Empty String??

[Gnawk]

This is just a part of a program

When my code asks me
Enter diamond size ("short", "average", or "tall"):
how do i make it so that if i don't put anything and press enter it asks me exactly same questions again??

i tried putting
input.length() == 0 or input.equals("") or input == null but none of them seems to work

import java.util.Scanner;
public class Pattern 
{
    public static void main(String[] args) 
    {

        Scanner kb = new Scanner(System.in);
        String input;
        String character;
      
        do 
        {
            System.out.print("Enter diamond size (\"short\", \"average\","); 
	        System.out.print(" or \"tall\"): ");
            input = kb.next().toLowerCase();
        }
        while (checkSize(input) == -1 || input.length() == 0);
        do 
        {
            System.out.print("Enter pattern character: ");
            character = kb.next();
        } 
        while (character.length() > 1);
      
          char ch = character.charAt(0);
      
          displayPattern(checkSize(input), ch);
    }
    public static int checkSize(String size) 
    {
        if (size.equals("short"))
	{
            return 6;
        }
        else if (size.equals("average")) 
        {
            return 12;
        }
        else if (size.equals("tall")) 
	    {
            return 22;
        }
 
        return -1;
    }

[Gnawk]

figured it by myself.. just had to change
input = kb.next().toLowerCase();
to
input = kb.nextLine().toLowerCase();