How to replace all characters in a user input string except one

Question

I'm currently in an introductory level Java class, and am working on the classic phrase guess assignment. The object is for one user to enter a secret phrase, and another to guess it one letter at a time. Between guesses, the phrase must be displayed as all question marks except the letters that were guessed correctly. Our class has only really covered some very basic methods, if-else statements and loops up to this point, but I'm trying to research some string methods that may make this a bit easier.

I know of the replace(), replaceAll() and contains() methods, but was wondering if there is a method which allows you to replace all but one character of your choice in a string.

Thanks in advance

Answer 1

The easiest way is probably to use String.replaceAll():

String out = str.replaceAll("[^a]", "?");

This will leave all letters a intact and will replace all other characters with question marks.

This can be easily extended to multiple characters, like so:

String out = str.replaceAll("[^aeo]", "?");

This will keep all letters a, e and o and will replace everything else.