Java基础之在窗口中绘图——显示曲线的控制点（CurveApplet 2 displaying control points）

Applet程序。

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

 @SuppressWarnings("serial")
 public class CurveApplet extends JApplet {
   // Initialize the applet
   @Override
   public void init() {
     pane = new CurvePane();                                            // Create pane containing curves
     Container content = getContentPane();                              // Get the content pane

     // Add the pane displaying the curves to the content pane for the applet
     content.add(pane);                                                 // BorderLayout.CENTER is default position
   }

   // Class defining a pane on which to draw
   class CurvePane extends JComponent {
     // Constructor
     public CurvePane() {
       quadCurve = new QuadCurve2D.Double(                              // Create quadratic curve
                       startQ.x, startQ.y,                              // Segment start point
                       control.x, control.y,                            // Control point
                       endQ.x, endQ.y);                                 // Segment end point

       cubicCurve = new CubicCurve2D.Double(                            // Create cubic curve
                       startC.x, startC.y,                              // Segment start point
                       controlStart.x, controlStart.y,                  // Control pt for start
                       controlEnd.x, controlEnd.y,                      // Control point for end
                       endC.x, endC.y);                                 // Segment end point
     }

     @Override
     public void paint(Graphics g) {
       Graphics2D g2D = (Graphics2D)g;                                  // Get a 2D device context

       // Draw the curves
       g2D.setPaint(Color.BLUE);
       g2D.draw(quadCurve);
       g2D.draw(cubicCurve);

       // Create and draw the markers showing the control points
       g2D.setPaint(Color.red);                                         // Set the color
       ctrlQuad.draw(g2D);
       ctrlCubic1.draw(g2D);
       ctrlCubic2.draw(g2D);
       // Draw tangents from the curve end points to the control marker centers
       Line2D.Double tangent = new Line2D.Double(startQ, ctrlQuad.getCenter());
       g2D.draw(tangent);
       tangent = new Line2D.Double(endQ, ctrlQuad.getCenter());
       g2D.draw(tangent);

       tangent = new Line2D.Double(startC, ctrlCubic1.getCenter());
       g2D.draw(tangent);
       tangent = new Line2D.Double(endC, ctrlCubic2.getCenter());
       g2D.draw(tangent);
     }
   }

   // Inner class defining a control point marker
   private class Marker {
     public Marker(Point2D.Double control)  {
       center = control;                                                // Save control point as circle center

       // Create circle around control point
       circle = new Ellipse2D.Double(control.x-radius, control.y-radius,
                                     2.0*radius, 2.0*radius);
     }

       // Draw the marker
       public void draw(Graphics2D g2D) {
         g2D.draw(circle);
       }

      // Get center of marker - the control point position
       Point2D.Double getCenter() {
         return center;
     }

     Ellipse2D.Double circle;                                           // Circle around control point
     Point2D.Double center;                                             // Circle center - the control point
     static final double radius = 3;                                    // Radius of circle
   }

   // Points for quadratic curve
   private Point2D.Double startQ = new Point2D.Double(50, 75);          // Start point
   private Point2D.Double endQ = new Point2D.Double(150, 75);           // End point
   private Point2D.Double control = new Point2D.Double(80, 25);         // Control point

   // Points for cubic curve
   private Point2D.Double startC = new Point2D.Double(50, 150);         // Start point
   private Point2D.Double endC = new Point2D.Double(150, 150);          // End point
   private Point2D.Double controlStart = new Point2D.Double(80, 100);   // 1st cntrl point
   private Point2D.Double controlEnd = new Point2D.Double(160, 100);    // 2nd cntrl point
   private QuadCurve2D.Double quadCurve;                                // Quadratic curve
   private CubicCurve2D.Double cubicCurve;                              // Cubic curve
   private CurvePane pane = new CurvePane();                            // Pane to contain curves

    // Markers for control points
    private Marker ctrlQuad = new Marker(control);
    private Marker ctrlCubic1 = new Marker(controlStart);
    private Marker ctrlCubic2 = new Marker(controlEnd);

 }

HTML文件与上一例同。