Editing Singleton pattern (section)

=== Lazy initialization ===

A singleton implementation may use [[lazy initialization]] in which the instance is created when the static method is first invoked. In [[Multithreading (software)|multithreaded]] programs, this can cause [[Race condition|race conditions]] that result in the creation of multiple instances. The following [[Java_version_history#Java_5|Java 5+]] example<ref>{{Cite book|author=Eric Freeman, Elisabeth Freeman, Kathy Sierra, and Bert Bates|title=Head First Design Patterns|publisher=O'Reilly Media, Inc|date=October 2004|edition=First|chapter=5: One of a Kind Objects: The Singleton Pattern|page=182|isbn=978-0-596-00712-6|chapter-url=https://books.google.com/books?id=GGpXN9SMELMC&pg=PA182}}</ref> is a [[Thread safety|thread-safe]] implementation, using lazy initialization with [[double-checked locking]].

<syntaxhighlight lang="java">
public 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;
    }
}
</syntaxhighlight>