Singleton

In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized to systems that operate more efficiently when only one object exists, or that restrict the instantiation to a certain number of objects.



Singleton in Java recognizable by creational methods returning the same instance (usually of itself) every time

• java.lang.Runtime#getRuntime()
• java.awt.Desktop#getDesktop()
• java.lang.System#getSecurityManager()

public final class Singleton {
	private static final Singleton INSTANCE = new Singleton();

	private Singleton() {}

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

// -- OR --

public final class Singleton {
	private static volatile Singleton instance = null;

	private Singleton() {}

	public static Singleton getInstance() {
		if(instance == null) {
			synchronized(Singleton.class) {
				if(instance == null) {
					instance = new Singleton();
				}
			}
		}
		return instance;
	}
}