You could use something like the following algorithm to accomplish this:
// Our input string.
String input = "I love fish";
// Contains the "encrypted" output string.
StringBuilder encrypted = new StringBuilder();
// Process each character in the input string.
for (char c : input.toCharArray()) {
if (Character.toLowerCase(c) != 'a' && Character.isLetter(c)) {
// If the character is a letter that's not 'a', convert it to the previous letter.
char previous = (char) ((int) c - 1);
encrypted.append(previous);
} else {
// Otherwise just append the original character.
encrypted.append(c);
}
}
// Prints the output to stdout.
System.out.println(encrypted.toString());
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solved How to convert a letter to an another letter in Java [duplicate]