Java基础之在窗口中绘图——绘制曲线（CurveApplet 1）

Applet程序。

定义自由曲线的类有两个，其中一个定义二次曲线，另一个定义三次曲线。这些自由曲线是用一系列线段定义的参数化曲线。二次曲线段用方程定义，方程包含独立变量x的平方。三次曲线也用方程定义，方程包含独立变量x的立方。

QuadCurve2D:二次曲线的抽象基类，曲线用两个端点和一个用来定义两端切线的控制点来定义。切线是从端点到控制点的直线。

CubicCurve2D:三次曲线的抽象基类，曲线用两个端点和两个用来定义两端切线的控制点来定义。切线是从端点到对应控制点的直线。

 1 import javax.swing.*;
 2 import java.awt.*;
 3 import java.awt.geom.*;
 4 
 5 @SuppressWarnings("serial")
 6 public class CurveApplet extends JApplet {
 7   // Initialize the applet
 8   @Override
 9   public void init() {
10     pane = new CurvePane();                                            // Create pane containing curves
11     Container content = getContentPane();                              // Get the content pane
12 
13     // Add the pane displaying the curves to the content pane for the applet
14     content.add(pane);                                                 // BorderLayout.CENTER is default position
15   }
16 
17   // Class defining a pane on which to draw
18   class CurvePane extends JComponent {
19     // Constructor
20     public CurvePane() {
21       quadCurve = new QuadCurve2D.Double(                              // Create quadratic curve
22                       startQ.x, startQ.y,                              // Segment start point
23                       control.x, control.y,                            // Control point
24                       endQ.x, endQ.y);                                 // Segment end point
25 
26       cubicCurve = new CubicCurve2D.Double(                            // Create cubic curve
27                       startC.x, startC.y,                              // Segment start point
28                       controlStart.x, controlStart.y,                  // Control pt for start
29                       controlEnd.x, controlEnd.y,                      // Control point for end
30                       endC.x, endC.y);                                 // Segment end point
31     }
32 
33     @Override
34     public void paint(Graphics g) {
35       Graphics2D g2D = (Graphics2D)g;                                  // Get a 2D device context
36 
37       // Draw the curves
38       g2D.setPaint(Color.BLUE);
39       g2D.draw(quadCurve);
40       g2D.draw(cubicCurve);
41     }
42   }
43 
44   // Points for quadratic curve
45   private Point2D.Double startQ = new Point2D.Double(50, 75);          // Start point
46   private Point2D.Double endQ = new Point2D.Double(150, 75);           // End point
47   private Point2D.Double control = new Point2D.Double(80, 25);         // Control point
48 
49   // Points for cubic curve
50   private Point2D.Double startC = new Point2D.Double(50, 150);         // Start point
51   private Point2D.Double endC = new Point2D.Double(150, 150);          // End point
52   private Point2D.Double controlStart = new Point2D.Double(80, 100);   // 1st cntrl point
53   private Point2D.Double controlEnd = new Point2D.Double(160, 100);    // 2nd cntrl point
54   private QuadCurve2D.Double quadCurve;                                // Quadratic curve
55   private CubicCurve2D.Double cubicCurve;                              // Cubic curve
56   private CurvePane pane = new CurvePane();                            // Pane to contain curves
57 }

 1 <html>
 2     <head>
 3     </head>
 4     <body bgcolor="000000">
 5         <center>
 6             <applet
 7                 code    = "CurveApplet.class"
 8                 width    = "300"
 9                 height    = "300"
10                 >
11             </applet>
12         </center>
13     </body>
14 </html>