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The code in the main method instantiates an object of the GUI class. The code showing in the GUI class definition declares several instance variables, initializing some of them.
Beginning of the constructor for the GUI class
Listing 2 shows the beginning of the constructor for the GUI class.
| Listing 2 . Beginning of the constructor for the GUI class. |
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GUI(){//constructor
//Set JFrame size, title, and close operation.setSize(hSize,vSize);
setTitle("Copyright 2008,R.G.Baldwin");setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);//Create a new drawing canvas and add it to the
// center of the JFrame.myCanvas = new MyCanvas();
this.getContentPane().add(myCanvas);//This object must be visible before you can get an// off-screen image. It must also be visible before
// you can compute the size of the canvas.setVisible(true);
osiWidth = myCanvas.getWidth()/2;osiHeight = myCanvas.getHeight(); |
The code in Listing 2 is straightforward and shouldn't require an explanation beyond the embedded comments.
Create two off-screen images
Here is where things begin to get interesting. Listing 3 creates two off-screen images and gets a graphics context on each of them.
| Listing 3 . Create two off-screen images. |
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osiA = createImage(osiWidth,osiHeight);
Graphics2D g2Da = (Graphics2D)(osiA.getGraphics());osiB = createImage(osiWidth,osiHeight);Graphics2D g2Db = (Graphics2D)(osiB.getGraphics()); |
Java programs are not constrained to simply draw on the screen. It is possible to create an off-screen image, get a graphics context on it,and use methods of the Graphics and/or Graphics2D classes to draw on the off-screen image.
The off-screen image can then be used for a variety of purposes including:
Will copy off-screen images to the screen
In this program, we will copy each of the off-screen images to a Canvas object in a side-by-side format (see Figure 1 ) and display the Canvas object on the content pane in a JFrame object.
While animation is not the purpose of using off-screen images in this program, such off-screen images are often used to produce clean animation. Code isexecuted to produce an off-screen image and then the entire image is copied to the screen in a single blast. This prevents the user from seeing thedevelopment of the image that might otherwise be visible if the code was executed to produce the image directly onto the screen.
The createImage method
There are seven different versions of the createImage method in the JDK 1.7 standard library. The version of the method used in this programis defined in the Component class and inherited into the JFrame class. According to the Sun documentation, this method creates anoff-screen drawable image to be used for double buffering.
The two required parameters are width and height. If the image is later displayed on the screen, the units of width and height are measured in pixels.The values contained in the width and height parameters in Listing 3 were computed in Listing 2 . The width of each off-screen image is set to half the width of the canvas. The height of each off-screen image is set to theheight of the canvas. This facilitates the display of the two images in the side-by-side format shown in Figure 1 .
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