How to switch a tab with multiple spaces without the replace() methods?

I am trying to get my method to swap a tabs from an input file with x amount of spaces and print to a new file, and all it is doing is reprinting the exact same thing into the output file. However, I can't use the replace() methods.

So far I have for the conversion:

public static String expandLine(String line, int numberOfSpaces) {
   String tab = "\t";
   String formattedLine = "";
   String space = spaces(numberOfSpaces);
   for (int i = 0; i < line.length(); i++) {
      if (line.charAt(i) == tab.charAt(0)) {
         formattedLine += space;
      } else {
         formattedLine += line.charAt(i);
      }
   } 
   return formattedLine;
}