I did solve it,but i didn't use extra checker "if String is NOT equal to Zero" (if (string!=null) { result.add(s);, and after it has passed the validator - I didn't get why do we need it? Also i do have an issues with debugging multithreading Solutions, is anyone could suggest anything - how you could see every thread is going through the program - not just Main thread? Because default IntelijIdea debugger just not showing what is going with other Threads.