Software Engineering for Students A Programming Approach

156 Chapter 12 ■ Design patterns
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createImage is class (static) method of the class Image. It looks at the extension of
the file name parameter and creates an appropriate object. For example, if the file type
is jpeg, it creates a subclass of Image suitable for jpeg images. The sole purpose of this
method is to create the appropriate object, making the choice at run time. So now users
can treat all image file types in the same manner. The code for the factory method is:
class Image {
}
public static Image createImage(String fileName) {
String extension = getExtension(fileName);
if (extension.equals("jpeg"))
return (new JpegImage(fileName));
if (extension.equals("gif"))
return (new GifImage(fileName));
}
Here we have used a method getExtension (not shown) that returns the extension
part of a file name.
We have buried the code that creates an appropriate class within the factory method,
providing a simple interface to the users.
Why could we not simply use the constructor method of class Image? The answer is
that a constructor can only return an object of its own class. This is a limitation of constructors
and, if we need to do anything different, we need to use a factory method.
12.6 ● Façade
Suppose you write a group of classes that perform some useful functions. It could be a
filing system that allows other classes (users) to open a file, close a file, read and write
information. One option is to tell users which classes to use and how (Figure 12.3, lefthand
diagram). However, this means that a user needs to understand the structure of
the subsystem to use it effectively. In addition, changes to the structure of the subsystem
may require changes to its users.
A better option (Figure 12.3, right-hand diagram) is to tell users to use one class –
a façade class. The façade class presents a clean and simple interface for users. It has
Figure 12.3 The Façade pattern
User
User
Façade
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