14.7. Key Terms

[Page 438]

13.9. The Polygon Class and Drawing Polygons and Polylines

The Polygon class encapsulates a description of a closed two-dimensional region within a coordinate space. This region is bounded by an arbitrary number of line segments, each of which is one side (or edge) of the polygon. Internally, a polygon comprises a list of (x, y) coordinate pairs in which each pair defines a vertex of the polygon, and two successive pairs are the endpoints of a line that is a side of the polygon. The first and final pairs of (x, y) points are joined by a line segment that closes the polygon.

The two constructors given below are used to create a Polygon object.

• public Polygon()

  Constructs an empty polygon.

• public Polygon( int [] xpoints, int [] ypoints, int npoints)

  Constructs and initializes a Polygon with specified points. Parameters xpoints and ypoints are arrays representing x -coordinates and y -coordinates, and npoints indicates the number of points.

To append a point to the polygon, use the addPoint(int x, int y) method. The Polygon class has the public data fields xpoints , ypoints , and npoints , which represent the array of x -coordinates and y -coordinates, and the total number of points.

Here is an example of creating a polygon and adding points into it:

 Polygon polygon =   new   Polygon(); polygon.addPoint(   40   ,   20   ); polygon.addPoint(   70   ,   40   ); polygon.addPoint(   60   ,   80   ); polygon.addPoint(   45   ,   45   ); polygon.addPoint(   20   ,   60   ); 

To draw or fill a polygon, use one of the following methods :

 drawPolygon(Polygon polygon); fillPolygon(Polygon polygon); drawPolygon(   int   [] xpoints,   int   [] ypoints,   int   npoints); fillPolygon(   int   [] xpoints,   int   [] ypoints,   int   npoints); 

For example:

 int x[] = {   40   ,   70   ,   60   ,   45   ,   20   }; int y[] = {   20   ,   40   ,   80   ,   45   ,   60   }; g.drawPolygon(x, y, x.length); 

---

[Page 439]

The drawing method opens the polygon by drawing lines between point (x[i], y[i]) and point (x[i+1], y[i+1]) for i = 0, ... , x.length-1 ; it closes the polygon by drawing a line between the first and last points (see Figure 13.14(a)).

Figure 13.14. The drawPolygon method draws a polygon, and the polyLine method draws a polyline.

To draw a polyline, use the drawPolyline(int[] x, int[] y, int nPoints) method, which draws a sequence of connected lines defined by arrays of x and y coordinates. For example, the following code draws the polyline, as shown in Figure 13.14(b):

 int x[] = {   40   ,   70   ,   60   ,   45   ,   20   }; int y[] = {   20   ,   40   ,   80   ,   45   ,   60   }; g.drawPolygon(x, y, x.length); 

Listing 13.7 is an example of how to draw a hexagon, with the output shown in Figure 13.15.

Figure 13.15. The program uses the drawPolygon method to draw a polygon.

Listing 13.7. DrawPolygon.java

(This item is displayed on pages 439 - 440 in the print version)

 1   import   javax.swing.JFrame; 2   import   javax.swing.JPanel; 3   import   java.awt.Graphics; 4   import   java.awt.Polygon; 5 6   public class   DrawPolygon   extends   JFrame { 7   public   DrawPolygon() { 8 setTitle(   "DrawPolygon"   ); 9 add(   new   PolygonsPanel()); 10 } 11 12  /** Main method */  13   public static void   main(String[] args) { 14 DrawPolygon frame =   new   DrawPolygon(); 

---

[Page 440]

 15 frame.setLocationRelativeTo(   null   );  // Center the frame  16 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 17 frame.setSize(   200   ,   250   ); 18 frame.setVisible(   true   ); 19 } 20 } 21 22  // Draw a polygon in the panel  23   class   PolygonsPanel   extends   JPanel { 24    protected void   paintComponent(Graphics g)  { 25    super   .paintComponent(g);  26 27   int   xCenter = getWidth() /   2   ; 28   int   yCenter = getHeight() /   2   ; 29   int   radius = (   int   )(Math.min(getWidth(), getHeight()) *     .   4   ); 30 31  // Create a Polygon object  32 Polygon polygon =   new   Polygon(); 33 34  // Add points to the polygon  35  polygon.addPoint)  (xCenter + radius, yCenter); 36  polygon.addPoint)  ((   int   )(xCenter + radius * 37 Math.cos(   2   * Math.PI /   6   )), (   int   )(yCenter “ radius * 38 Math.sin(   2   * Math.PI /   6   ))); 39 polygon.addPoint((   int   )(xCenter + radius * 40 Math.cos(   2   *   2   * Math.PI /   6   )), (   int   )(yCenter “ radius * 41 Math.sin(   2   *   2   * Math.PI /   6   ))); 42 polygon.addPoint((   int   )(xCenter + radius * 43 Math.cos(   3   *   2   * Math.PI /   6   )), (   int   )(yCenter “ radius * 44 Math.sin(   3   *   2   * Math.PI /   6   ))); 45 polygon.addPoint((   int   )(xCenter + radius * 46 Math.cos(   4   *   2   * Math.PI /   6   )), (   int   )(yCenter “ radius * 47 Math.sin(   4   *   2   * Math.PI /   6   ))); 48 polygon.addPoint((   int   )(xCenter + radius * 49 Math.cos(   5   *   2   * Math.PI /   6   )), (   int   )(yCenter “ radius * 50 Math.sin(   5   *   2   * Math.PI /   6   ))); 51 52  // Draw the polygon  53  g.drawPolygon(polygon);  54 } 55 }