Draw a circle with a radius and points around the edge

Question

I'm really stuck on how to go about programming this. How to draw a circle in Java with a radius and points around the edge?

I need to draw a circle within a JFrame with a radius and points around the circumference. i can mathematically calculate how to find the coordinates of the point around the edge but i cant seem to be able to program the circle. I am currently using a Ellipse2D method but that doesn't seem to work and doesn't return a radius, as under my understanding, it doesn't draw the circle from the center rather from a starting coordinate using a height and width.

My current code is on a separate frame but I need to add it to my existing frame.

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

public class circle extends JFrame { 
  public circle() { 
     super("circle"); 
     setSize(410, 435); 
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     Panel sp = new Panel(); 
     Container content = getContentPane(); 
     content.add(sp); 
     setContentPane(content); 
     setVisible(true); 
 } 

 public static void main (String args[]){
  circle sign = new circle(); 
 } 
} 

class Panel extends JPanel { 
 public void paintComponent(Graphics comp) { 
     super.paintComponent(comp); 
     Graphics2D comp2D = (Graphics2D) comp; 

     comp2D.setColor(Color.red); 
     Ellipse2D.Float sign1 = new Ellipse2D.Float(0F, 0F, 350F, 350F); 
     comp2D.fill(sign1); 
 } 
}

Answer 1

Points on a circle may be specified as a function of the angle θ:

x = a + r cos(θ)
y = b + r sin(θ)

Here, increments of 2π/8 are shown.

Addendum: As suggested in a comment by @Christoffer Hammarström, this revised example reduces the number of magic numbers in the original. The desired number of points becomes a parameter to the constructor. It also adapts the rendering to the container's size.

/** @see https://stackoverflow.com/questions/2508704 */
public class CircleTest extends JPanel {

    private static final int SIZE = 256;
    private int a = SIZE / 2;
    private int b = a;
    private int r = 4 * SIZE / 5;
    private int n;

    /** @param n  the desired number of circles. */
    public CircleTest(int n) {
        super(true);
        this.setPreferredSize(new Dimension(SIZE, SIZE));
        this.n = n;
    }

    @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.black);
        a = getWidth() / 2;
        b = getHeight() / 2;
        int m = Math.min(a, b);
        r = 4 * m / 5;
        int r2 = Math.abs(m - r) / 2;
        g2d.drawOval(a - r, b - r, 2 * r, 2 * r);
        g2d.setColor(Color.blue);
        for (int i = 0; i < n; i++) {
            double t = 2 * Math.PI * i / n;
            int x = (int) Math.round(a + r * Math.cos(t));
            int y = (int) Math.round(b + r * Math.sin(t));
            g2d.fillOval(x - r2, y - r2, 2 * r2, 2 * r2);
        }
    }

    private static void create() {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(new CircleTest(9));
        f.pack();
        f.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                create();
            }
        });
    }
}