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

unicorns.java

import org.junit.Test;

import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class Unicorns {
    @Test
    public void patUnicorns() {
        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");
    }

    public static class Unicorn {
        /** The set of stack traces that we've seen */
        private static Set<List<StackTraceElement>> stacks = new HashSet<List<StackTraceElement>>();

        /**
         * Pat the unicorn. Return true if the caller has never patted us before. This includes the exact line of code
         * as the caller and not just the class/method.
         * Note that this implementation is truly horrific, and uses the fact that an Exception knows its stack trace
         * and the stack trace knows the calling line numbers of every method in the stack above us. We then remember
         * the stack trace of every caller, and we return true only if the stack trace is one we've never seen before.
         * @return true if the line number has never patted us before. False if they have
         */
        public static boolean pat() {
            try {
                throw new Exception();
            }
            catch (Exception e) {
                e.fillInStackTrace();
                List<StackTraceElement> ste = Arrays.asList(e.getStackTrace());
                boolean result = true;
                if (stacks.contains(ste)) {
                    result = false;
                }
                stacks.add(ste);
                return result;
            }
        }
    }
}