Substitution cipher in java

Question

My teacher wants me to do it using only the replace() and charAt() methods, basic loops, and conditional statements. I have given it a shot, but my code doesn't always get it right and frankly, I don't know why.
my current attempt:
https://code.sololearn.com/ciuIeDPHV1zo/#java

Answer 1

If you put System.out.println(Klartext); inside the loop it will show you what is happening each iteration. If you run it with "abcdefghijklmnopqrstuvwxyz" as the input you will see an issue

Answer 2

Ok. Now I know the issue but it turns out to be the same issue I have been failing to fix the entire time. You have a character more than one time? The replace method will mess up the encryption. What do you do?

Answer 3

convert Klartext to uppercase