CSE Lecture 25/Jan/2008

Singleton Class

Before, we had utility classes, no instances were allowed.

Singleton class -> only one object is allowed to be instantiated.

Good for:

- Things that need to be globally constant, e.g. configuration and system status details.
- The key is the access specifier of the constructor: it must be private.

Object creation will take place within the class ---> created instances will be returned using a method other than the constructor...e.g getInstance()

Attempt 1
- Use static features to count # of instances created -> instanceCount variable.
- getInstance() checks instanceCount.
- If 0, create an instance and return it, set instance to 1
- If 1, return null.

Better way
- get Instance returns the single instance whenever it is called.
- class has to keep track of the object reference - via a private static variable.

Attempt 2a
- getInstance() checks instanceCount
-If 0, creates a new instance and saves it to a private static attribute and increments instanceCount
-returns the reference to the object.

Attempt 2b
-Don't need an instance counter
-Initialize instance when declared.

-This attempt is a design pattern for singleton classes.

Singleton Design Pattern

-private static instance holder --> creates and holds the only instance
-public static getInstance() -> returns instance
-private constructor -> prevents instances from being created by the client.
-no mutator for the instance holder, instance is final.