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

MagicalLand1.java

import java.util.HashSet;
import java.util.Set;

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");
  }
}

class Unicorn {

	private static Set<Integer> sourceLinesVisited = new HashSet<>();

	public static boolean pat() {
		final int callerLineNumber = Thread.currentThread().getStackTrace()[2].getLineNumber();
		return sourceLinesVisited.add(callerLineNumber);
	}
}

MagicalLand2.java

import java.io.OutputStream;
import java.io.PrintStream;
import java.util.HashSet;
import java.util.Set;

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");
  }
}

class Unicorn {

	static {
		System.setOut(new NoDuplicateOutputStream(System.out));
	}

	public static boolean pat() {
		return true;
	}

}

class NoDuplicateOutputStream extends PrintStream {

	private final Set<String> printedMessages = new HashSet<>();

	public NoDuplicateOutputStream(OutputStream out) {
		super(out);
	}

	@Override
	public void println(String x) {
		if (printedMessages.add(x)) {
			super.println(x);
		}
	}
}

MagicalLand3.java

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");
  }
}


class Unicorn {
	public static boolean pat() {
		return true;
	}
}

class Math {

	public static double random() {
		return -1.0 / 500;
	}

}

MagicalLand4.java

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");
  }
}


class Unicorn {

	static {
		System.out.println("UNICORN #1: PAT THIS UNICORN ONCE");
		System.out.println("UNICORN #2: PAT THIS UNICORN ONCE");
	}

	public static boolean pat() {
		return false;
	}

}

MagicalLand5.java

import java.util.concurrent.atomic.AtomicBoolean;

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");
  }
}

class Unicorn {

	private static AtomicBoolean printed = new AtomicBoolean();

	public static boolean pat() {
		if (!printed.getAndSet(true)) {
			System.out.println("UNICORN #1: PAT THIS UNICORN ONCE");
			System.out.println("UNICORN #2: PAT THIS UNICORN ONCE");
		}
		return false;
	}

}