Demonstration of the Singleton design pattern in Java.

Singleton.java

package de.florianwolters.pattern.singleton;

/**
 * Demonstration of the <i>Singleton</i> design pattern in Java.
 *
 * This is the correct implementation of the <i>Singleton</i> creational design
 * pattern in the Java programming language.
 *
 * - Thread-safe without special language constructs (e.g.
 *   <tt>synchronized</tt> or <tt>volatile</tt>).
 * - Prevents cloning of an instance via the <tt>Object.clone()</tt> method.
 *
 * @author Florian Wolters <[email protected]>
 */
final public class Singleton {
    // Declaring the class as "final" is required since one subclass would lead
    // to unexpected behaviour.

    /**
     * Returns the <i>Singleton</i> instance of this class.
     *
     * @return The <i>Singleton</i> instance.
     */
    public static Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }

    /**
     * Holds the <i>Singleton</i> instance of the class {@link Singleton}.
     *
     * The class {@link SingletonHolder} is loaded on the first execution of the
     * method {@link Singleton.getInstance()} or the first access to the
     * property {@link SingletonHolder.INSTANCE}.
     */
    private static class SingletonHolder {
        /**
         * The <i>Singleton</i> instance of the class {@link Singleton}.
         */
        private static final Singleton INSTANCE = new Singleton();

        /**
         * Private constructor.
         */
        private SingletonHolder() {
        }
    }

    /**
     * Throws a CloneNotSupportedException to prevent cloning of an instance of
     * this class.
     *
     * @return Never. Always throws a CloneNotSupportedException.
     * @throws CloneNotSupportedException Always.
     */
    @Override
    final public Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }

    /**
     * Protected constructor to prevent creating a new instance of this class
     * via the <tt>new</tt> operator.
     */
    protected Singleton() {
        // The constructor could also be declared with the visibility "private".
    }

}