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CMSC 433 – Programming Language
Technologies and Paradigms
Spring 2007
Singleton Pattern
Mar. 13, 2007
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What is it?
• If you need to make sure that there can be one and
only one instance of a class.
– For example, your system can have only one window
manager or print spooler, or a singl e point of access to a
database engi ne.
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Approach 1
• Embed a static variable inside the class that we set
on the first instance and check for each time we
enter the constructor.
– A static variable is one for which there is on ly one
instance, no matter how many instances there are of the
class.
• static boolean instance_flag = false;
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Disadvantage of Approach 1
• How to find out whether creating an instance was
successful or not
– Remember constructors do not return values.
• One way would be to call a method that checks for
the success of creation, and which simply returns
some value derived from the static variable.
– This is inelegant and prone to error
• because there is nothing to keep you from creating many instances of
such non-functional classes and forgetting to check for this error
conditio n.
