Reviews : Java Books :
Beginning Java Objects : Chapters 2 and 3

Buy this book Title: Beginning Java Objects ISBN: 1861004176 US Price: $ 39.99 Canadian Price: C$ 59.95 UK Price: £ 28.99 Publication Date: November 2000 Pages: 800 © Wrox Press Limited, US and UK.

Memory allocation using handles might look something like this conceptually:

With this approach, each object is allocated in memory only once; the Student object knows how to find and communicate with its advisor (Professor) object whenever it needs to through its handle, and vice versa.

What do we gain by defining the Student's advisor attribute as a reference to a Professor object, instead of merely storing the name of the advisor as a String attribute of the Student object?

• For one thing, we can ask the Professor object its name whenever we need it by invoking its getName method. Why is this important? To avoid data redundancy and the potential for loss of data integrity. If the Professor object's name changes for some reason, the name will only be stored in one place: encapsulated as an attribute within the Professor object that 'owns' the name, which is precisely where it belongs. If we instead were to store the Professor's name both as a String attribute of the Professor object and as a String attribute of the Student object, we'd have to remember to update the name in two places any time the name changed (or three, or four, or however many places this Professor's name is referenced as an advisor of countless Students). If we were to forget to do so, then the name of the Professor would be 'out of synch' from one instance to another.
• Just as importantly, by maintaining a handle on the Professor object via the advisor attribute of Student, the Student object can also request other services of this Professor object via whatever methods are defined for the Professor class. A Student object may, for example, ask its advisor (Professor) object where the Professor's office is located, or what classes the Professor is teaching so that the Student can sign up for one of them.
• Another advantage of using object handles from an implementation standpoint is that they also reduce memory overhead. Storing a handle to an object only requires 32 or 64 bits of memory, depending on the programming language, instead of however many bytes of storage the referenced object as a whole occupies in memory. If we were to have to make a copy of an entire object every place we needed to refer to it in our application, we'd quickly exhaust available memory.
  
  
  
  
  
  

  Applet Index (sorted alphabetically) A B C D E F G H I J K L M N O P Q R S T U V W X Y Z #s

  The Java Source (applets w/source code) A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

  How to Add Java Applets to Your Site

  New on the Java Boutique:

  New Review:
  

  Time Management Made Easy with the Quartz Enterprise Job Scheduler
  Why not just use the Java timer API? This open source scheduling API boasts simplicity, ease-of-integration, a well-rounded feature set, and it's free!

  New Applet:
  

  Reverse Complement
  Reverse Complement is a simple applet that converts DNA or RNA sequences into three useful formats.

  Elsewhere on internet.com:

  WebDeveloper Java
  Lots of Java information on webdeveloper.com

  WDVL Java
  Thorough Java resource at the Web Developer's Virtual Library.

  ScriptSearch Java
  Hundreds of free Java code files to download.

  jGuru: Your View of the Java Universe
  Customizable portal with online training, FAQs, regular news updates, and tutorials.