Design Patterns: Creational - Singleton

Singleton Pattern:

• a class manages the creation of its one and only instance through a static variable
• no Public constructors, constructors are declared as Private
• a Singleton object is not instantiated using the new() operator but calling a static method to request for an instance
• may implement "lazy instantiation", if an instance is not needed then it doesn't get created
• "eager instantiation" may be implemented if the overhead of creation and the runtime aspects of the Singleton is not burdensome
• Singletons are meant to be used sparingly

Lazy Instantiation:

in Java:

public class Singleton {
private static Singleton uniqueInstance;

private Singleton() {}

public static Singleton getInstance() {
if (uniqueInstance == null) {
uniqueInstance = new Singleton();
}
return uniqueInstance;
}
}

Eager Instantiation:

in C#:

public sealed class Singleton
{
static readonly Singleton uniqueInstance = new Singleton();

Singleton() {}

public static Singleton getInstance {
get { return uniqueInstance; }
}
}

in Java:

public class Singleton {
private static Singleton uniqueInstance = new Singleton();

private Singleton() {}

public static Singleton getInstance() {
return uniqueInstance;
}
}