I would rather place an if statement in your for loop to only edit the characters which are alphabetical. do something like if(Character.isAlphabetical(string.getChar(i))) { }. This means the for loop will not do anything with the blank spaces so long as you're adding to the array in the if statement.

I know this is 7 months ago but - if you're trying to change characters in a string, remember strings are immutable. Meaning you need to reinitialise it to have the edits take affect. I made the same mistake.