Solution for http://zeroturnaround.com/fun/magical-java-puzzle-pat-the-unicorns/

MagicalLand.java

import java.util.*;

public class MagicalLand {
  public static void main(String[] args) {
		for (int i = 0; i < (Math.random() * 500) + 2; i++) {
			if (Unicorn.pat()) {
				System.out.println("UNICORN #1: PAT THIS UNICORN ONCE");
			}
		}
 
		for (int i = 0; i < (Math.random() * 500) + 2; i++) {
			if (Unicorn.pat()) {
				System.out.println("UNICORN #2: PAT THIS UNICORN ONCE");
			}
		}
		System.out.println("END OF PROGRAM");
	}

	private static class Unicorn {
			
		private static Set<List<StackTraceElement>> pats = new HashSet<List<StackTraceElement>>();
		
		public static boolean pat() {
			Throwable t = new Throwable();
			StackTraceElement[] stackTrace = t.getStackTrace();
			List<StackTraceElement> trace = Arrays.asList(stackTrace);
			if(pats.contains(trace)) {
				return false;
			}
			pats.add(trace);
			return true;
		}
	}
}