Adapting in the Java API
Java has two interfaces for traversing a container—the older Enumeration interface, and the newer Iterator interface. In the book Head First Design Patterns, written by Eric & Elisabeth Freeman (along with Kathy Sierra and Bert Bates), the authors challenge you to write the following code:
- Adapt an iterator to an enumeration.
- Write a main method that uses this new adapted enumeration to traverse an
ArrayList. (Remember, the ArrayList class doesn't support enumerations.)
Listings 9 and 10 contain a version of the iterator adapter and the main method.
Like the DeviceAdapter, the IteratorAdapter implements the adaptee's interface (the Enumeration interface). The IteratorAdapter stores an instance of Iterator, and uses this instance to call the hasNext() method in the implementation of hasMoreElements(). The main method (Listing 10) creates an ArrayList to hold some strings. The ArrayList class doesn't support the Enumeration interface. Even so, Listing 10 uses an enumeration to traverse an ArrayList.
The following code displays the output of the iterator adapter test application:
-- Iterator Adapter Test Application --
Enumerating Through the JavaBoutique Articles
Written by Barry Burd and Michael Redlich
1: James: The Java Apache Mail Enterprise Server, September 30, 2005
2: Avoid Excessive Subclassing with the Decorator Design Pattern, January 27, 2006
3: Keeping Your Java Objects Informed with the Observer Design Pattern, June 19, 2006
4: Manufacturing Java Objects with the Factory Design Pattern, August 14, 2006
Object vs. Class Adapters
There are actually two ways to implement the Adapter design pattern. The examples in this article demonstrate the use of the Adapter design pattern as an object adapter. The object adapter uses composition to accomplish its goal.
The class adapter is similar. But the class adapter uses inheritance (more specifically, multiple inheritance) instead of composition. Java doesn't support multiple inheritance, so you can't implement a class adapter in Java. (To implement a class adapter, you must dirty your hands with C++.)
Resistance is Futile
For the Borg on Star Trek, and for application software, assimilation is crucial. The adapting of one system to another system's interface is an important tool in the development process. If the Borg's computers run Java (and we're certain that they do) then the computers connect with other species using the Adapter design pattern. The pattern provides an elegant, orderly procedure for making one system's interface match another system's expectations.
Resources
About the Authors
Barry Burd is a professor in the Department of Mathematics and Computer Science at Drew University in Madison, New Jersey. When he's not lecturing at Drew University, Dr. Burd leads training courses for professional programmers in business and industry. He has lectured at conferences in America, Europe, Australia and Asia. He is the author of several articles and books, including "Java 2 For Dummies" and "Eclipse For Dummies," both published by Wiley.
Michael P. Redlich is a Senior Research Technician (formerly a Systems Analyst) at ExxonMobil Research & Engineering, Co. in Clinton, New Jersey with extensive experience in developing custom web and scientific laboratory applications. Mike also has experience as a Technical Support Engineer for Ai-Logix, Inc. where he developed computer telephony applications. He holds a Bachelor of Science in Computer Science from Rutgers University. In his spare time, Mike facilitates the ACGNJ Java Users Group and serves as ACGNJ Secretary. Mike has co-written several articles for Java Boutique, and his computing experience includes computer security, relational database design and development, object-oriented design and analysis, C/C++, Java, Visual Basic, FORTRAN, Pascal, MATLAB, HTML, XML, ASP, VBScript, and JavaScript in both the PC and UNIX environments.
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