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The code is testing the value of i. That is the int that controls the for loop. It is NOT a char to be tested by the Character class's methods.
Where is the char values that you want to test?...

If you are working with an array, you could use a loop, test each element of the array and change that element of the array by assigning it the new value. The code would not use the replace()...

Are you working with a String or an array?
With a String I think a regex might work.
With an array, a loop and if statement would do it.

That sounds like something that a regular expression could do if the contents of the array were in a String. I'm no good with regex so I'll leave it to someone that is.
If the values are in an...