16.1.1 Drawing Methods

Methods of the Graphics class operate in a standard coordinate system. The origin of a newly created graphics context is the top left pixel of the component's drawing area, as shown in Figure 16.2, which illustrates the default coordinate system. The point (0,0) is at the top left corner of the drawing area; the point (width, height) is just outside the drawing area at the bottom right corner. The point at the bottom right corner within the drawing area has coordinates (width-1, height-1). This gives you a drawing area that is width pixels wide and height pixels high.

Figure 16.2: Graphics coordinate system

The coordinate system can be translated (shifted) with the translate() method to specify a new point as the origin. The drawable area of the graphics context can be limited to a region with the setClip() method.

The basic drawing and painting methods should seem familiar to you if you've done any graphics programming. The following applet, TestPattern, exercises most of the simple shape drawing commands; it's shown in Figure 16.3.

Figure 16.3: The TestPattern applet

import java.awt.*;
import java.awt.event.*;
public class TestPattern extends java.applet.Applet { 
    int theta = 45;
    public void paint( Graphics g ) {
        int Width = size().width;
        int Height = size().height;
        int width = Width/2;
        int height = Height/2;
        int x = (Width - width)/2;
        int y = (Height- height)/2;
        int [] polyx =  { 0, Width/2, Width, Width/2 };
        int [] polyy =  { Height/2, 0, Height/2, Height };
        Polygon poly = new Polygon( polyx, polyy, 4 );
        
        g.setColor( Color.black );
        g.fillRect( 0, 0, size().width, size().height );
        g.setColor( Color.yellow );
        g.fillPolygon( poly );
        g.setColor( Color.red );
        g.fillRect( x, y, width, height );
        g.setColor( Color.green );
        g.fillOval( x, y, width, height );
        g.setColor( Color.blue );
        int delta = 90;
        g.fillArc( x, y, width, height, theta, delta );
        g.setColor( Color.white );
        g.drawLine( x, y, x+width, x+height );
    }
    public void init() {
        addMouseListener( new MouseAdapter() {
            public void mousePressed( MouseEvent e ) {
                theta = (theta + 10) % 360;
                repaint();
            }
        } );
    }
}

TestPattern draws a number of simple shapes and responds to mouse clicks by rotating the filled arc and repainting. Compile it and give it a try. If you click repeatedly on the applet, you may notice that everything flashes when it repaints. TestPattern is not very intelligent about redrawing; we'll examine some better algorithms in the upcoming section on drawing techniques.

With the exception of fillArc() and fillPolygon(), each method takes a simple x, y coordinate for the top left corner of the shape and a width and height for its size. We have picked values that draw the shapes centered, at half the width and height of the applet.

The most interesting shape we have drawn is the Polygon, a yellow diamond. A Polygon object is specified by two arrays that contain the x and y coordinates of each vertex. In our example, the coordinates of the points in the polygon are (polyx[0], polyy[0]), (polyx[1], polyy[1]), and so on. There are simple drawing methods in the Graphics class that take two arrays and draw or fill the polygon, but we chose to create a Polygon object and draw it instead. The reason is that the Polygon object has some useful utility methods that we might want to use later. A Polygon can, for instance, give you its bounding box and tell you if a given point lies within its area.

AWT also provides a Shape interface, which is implemented by different kinds of 2D objects. Currently, it is implemented only by the Rectangle and Polygon classes, but it may be a sign of things to come, particularly when JavaSoft releases the extended 2D drawing package. The setClip() method of the Graphics class takes a Shape as an argument, but for the time being, it works only if that Shape is a Rectangle.

The fillArc() method requires six integer arguments. The first four specify the bounding box for an oval--just like the fillOval() method. The final two arguments specify what portion of the oval we want to draw, as a starting angle and an offset. Both the starting angle and the offset are specified in degrees. Zero degrees is at three o'clock; a positive angle is clockwise. For example, to draw the right half of a circle, you might call:

g.fillArc(0, 0, radius * 2, radius * 2, -90, 180); 

Table 16.1 shows the shape-drawing methods of the Graphics class. As you can see, for each of the fill() methods in the example, there is a corresponding draw() method that renders the shape as an unfilled line drawing.

draw3DRect() automatically chooses colors by "darkening" the current color. So you should set the color to something other than black, which is the default (maybe gray or white); if you don't, you'll just get black on both sides. For an example, see the PictureButton in Chapter 14.

A few important drawing methods are missing from Table 16.1: for example, the drawString() method, which draws text, and the drawImage() method, which draws an image. We'll discuss these methods in detail in later sections.