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

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);

}