What is Singleton Category: Creational pattern A global object (application-wide visibility) Benefits of static methods + Benefits of objects Concealed object construction process Restrict object creation, and control in a single location

How to use Singleton Implementation Details Usage Details Private constructor ‘getInstance()’ static method ‘instance’ class variable Usage Details Get object via ‘getInstance()’ method instead of constructor.

How to use Singleton Traditional Objects A Singleton Object Implementation public class MyClass { public MyClass() { } public void printMessage(String msg) { System.out.println(message); Usage // create object MyClass m = new MyClass(); // use object m.printMessage(“Hello world!”); A Singleton Object Implementation public class MyClass { private static final MyClass instance = new MyClass(); private MyClass() { } public static MyClass getInstance() { return instance; public void printMessage(String msg) { System.out.println(message); Usage MyClass m = MyClass.getInstance(); m.printMessage(“Hello World”);

From (Stephen Stelting, Olav Maassen, Applied Java™ Patterns) Class Diagram From (Stephen Stelting, Olav Maassen, Applied Java™ Patterns)

Why Singleton … instead of single objects Functionality can be common across application Data can be common across application Actual instance type does not have to be known by developer … instead of static fields/methods Pre-instantiation preparation can be performed (such as constructor code) Access to object-oriented techniques (inheritance, polymorphism, design patterns!) Control of amount and type of instances provided to class users.

When to use Singleton Class functionality is useful across application Different application components share common data sets Memory usage is at a premium Communication must occur between many portable components

In the Real World Singletons often serve as common locations for these application services: Logging Data caching/storage Application properties/preferences