius, radius * 2, radius * 2);
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        CircleDrawer panel = new CircleDrawer();
        frame.add(panel);
        frame.setSize(400, 400);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

  • centerX - radius, centerY - radius shifts the top-left corner so the circle is centered.

  • radius * 2 ensures the circle has the correct diameter.

3

For anti-aliased circles (smoother edges), use Graphics2D:

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

public class SmoothCircleDrawer extends JPanel {
    private int centerX = 200;
    private int centerY = 200;
    private int radius = 100;

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2d = (Graphics2D) g;
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2d.setColor(Color.RED);
        g2d.drawOval(centerX - radius, centerY - radius, radius * 2, radius * 2);
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        SmoothCircleDrawer panel = new SmoothCircleDrawer();
        frame.add(panel);
        frame.setSize(400, 400);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}
  • Graphics2D improves rendering quality using anti-aliasing.
  • Ideal for high-resolution or scientific visualizations.
4

If you want a solid-filled circle, use fillOval():

g.setColor(Color.GREEN);
g.fillOval(centerX - radius, centerY - radius, radius * 2, radius * 2);

This will draw a filled circle instead of just an outline.