Singleton5.java

public class Singleton5 {
	private static volatile Singleton5 INSTANCE = null;
	
	private Singleton5(){
		if (null != INSTANCE){
			throw new IllegalArgumentException("Already instantiated.");
		}
	}
	
	public static Singleton5 getInstance(){
		if (null == INSTANCE){
			synchronized (Singleton5.class) {
				if (null == INSTANCE){
					INSTANCE = new Singleton5();
				}
			}
		}
		return INSTANCE;
	}
}