Software Engineering for Students A Programming Approach

12.5 Factory method 155
The following coding shows how a singleton class Demo can be written in Java.
public class Demo {
}
private static final Demo demo = new Demo();
private Demo() {
}
private static Demo getInstance() {
return demo;
}
SELF-TEST QUESTION
12.2 Why is the constructor Demo labeled as private?
12.5 ● Factory method
Suppose that we are writing some software to handle images. An image resides in a file
and there are several different common file formats – for example, jpeg and gif. We would
like the facility to create an object corresponding to a graphics image, in this manner:
Image image = new Image("picture.jpeg");
Once we have created the object, we can perform such operations as:
image.rotate(90);
image.display();
image.enlarge(2, 2);
The problem is that all of these methods will be different for each different graphics
file format. Indeed, the graphics object will be different for each file format. So one single
class – Image – will not suffice; we need a different class for each file format. One
approach would be to provide a number of classes and expect a user to call the constructor
for the relevant class like this:
JpegImage image = new JpegImage("fileName.jpeg");
But this is clumsy. The alternative is to create an abstract class Image that has a factory
method createImage. Now we create an image object conveniently like this:
Image image = Image.createImage("fileName.jpeg");
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