Replace one set of characters with another set of characters

I am writing code to convert Roman Numbers to Decimal Numbers.

I was wondering if this line could be optimised?

String s = "MCMXCIV";
String code = s.replaceAll("I", "a")
               .replaceAll("V", "b")
               .replaceAll("X", "c")
               .replaceAll("L", "d")
               .replaceAll("C", "e")
               .replaceAll("D", "f")
               .replaceAll("M", "g");

Expected value of code: gegceab

EDIT 1: Can the same code can be written in a shorter way?

EDIT 2:

Two conditions I want

1. Avoid expensive string concatenation
2. replace each character with corresponding code in O(1)

I have no idea how this helps you translate roman numbers to decimal numbers, but, sure, this can be simplified!

For starters, .replace does this job strictly better ('replaceAll is a stupid name; the first argument is a regular expression. replace, without the all, also replaces all, and takes an actual raw string, which is what you wanted – java doesn't like breaking backwards compatibility; the silly name is unlikely to be corrected).

Secondly, 'shorter' is mostly meaningless. The relevant goals when writing code are readability and flexibility, and rarely efficiency (performance); shorter is on its own not a relevant goal.

Here's an alternative take; it's significantly more efficient for large strings. For small strings, it's 2020, the time taken is going to round down to 0.

private static final String LETTERS = "IVXLCDM";

char[] cs = s.toCharArray();
for (int i = 0; i < cs.length; i++) {
    cs[i] = 'a' + LETTERS.indexOf(cs[i]);
}
String code = new String(cs);

You can use two arrays and loop through them, replacing from[i] with to[i].

String s = "MCMXCIV";
char[] from = new char[]{'I', 'V', 'X', 'L', 'C', 'D', 'M'};
char[] to = new char[]{'a', 'b', 'c', 'd', 'e', 'f', 'g'};

for (int i = 0; i < from.length; i++){
    s = s.replace(from[i], to[i]);
}