先上图：
    image.png
    图中的D、C、F点都会随着P点的移动而移动，拖动下面的滑块即可移动P点。

    是不是看起来像初中数学几何题？没错，就是因为题目做了出来，想看一下运动轨迹，才写了这个程序。

    代码简单，有过一丢丢开发经验的人一看就懂：

    1. // By woffee. May 7, 2019.
    2. import javax.swing.*;
    3. import java.awt.*;
    4. import javax.swing.event.*;
    6. class Test {
    7.     public static void main(String args[]) {
    8.         TheWindow w = new TheWindow();
    9.         w.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    10.         w.setSize(800, 600);
    11.         w.setVisible(true);
    12.     }
    13. }
    15. class Point{
    16.     public int x = 0;
    17.     public int y = 0;
    18.     public Point(int x, int y){
    19.         this.x = x;
    20.         this.y = y;
    21.     }
    22. }
    24. class DrawOval extends JPanel {
    26.     private int SIZE = 20;
    27.     private int WIDTH = 760;
    28.     private int HEIGHT = 500;
    30.     private Point ip = new Point(SIZE,SIZE + HEIGHT);
    31.     private int dd = 230; //SET START SIZE OF CIRCLE
    33.     private Point o,a,b,c,d,p,f;
    35.     public DrawOval()
    36.     {
    37.         this.o = new Point(0,0);
    38.         this.a = new Point(400,0);
    39.         this.b = new Point(200,(int)(200 * Math.sqrt(3)));
    40.         this.c = new Point(0,0);
    41.         this.d = new Point(0,0);
    42.         this.p = new Point(0,0);
    43.         this.f = new Point(0,0);
    44.     }
    47.     public void paintComponent(Graphics g) {
    48.         super.paintComponent(g); //CALL JPANEL PAINT.COMPONENT METHOD
    50.         // 已知等边三角形两点d和p，求另一点c。  参考：https://blog.csdn.net/lkj345/article/details/78375862
    51.         p.x = dd;
    52.         p.y = 0;
    54.         d.x = (int)( (b.x+p.x)/2 );
    55.         d.y = (int)( (b.y+p.y)/2 );
    57.         c.x = (int)( p.x + (d.x-p.x)/2.0 + (d.y-p.y)/2.0*Math.sqrt(3) );
    58.         c.y = (int)( p.y + (d.y-p.y)/2.0 - (d.x-p.x)/2.0*Math.sqrt(3) );
    60.         f.x = c.x;
    61.         f.y = 0;
    63.         g.setColor(Color.gray);
    64.         g.fillRect(SIZE,SIZE,WIDTH,HEIGHT);
    66.         g.setColor(Color.black);
    67.         myDrawLine(g, o, a);
    68.         myDrawLine(g, o, b);
    69.         myDrawLine(g, a, b);
    70.         myDrawLine(g, b, p);
    72.         myDrawLine(g, d, c);
    73.         myDrawLine(g, p, c);
    74.         myDrawLine(g, f, c);
    76.         g.setColor(Color.GREEN);
    77.         myDrawPoint(g,"D", d);
    78.         myDrawPoint(g,"P", p);
    79.         myDrawPoint(g,"C", c);
    80.         myDrawPoint(g,"F", f);
    82.         myDrawPoint(g,"O", o);
    83.         myDrawPoint(g,"A", a);
    84.         myDrawPoint(g,"B", b);
    86.         //g.setColor(Color.green);
    87.         //g.fillOval(10, 10, dd, dd);  // DRAW CIRCLE
    89.     }
    91.     public void myDrawPoint(Graphics g, String s, Point aa){
    92.         g.drawString(s + ":"+aa.x+","+aa.y, ip.x + aa.x + 10, ip.y - aa.y -10);
    93.     }
    95.     public void myDrawLine(Graphics g, Point aa, Point bb)
    96.     {
    97.         g.drawLine(ip.x+aa.x, ip.y-aa.y, ip.x+bb.x, ip.y-bb.y);
    98.     }
    100.     // CREATE SETTER METHOD TO CHANGE VARIABLE AND REPAINT
    101.     public void setD(int newD) {
    102.         dd = (newD >= 0 ? newD : 230);
    103.         repaint();
    104.     }
    106.     public Dimension getPreferredSize() {
    107.         return new Dimension(200, 200);
    108.     }
    110.     public Dimension getMinimumSize() {
    111.         return getPreferredSize();
    112.     }
    113. }
    115. class TheWindow extends JFrame {
    116.     private JSlider slider; // INSTANTIATE JSLIDER
    117.     private DrawOval myPanel; // INSTANTIATE DRAWOVAL CLASS
    119.     public TheWindow() {
    120.         super("The Slider Demo");
    121.         myPanel = new DrawOval();
    122.         myPanel.setBackground(Color.white);
    124.         slider = new JSlider(SwingConstants.HORIZONTAL, 0, 400, 230);
    125.         slider.setMajorTickSpacing(10);
    126.         slider.setPaintTicks(true);
    128.         // ADD LISTERNER TO SLIDER
    129.         slider.addChangeListener(new ChangeListener() {
    130.             public void stateChanged(ChangeEvent e) {
    131.                 myPanel.setD(slider.getValue()); //CALL SETTER METHOD
    132.             }
    133.         });
    134.         add(slider, BorderLayout.SOUTH); // ADD SLIDER + LAYOUT
    135.         add(myPanel, BorderLayout.CENTER); // ADD PANEL TO FRAME
    136.     }
    137. }