Platinum Edition Using HTML 4, XML, and Java 1.2
(Publisher: Macmillan Computer Publishing)
Author(s): Eric Ladd
ISBN: 078971759x
Publication Date: 11/01/98

You pass the coordinates of the upper-left corner of the imaginary surrounding rectangle and the width and height of the oval to drawOval() or fillOval(). The width and height are equal to the width and height of the imaginary surrounding rectangle. The Ovals applet in Listing 39.5 draws a circle and a filled oval. Figure 39.8 shows the output from the Ovals applet.

Listing 39.5 Ovals.java—You Can Use the Oval Methods to Draw a Circle

import java.awt.Graphics;

// This applet draws an unfilled circle and a filled oval


public class Ovals extends java.applet.Applet
{
   public void paint(Graphics g)
   {
     // Draw a circle with a diameter of 150 (width=150, height=150)
     // With the enclosing rectangle’s upper-left corner at (20, 20)
     g.drawOval(20, 20, 150, 150);
     // Fill an oval with a width of 150 and a height of 80
     // The upper-left corner of the enclosing rectangle is at (200, 20)
     g.fillOval(200, 20, 150, 80);
   }
}

FIGURE 39.8  Draw ovals and circles by using the Graphics class drawOval() and fillOval() methods.

Drawing Arcs

An arc is a segment of the line that forms the perimeter of an oval, as demonstrated in Figure 39.9. This output is generated by the ArcDemo1 applet shown in Listing 39.6.

Listing 39.6 ArcDemo1.java—Think of an Arc as a Wedge of an Oval

import java.awt.Graphics;
import java.awt.Color;

public class ArcDemo1 extends java.applet.Applet
{
   public void paint(Graphics g)
   {
     // Draw a gray oval 200 pixels wide and 100 pixels
     //  high with the upper left corner of its
     //  enclosing rectangle at (20, 20).
     g.setColor(Color.gray);
     g.drawOval(20, 20, 200, 100);
     // Draw a black arc from 0 to 90 degrees inside
     //  the oval created above.
     g.setColor(Color.black);
     g.drawArc(20, 20, 200, 100, 0, 90);
     // Draw the same arc as above but to the right
     //  with the upper left corner of its enclosing
     //  rectangle at (240, 20).
     g.drawArc(240, 20, 200, 100, 0, 90);
   }
}

FIGURE 39.9  At the left are the arc and its associated oval, and at the right is the arc alone.

Two Graphics class methods are provided for drawing arcs: the drawArc() and fillArc() methods. Their complete definitions are as follows:

public abstract void drawArc(int  x, int  y, int  width,
  ⇒int  height, int  startAngle, int  arcAngle)

public abstract void fillArc(int  x, int  y, int  width,
 ⇒int  height, int  startAngle, int  arcAngle)

Use the first four parameters just as you did with the oval methods. In fact, you are drawing an invisible oval and the arc is a segment of the oval’s perimeter defined by startAngle and arcAngle, the last two parameters.

The startAngle parameter defines where your arc starts along the invisible oval’s perimeter. In Java, angles are set around a 360° circle as follows:

•  0° is at 3 o’clock.

•  90° is at 12 o’clock.

•  180° is at 9 o’clock.

•  270° is at 6 o’clock.

The arcAngle parameter defines the distance, in degrees, that your arc traverses along the invisible oval’s perimeter. Angles are positive in the counterclockwise direction and negative in the clockwise direction.

The arc you saw in Figure 39.9 began at 0°, or at 3 o’clock, and traversed the invisible oval 90° in the positive, or counterclockwise, direction. The relevant line in Listing 39.6 is reproduced here:

g.drawArc(20, 20, 200, 100, 0, 90);

Notice that the last parameter is given in the angle traversed and not the angle at which the arc ends. Therefore, if you want an arc that starts at 45° and ends at 135°, you must provide a startAngle parameter value of 45° and an arcAngle parameter value of 90°.

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NOTE:  When you use a negative arcAngle parameter value, the arc sweeps clockwise along the invisible oval’s perimeter.

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Using the fillArc() method results in a filled, pie-shaped wedge defined by the center of the invisible oval and the perimeter segment traversed by the arc.

Drawing Polygons

The Java Graphics class provides four methods for building polygons: two versions of the drawPolygon() method and two versions of the fillPolygon() method. Each has two methods so that you can either pass two arrays containing the x , y coordinate of the points in the polygon, or you can pass an instance of a Polygon class. The Polygon class is defined in the java.awt package.

Making a Polygon by Passing Coordinate Arrays First look at how to make a polygon by using two arrays. The full definitions of the drawPolygon() and fillPolygon() methods for using arrays are

public abstract void drawPolygon(int  xPoints[], int yPoints[],
 ⇒int  nPoints)

public abstract void fillPolygon(int  xPoints[], int  yPoints[],
 ⇒int  nPoints)

The DrawPoly applet in Listing 39.7 draws a polygon using an array of x coordinates (xCoords) and an array of y coordinates (yCoords). Each x,y pair, the first x (50) and the first y (100) pair, for instance, defines a point on a plane (50, 100). Use the drawPolygon method to connect each point to the following point in the list. The first pair is (50, 100) and connects by a line to the second pair (200, 0), and so on. The drawPolygon method’s third parameter, nPoints, is the number of points in the polygon and should equal the number of pairs in the x and y arrays. Figure 39.10 shows the DrawPoly applet’s output.

Listing 39.7 DrawPoly.java—This Applet Draws Polygons Based on Arrays of Coordinates

import java.awt.Graphics;

public class DrawPoly extends java.applet.Applet
{
   int xCoords[] = { 50, 200, 300, 150, 50 };
   int yCoords[] = { 100, 0, 50, 300, 200 };

   int xFillCoords[] = { 450, 600, 700, 550, 450 };

   public void paint(Graphics g)
   {
     // Draw the left polygon.
     g.drawPolygon(xCoords, yCoords, 5);
     // Draw the right filled polygon.
     g.fillPolygon(xFillCoords, yCoords, 5);
   }
}

	The applets in this chapter assume that you have a graphics resolution of at least 800 pixels across by 600 pixels top to bottom (800×600). If your monitor displays a smaller number of pixels (640×480, for example), you can either use your browser’s scroll bars to see the rest of the applet window, or you can change some of the coordinates in the example so that they are no larger than your largest screen coordinates.