Singleton

public class SingletonMain {
	public static void main(String []argv) {
		EnumSingleton es = EnumSingleton.INSTANCE;
		ClassSingleton cs = ClassSingleton.getInstance();
	}
}

enum EnumSingleton {
	INSTANCE;

	public void exec() {}
}

class ClassSingleton {
	private static ClassSingleton INSTANCE = null;

	private ClassSingleton(){}

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