Singleton pattern example

Singleton

	/**
	 * 
	 * thread-safe
	 * 
	 */
	public class Singleton {
		private static final Singleton INSTANCE = new Singleton();

		private Singleton() {
		}

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

	/**
	 * 
	 * lazy init non-thread-safe
	 * 
	 */
	public class Singleton {
		private static Singleton sInstance;

		private Singleton() {
		}

		public static Singleton getInstance() {
			if (sInstance == null) {
				sInstance = new Singleton();
			}
			return sInstance;
		}
	}

	/**
	 * 
	 * lazy init thread-safe low performance
	 * 
	 */
	public class Singleton {
		private static Singleton sInstance;

		private Singleton() {
		}

		public synchronized static Singleton getInstance() {
			if (sInstance == null) {
				sInstance = new Singleton();
			}
			return sInstance;
		}
	}

	/**
	 * 
	 * lazy init & thread-safe (Java 5 or later)
	 * 
	 */
	public class Singleton {
		private volatile static Singleton sInstance;

		private Singleton() {
		}

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

	
	/**
	 * 
	 * lazy init & thread-safe (for all Java version)
	 *
	 */
	public class Singleton {
		private Singleton() {
		}

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

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