First you would need to create a Map of all the letters:
Hashmap<String, String> map = new Hashmap<String, String>();
map.put("a", "c");
map.put("b", "f");
...
To get the translation of each letter you simply get the value from the map:
String translatedLetter = map.get(letter);
So now you would need to create a loop to translate the whole word one letter at a time. I would use a StringBuilder to keep track of each translated letter.
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solved How to change letters to other letters Java [closed]