How would you add an invalid input to this code in java,

 public static void main(String[] args)

   {

  String playagain;

               String name1;

               String name2;

  char playAgainOrNot;

  int user1Input, user2Input;

  //Rock = 1

  //Paper = 2

  //Scissors = 3

       Scanner keyboard = new Scanner(System.in);

       Scanner input = new Scanner(System.in);

       do

       {

           System.out.println("Player One enter your first name");

           name1 = input.next();

           System.out.println("Player Two enter your first name");

           name2 = input.next();

           System.out.println("Instructions: Type 1 for Rock, 2 for Paper, or 3 for Scissors");

           System.out.println (name1 + " would you like to play rock, paper, or scissors?");

           

           user1Input = keyboard.nextInt();

           System.out.println (name2 + " would you like to play rock, paper, or scissors?");

           

           user2Input = keyboard.nextInt();

           //if the users choices are both equal

           if (user1Input == user2Input)

           {

               System.out.println("It's a tie!");

           }

           else if (user1Input != user2Input)

           {

               //rock breaks scissors

              if (user1Input == 1 && user2Input == 3)

               {

                   System.out.println(name1 + " chose Rock");

                   System.out.println(name2 + " chose Scissors");

                   System.out.println("Rock Breaks Scissors " + name1 + " wins!");

               }

               else if (user2Input == 1 && user1Input == 3)

               {

                   System.out.println(name1 + " chose Scissors");

                   System.out.println(name2 + " chose Rock");

                   System.out.println("Rock Breaks Scissors " + name2 + " wins!");

               }

               //paper covers rock

               else if (user1Input == 2 && user2Input == 1)

               {

                   System.out.println(name1 + " chose Paper");

                   System.out.println(name2 + " chose Rock");

                   System.out.println("Paper Covers Rock " + name1 + " wins!");

               }

               else if (user2Input == 2 && user1Input == 1)

               {

                   System.out.println(name1 + " chose Rock");

                   System.out.println( name2 + " chose Paper");

                   System.out.println("Paper Covers Rock " + name2 + " wins!");

               }

               //scissors cut paper

               else if (user1Input == 3 && user2Input == 2)

               {

                   System.out.println(name1 + " chose Scissors");

                   System.out.println(name2 + " chose Paper");

                   System.out.println("Scissors cut Paper " + name1 + " wins!");

               }

               else if (user2Input == 3 && user1Input == 2)

               {

                   System.out.println(name1 + " chose Paper");

                   System.out.println(name2 + " chose Scissors");

                   System.out.println("Scissors cut Paper " + name2 + " wins!");

            }

    }

    System.out.println("Would you like to play again ");

             System.out.println("Please enter Y/N");

             playagain = keyboard.next();

             playagain = playagain.toUpperCase();

             playAgainOrNot = playagain.charAt(0);

         }

         //keep the loop running while the user enter Y

         while (playAgainOrNot == 'Y');