Question

I have this Java function that’s supposed to convert the string s to title case. It should return a copy of s to the caller, leaving s preserved.

Currently, rv ends up as an empty string. Can anyone tell me why?

private static String titleCase(String s) {
        String rv = new String();
        StringTokenizer strtok = new StringTokenizer(s);
        // handle the potential null error: (should really output a runtime warning here)
        if(s == null) return null;
        while(strtok.hasMoreTokens()) {
            String word = strtok.nextToken();
            String firstLetter = word.substring(0,1);
            String restOfWord = word.substring(1);
            rv.concat(firstLetter.toUpperCase() + restOfWord.toLowerCase());
        }
        return rv;
    }

Answer 1

Strings being immutable in Java, once you have declared rv as “” (= new String()) it won’t change unless you allocate a new String to it.
You can either replace the concat line by:

rv = rv.concat(firstLetter.toUpperCase() + restOfWord.toLowerCase());

Or better, use a StringBuilder instead of a String (not tested):

StringBuilder rv = new StringBuilder();
// [...]
rv.append(firstLetter.toUpperCase()).append(restOfWord.toLowerCase());
// [...]
return rv.toString();