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This chapter is from the book

This chapter is from the book

10.2 Drawing Shapes

With the AWT, you generally drew a shape by calling the drawXxx or fillXxx method of the Graphics object. In Java 2D, you generally create a Shape object, then call either the draw or fill method of the Graphics2D object, supplying the Shape object as an argument. For example:

public void paintComponent(Graphics g) {
 super.paintComponent(g);
 Graphics2D g2d = (Graphics2D)g;
 // Assume x, y, and diameter are instance variables.
 Ellipse2D.Double circle =
  new Ellipse2D.double(x, y, diameter, diameter);
 g2d.fill(circle);
 ...
}

Most of the Shape classes define both a Shape.Double and a Shape.Float version of the class. Depending on the version of the class, the coordinate locations are stored as either double precision numbers (Shape.Double) or single precision numbers (Shape.Float). The idea is that single precision coordinates might be slightly faster to manipulate on some platforms. You can still call the familiar drawXxx methods of the Graphics class if you like; the Graphics2D object inherits from the Graphics object. This approach is necessary for drawString and drawImage and possibly is convenient for draw3DRect.

Shape Classes

Arguments to the Graphics2D draw and fill methods must implement the Shape interface. You can create your own shapes, of course, but you can also use major built-in classes: Arc2D, Area, CubicCurve2D, Ellipse2D, Gener_alPath, Line2D, QuadCurve2D, Rectangle2D, and RoundRectangle2D. Each of these classes is contained in the java.awt.geom package. Each of these classes, except for Area, Polygon, and Rectangle, has float and dou_ble constructors.

The classes Polygon and Rectangle, holdovers from Java 1.1, also implement the Shape interface. These two shapes are covered in Section 9.11 (Graphics Operations).

The most common constructors for these Shapes follow.

public Arc2D.Float(float left, float top, float width, float height, 								 
                           float startAngle, float deltaAngle, 								 
                           int closure) 
public Arc2D.Double(double left, double top, double width, double height, 
                    double startAngle, double deltaAngle, 
                    int closure)

These constructors create an arc by selecting a portion of a full ellipse whose bounding rectangle has an upper-left corner located at the (left, top). The vertex of the arc (ellipse) is located at the origin of the bounding rectangle. The reference for the start angle is the positive x-axis. Angles are specified in degrees and represent arc degrees, not true degrees. Arc angles are defined such that the 45 degree line runs from the ellipse center to the upper-right corner of the bounding rectangle. The arc closure is one of Arc2D.CHORD, Arc2D.OPEN, or Arc2D.PIE.

public Area(Shape shape)

This constructor creates an Area with the given Shape. Areas support geometrical operations, for example: add, subtract, intersect, and e_xclusiveOr.

public CubicCurve2D.Float(float xStart, float yStart, 
                               float pX, float pY, 
                               float qX, float qY, \
                               float xEnd, float yEnd)
public CubicCurve2D.Double(double xStart, double yStart, 
                           double pX, double pY, 
                           double qX, double qY, 
                           double xEnd, double yEnd)

These constructors create a CubicCurve2D shape representing a curve (spline) from (xStart, yStart) to (xEnd, yEnd). The curve has two control points (pX, pY) and (qX, qY) that impact the curvature of the line segment joining the two end points.

public Ellipse2D.Float (float left, float top, float width, 
                             float height)
public Ellipse2D.Double(double left, double top, 
                        double width, 
                           double height)

These constructors create an ellipse bounded by a rectangle of dimen_sion width by height. The Ellipse2D class inherits from the RectangularShape class and contains the same methods as common to Rectangle2D and RoundRectangle2D.

public GeneralPath()

A GeneralPath is an interesting class because you can define all the line segments to create a brand-new Shape. This class supports a handful of methods to add lines and Bézier (cubic) curves to the path: closePath, curveTo, lineTo, moveTo, and quadTo. Appending a path segment to a GeneralPath without first performing an initial moveTo generates an IllegalPathStateException. An example of creating a GeneralPath follows:

GeneralPath path = new GeneralPath();
path.moveTo(100,100); 
path.lineTo(300,205);
path.quadTo(205,250,340,300);
path.lineTo(340,350);
path.closePath();

public Line2D.Float(float xStart, float yStart, float xEnd,
 							   float yEnd) 
public Line2D.Double(double xStart, double yStart,
                     double xEnd, double yEnd)

These constructors create a Line2D shape representing a line segment from (xStart, yStart) to (xEnd, yEnd).

public Line2D.Float(Point p1, Point p2) public Line2D.Double(Point p1, Point p2)

These constructors create a Line2D shape representing a line segment from Point p1 to Point p2.

public QuadCurve2D.Float(float xStart, float yStart, 
                              float pX, double pY, 
                              float xEnd, float yEnd)
public QuadCurve2D.Double(double xStart, double yStart,
                          double pX, double pY, 
                             double xEnd, double yEnd)

These constructors create a Shape representing a curve from (xStart, yStart) to (xEnd, yEnd). The point (pX, pY) represents a control point impacting the curvature of the line segment connecting the two end points.

public Rectangle2D.Float(float top, float left, float width, 
                              float height)
public Rectangle2D.Double(double top, double left, 
                          double width, double height)

These constructors create a Rectangle2D shape with the upper-left corner located at (top, left) and a dimension of width by height.

public RoundRectangle2D.Float(float top, float left,
                                   float width, float height,
                                   float arcX, float arcY)
public RoundRectangle2D.Double(double top, double left, 
                               double width, double height,
                               double arcX, double arcY)

These two constructors create a RectangleShape with rounded corners. The upper-left corner of the rectangle is located at (top, left), and the dimension of the rectangle is width by height. The arguments arcX and arcY represent the distance from the rectangle corners (in the respective x direction and y direction) at which the rounded curve of the corners start.

An example of drawing a circle (Ellispse2D with equal width and height) and a rectangle (Rectangle2D) is presented in Listing 10.3. Here, the circle is filled completely, and an outline of the rectangle is drawn, both based on the default context settings of the Graphics2D object. Figure 10–1 shows the result. The method getCircle plays a role in other examples throughout this chapter. ShapeExample uses WindowUtilities in Listing 14.1 and ExitListener in Listing 14.2 to create a closable JFrame container for the drawing panel.

Most of the code examples throughout this chapter are presented as Java applications. To convert the examples to applets, follow the given template:

import java.awt.*;
import javax.swing.*;
public class YourApplet extends JApplet {
 public void init() {
  JPanel panel = new ChapterExample();
  panel.setBackground(Color.white);
  getContentPane().add(panel);
 }
}

The basic idea is to create a JApplet and add the chapter example, which is implemented as a JPanel, to the contentPane of the JApplet. Depending on the particular example you are converting, you may need to set the background color of the JPanel. Once the corresponding HTML file is created (with an applet of the same dimensions as the original JFrame), you can either use appletviewer or convert the HTML file to support the Java Plug-In. See Section 9.9 (The Java Plug-In) for details on converting the HTML file.

Listing 10.3 ShapeExample.java

import javax.swing.*;  // For JPanel, etc.
import java.awt.*;   // For Graphics, etc.
import java.awt.geom.*; // For Ellipse2D, etc.

/** An example of drawing/filling shapes with Java 2D in
 * Java 1.2 and later.
 */

public class ShapeExample extends JPanel {
 private Ellipse2D.Double circle =
  new Ellipse2D.Double(10, 10, 350, 350);
 private Rectangle2D.Double square =
  new Rectangle2D.Double(10, 10, 350, 350);

 public void paintComponent(Graphics g) {
  clear(g);
  Graphics2D g2d = (Graphics2D)g;
  g2d.fill(circle);
  g2d.draw(square);
 }
 // super.paintComponent clears off screen pixmap,
 // since we're using double buffering by default.
 protected void clear(Graphics g) {
  super.paintComponent(g);
 }

 protected Ellipse2D.Double getCircle() {
  return(circle);
 }

public static void main(String[] args) {
  WindowUtilities.openInJFrame(new ShapeExample(), 380, 400);
 }
} 

Figure 10–1 An ellipse (circle) drawn with a box outline in Java 2D.

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