The Singleton is a creational design pattern. It is used in the case where exactly one instance of an object is needed. The class keeps track of its one instance and makes sure that no other instances are created. This is accomplished with a global access point -- the constructor is protected or private, and clients can only access the class through a member function. The Singleton pattern is relatively straightforward and easy to use; however, it can lead to less modular code, and should only be used in cases where a single instance of an object in a class is actually required.
A practical use of the Singleton pattern could be for an inventory, since all transactions need to be from the same single shared instance of the inventory. The inventory in this example tracks the production and sale of widgets; at any given moment in time, the number of widgets in inventory is the number that is actually available. Here is a UML illustration of a Singleton Inventory class:
In this case, we create the Inventory class Inventory.java, and have it include a private static member variable called _instance that represents the single instance of the object. The single instance is always returned by the public get_instance function. The main and run methods in the driver file can call all of the Inventory class methods, but they cannot create a new instance of the object.
While there are multiple ways to implement the Singleton pattern in Java, this example uses a protected constructor, a static member variable containing the only instance of the inventory, and a static member function to obtain the instance. This is how the Singleton pattern is represented in the Gang of Four's original Design Patterns book. An alternative approach could be to use an enumeration to implement the inventory, since an enum is public, static, and final by default.
To run this example, first compile the source files with javac Inventory.java and javac driver.java; these create executable byte code in Inventory.class and driver.class. Then run the program with java driver. Notice that, in the run function, we must get the instance first in order to call the member functions associated with the Singleton instance of the inventory.
$ javac Inventory.java
$ javac driver.java
$ java driver
New instance of inventory created
Current Number of Items in Inventory: 0 // after instantiation
Current Number of Items in Inventory: 2 // after widgets have been added
Current Number of Items in Inventory: 1 // after one widget has been removed