Java - Replace Correct Number Into Letter

I'm trying to figure out the correct way to replace number into letter. In this case, I need two steps.

First, convert letter to number. Second, restore number to word.

Words list: a = 1, b = 2, f = 6 and k = 11.

I have word: "baafk"

So, for first step, it must be: "211611"

Number "211611" must be converted to "baafk".

But, I failed at second step.

Code I've tried:

public class str_number {
public static void main(String[] args){
String word = "baafk";
String number = word.replace("a", "1").replace("b","2").replace("f","6").replace("k","11");
System.out.println(word);

[Code] ...

Result for converting to number: baafk = 211611

But, result for converting above number to letter: 211611 = bkfk

How to distinguish if 11 is for "aa" and for "k"? Do you have any solutions or other ways for this case?