1. 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

2. 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.

3. 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”);

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

5. 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.

6. 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

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