Loop to find and replace multiple instances of a char

So im trying to create a hangman like game. Ill add to it later but right now I just need the basics.
I'm trying to create a loop to find the position that a guessed letter is in then replace the appropriate asterisk with said letter.
What I have so far lets me run it but it replaces all the asterisks if the letter is in the phrase.

import javax.swing.*;
public class GuessingGame
{
public static void main(String[]args)
{
//string and variable declerations
String guessingWord = "Dota is awsome";
String answer = "**** ** ******";
String guessedLetter;
char sub;
int length;
int i;

//in case the string length comes into play
length = guessingWord.length();

//Prompting the user to guess letter or phrase
guessedLetter = JOptionPane.showInputDialog(null, "Enter a letter or the full phrase\nif you think you know the answer");

//assigning variable to be the number of the position of the particular asterisk of the answer string
int a = guessingWord.indexOf(guessedLetter);

//if guessed letter is part of the asnwer
if(a != -1)
{
while(a >= 0)
{
sub = guessingWord.charAt(a);
answer = answer.replace(answer.charAt(a), sub);
a = guessingWord.indexOf(guessedLetter, ++a);

}

JOptionPane.showMessageDialog(null, "That letter is in the phrase!\n" + answer);

}
else
JOptionPane.showMessageDialog(null, "That letter is not in the phrase");

}
}

I had the code written in a way before that seemed like it worked but the GUI would crash after a correct letter guess was made.
I know i have some unused char's in there, Im going to remove it later once I get this part of my code to work, I can focus on the rest.