java - How do I check for an empty string in a Boolean statement? -

i have assignment have attach letters "un" word user inputs (unless inputted word has "un" in front of it, in case return inputted word). i'm testing method encountered 1 problem: program keeps returning error if test empty input. here code:

    scanner keyboard = new scanner(system.in);                                                   system.out.print("enter: ");     string input = keyboard.nextline();      if(input.substring(0,2).equalsignorecase("un"))     {         system.out.println(input);     }      else if(input.equals(""))     {         system.out.println("un");     }     else     {         system.out.println("un" + input);     } 

so wanted ask how can test empty input/blank string since, evidently, "" quotations not work.

you having problem because trying substring of string doesnt have required length. put empty string check first.

 if(input.equals("")||input.length==1)     {        system.out.println("un");      }      else if(input.substring(0,2).equalsignorecase("un"))     {         system.out.println(input);     }     else     {         system.out.println("un" + input);     }