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Singletons rule

nerd

发贴: 42
积分: 0

于 2003-05-27 20:58

发信人: NullPointer (空指针·莫之夭阏), 信区: JavaClub
标题: Singletons rule
发信站: BBS 水木清华站 (Fri Nov 8 20:10:28 2002), 转信

Singletons rule
Keep on the object-oriented track with singletons

By Tony Sintes

Q: I have seen singleton classes used in many places. My question: What is the
advantage of using a singleton over a class with static methods?

A: The difference between using a singleton over a class with static methods
boils down to effective object-oriented design. Singletons normally represent
a cleaner approach. A class of static methods, unfortunately, breaks down to
a simple list of functions, or utilities.

Utilities

You may ask, what's the problem with a list of functions?

Simple. With a list of functions you no longer perform object-oriented
programming. In fact, your work easily devolves into a classic procedural
program. Suddenly, objects no longer represent the focus of your program. You
begin to fall into a
data-centric programming mode. That is, instead of sending messages to
objects that encapsulate state and behavior, you begin to call functions that
act on data. Utilities lead to a clear division between behavior and data
that you should never allow
in an object-oriented design.

That's not to say that you should never use utilities. Object-oriented
utilities do exist. There are times that you might need to treat objects as
data. For example, I recently wrote a set of utilities that convert Java
objects into a CORBA equivalent.
It is difficult to turn those methods into a class since they really have no
state. I also didn't want to embed the conversion code into the Java classes
either. However, it is easy to mess it all up.

Singletons
Use singletons when you want only one instance of a certain class in your
system at any given time. An example could be a class that provides
load-balanced access to CORBA servers. You wouldn't want two such objects,

since each would have to get
references to the same servers. Grabbing redundant resources would be
wasteful. It is better to centralize the server access inside one instance.

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