Basic Hangman game in Java (mostly concerns string manipulation)

I am taking an intro class to Java and we have a project that deals with a hangman game. I have most of the code worked out but there is a bug that I can't seem to resolve.

First, the program prompts the user for a letter and one space where they think the letter goes, then the program displays a word in a series of hyphens and, if the user makes a correct guess, the letter in the corresponding hyphen is replaced with said letter.

For testing purposes, the word has been defaulted to narrow.

So if I were to guess the letter r and for space, I were to guess index 2, the program would give me:

Guess a letter: r 

Guess a space:  2

Guess:    --r---

The problem I am having is that when I guess the index 3 for space and try to guess the next r, the program just gives me the same output as before.

We are not allowed to use arrays or string builder because we have not talked about them yet.

Here are my variables:

    // default word for testing
    String blank = "narrow";

    // variables to manipulate the string
    String start = "";
    String end = "";
    String word = "";

    // variables for input and output
    // input is used for the user's letter guess
    String input; 
    // space is used for the user's index guess
    String space = "";
    // guess is used at the end to display the word to the user and set equal to word after
    // it has been manipulated
    String guess = "";

Here is the code where the string is being manipulated.

    for (int i = 0; i < blank.length(); i++) {
         if (i == blank.indexOf(input)) {
             start = guess.substring(0, i);
             end = guess.substring(i + 1);
             word = start.concat(input).concat(end);                    
         }
    }

I think it has to do with the if statement, but I have tried some other things and they have not worked. Any help would be appreciated.

Thank you.