Try to pat the Unicorns from http://zeroturnaround.com/fun/magical-java-puzzle-pat-the-unicorns/

Unicorn.java

import java.io.OutputStream;
import java.io.PrintStream;

public class Unicorn {

    private static String str1 = "UNICORN #1: PAT THIS UNICORN ONCE";
    private static String str2 = "UNICORN #2: PAT THIS UNICORN ONCE";

    static class Printer extends PrintStream {
        private PrintStream origin;

        private boolean first = false;
        private boolean second = false;

        public Printer(OutputStream out) {
            super(out);
            this.origin = new PrintStream(out);
        }

        @Override
        public void println(String x) {
            if (x.equals(str1)) {
                if (!first) {
                    origin.println(x);
                    first = true;
                }
            } else if (x.equals(str2)) {
                if (!second) {
                    origin.println(x);
                    second = true;
                }
            } else {
                origin.println(x);
            }
        }

    }

    static {
        PrintStream origin = System.out;
        System.setOut(new Printer(origin));
    }

    public static boolean pat() {
        return true;
    }

}