import javax.swing.*;
import java.awt.*;
import static java.lang.Math.*;

public class N_gon extends JPanel {
	public N_gon() {
		setPreferredSize(new Dimension(220, 220));
	}

	@Override  
	public void paintComponent(Graphics g) {
		super.paintComponent(g);

		int np = 7;
		int sc = 100;

		int i;
		double theta1, theta2;
		for(i = 0; i < np; i++) {
			// 単位 ラジアン
			theta1 = PI * 2 * i / np;        // 360 * i / n度
			theta2 = PI * 2 * (i + 1) / np;  // 360 * (i + 1) / n度
			g.drawLine((int)(sc * (1.1 + cos(theta1))), 
					(int)(sc * (1.1 + sin(theta1))),
					(int)(sc * (1.1 + cos(theta2))),
					(int)(sc * (1.1 + sin(theta2))));
		}
	}

	public static void main(String[] args) {
		SwingUtilities.invokeLater(() -> {
			JFrame frame = new JFrame("正多角形");
			frame.add(new N_gon());
			frame.pack();
			frame.setVisible(true);
			frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		});
	}
}