绘制n边形：用两个以上的控件来控制矩形的颜色、大小、位置及空实心。（n由键盘输入）

package lzy.di9zhang;

import java.awt.BorderLayout;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.FocusEvent;
import java.awt.event.FocusListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class Shiyan4 extends JFrame implements ActionListener, FocusListener {
private JLabel jl;
private JTextField jtf;
private JButton jb;
private JPanel jp2;
private MyPanel mp;

public Shiyan4() {
jl = new JLabel("请输入正多边形的边数：");
jtf = new JTextField(10);
jtf.addFocusListener(this);
jb = new JButton("确定");
jb.addActionListener(this);
mp = new MyPanel();
jp2 = new JPanel();
jp2.add(jl);
jp2.add(jtf);
jp2.add(jb);
this.add(mp);
this.add(jp2, BorderLayout.SOUTH);
this.setSize(500, 500);
this.setVisible(true);
this.setDefaultCloseOperation(EXIT_ON_CLOSE);
}

public static void main(String[] args) {
Shiyan4 shiyan4 = new Shiyan4();
}

@Override
public void actionPerformed(ActionEvent e) {
if (e.getSource() == jb) {
String bianshu = jtf.getText();
mp.bianshu = Integer.parseInt(bianshu);
mp.repaint();
}
}

@Override
public void focusGained(FocusEvent e) {
if (e.getSource() == jtf) {
jtf.setText("");
}
}

public void focusLost(FocusEvent e) {
}
}

class MyPanel extends JPanel {
int bianshu;
private int bianshuMax = 20;
private int[] x = new int[bianshuMax];
private int[] y = new int[bianshuMax];
MyPolygon mplg = new MyPolygon(x, y);

public void paint(Graphics g) {
g.clearRect(0, 0, this.getWidth(), this.getHeight());//因为repaint不调用update，所以自己清空面板
if (bianshu <= bianshuMax) {
mplg.posOfPoint(bianshu);
g.drawPolygon(x, y, bianshu);
} else {
bianshuMax += 20;
x = new int[bianshuMax];
y = new int[bianshuMax];
mplg = new MyPolygon(x, y);
paint(g);
}
}
}

class MyPolygon {// 求正多边形的顶点坐标
private int[] x;
private int[] y;
private int startX;// 顶点的X坐标
private int startY;// 顶点的Y坐标
private int r;// 外接圆的半径

public MyPolygon(int[] x, int[] y) {
this.x = x;
this.y = y;
startX = 200;
startY = 10;
r = 200;
}

public void posOfPoint(int bianshu) {
x[0] = startX;
y[0] = startY;
Point p = new Point();
for (int i = 1; i < bianshu; i++) {
p = nextPoint(((2 * Math.PI) / bianshu) * i);
x[i] = p.x;
y[i] = p.y;
}
}

public Point nextPoint(double arc) {// arc为弧度，在顶点处建立直角坐标系，用r和arc确定下一个点的坐标
Point p = new Point();
p.x = (int) (x[0] - r * Math.sin(arc));
p.y = (int) (y[0] + r - r * Math.cos(arc));
return p;
}
}

核心代码：
 public　　Point 　　nextPoint(double arc) {// arc为弧度，在顶点（x[0],y[0]）处建立直角坐标系，

　　　　　　　　　　　　　　　　　　　　　　//用 r 和 arc 确定下一个点的坐标。
  　　　　　　　　　　Point p = new Point();
 　　　　　　　　　　 p.x = (int) (x[0] - r * Math.sin(arc));
 　　　　　　　　　　 p.y = (int) (y[0] + r - r * Math.cos(arc));
 　　　　　　　　　　 return p;
　　　　　　　　　　 }

运行界面：