Graphics in Java

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The Graphics class is an abstract graphics device which is used in the Applet class Rather than define a graphic class for every type of client machine, Java defines an abstract class which the local Java system translates to the real graphics device when graphics operations occur The Graphics class is of most importance to the writing of applets, since almost all applets will modify their appearance using Graphics contexts The only exception to this is the AWT components which we have covered in this chapter, which operate in a slightly different manner 751 Using Graphics in a basic applet When using the Applet class, the Java system creates a default Graphics context and this is passed to the update() and paint() methods Here is a simple example: import javaappletApplet; import javaawtGraphics; public class simpleApplet extends Applet { public void paint(Graphics g) { gdrawRect(10,10,100,75); } } In this example a rectangle is drawn at offset (10,10) from the origin into the Graphics context We can also draw text and images, using the drawString and drawImage methods In fact all of the main graphics primitives are supported, including line, arc and polygon as well as different types of rectangle Since Graphics inherits directly from Object, there is no direct support for events Instead you must use the parent object to receive events generated from the Graphics context This is not entirely consistent with the rest of the AWT elements, but if you are following an event handling approach such as the one we outlined in this chapter, then you will not need to worry about it
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752 Double buffering It is also possible to create an offscreen Image and Graphics context and use them for screen buffering Buffering the display is useful when you have a series of discrete operations to apply to the display which consume a long period of time When applying a large number of operations the Java system will update periodically This means that the user is presented with a partially complete display Performing the operations on a display which is not shown to the user and then copying the final work to the main display leads to two positive things happening: first, it prevents the display updating before the tasks are complete and, second, stops the display flickering as the context is manipulated An applet which draws a large number of lines to the display is shown below: import javaappletApplet; import javaawt*; public class singleBuffer extends Applet { public void init() { } public void paint(Graphics g) { for (int x = 0; x < size()width; x += 2) { gdrawLine(x,0,x,size()height); } for (int y = 0; y < size()height; y += 2) { gdrawLine(0,y,size()width,y); } } } The code in the first for loop draws vertical lines which are the height of the applet display until the end of the width of the display has been met The drawLine method associated with the Graphics class has four arguments: the first two represent the start position of a line while the remaining two represent the end position of a line The second loop draws horizontal lines In this code, the paint method draws a large number of lines directly to the display On some machines (notably slower client machines) the amount of time that is taken to draw a large number of graphics operations will mean that the screen suffers from flicker as the system updates the screen On faster machines, this may not happen; however, it is wise not to make assumptions about the hardware at the client end even when you think you know who will be using the software The code for an applet which uses an offscreen Image and Graphics is displayed below: import javaappletApplet; import javaawt*; public class doubleBuffer extends Applet { Image offScrImage; Graphics offScrGr; public void init() { offScrImage = createImage(size()width,size()height); offScrGr = offScrImagegetGraphics(); } public void paint(Graphics g) { for (int x = 0; x < size()width; x += 2) { offScrGrdrawLine(x,0,x,size()height);
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} for (int y = 0; y < size()height; y += 2) { offScrGrdrawLine(0,y,size()width,y); } gdrawImage(offScrImage,0,0,this); } } In this version, the Init() method creates an Image and then gets the graphics context (also sometimes called a surface) associated with it When the paint method is called the graphics operations are performed on the offscreen context which is not subject to being displayed on the screen and coinciding with system updates When the lines have been drawn, the whole of the offscreen image is copied into the default Graphics context by the drawImage method (notice that the offscreen Graphics occupies the whole of the offscreen Image) and the display updates smoothly This approach to screen management is especially useful with animation and with tasks which generally take a long time or require a large number of screen operations
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