public static String substitute( String source, String searchString, String replaceString ) {
        int sl;
        if (source == null) {
            return null;
        }
        if (searchString == null || (sl = searchString.length()) == 0) {
            return source;
        } 
        if (replaceString == null) {
            replaceString = "";
        }
        int len = source.length();
        int rl = replaceString.length();
        int length;
        if (sl == rl) {
            length = len;
        } else {
            int c = 0;
            int s = 0;
            int e;
            while ((e = source.indexOf(searchString, s)) != -1) {
                c++;
                s = e + sl;
            }
            if (c == 0) {
                return source;
            }
            length = len - (c * (sl - rl));
        }
 
        int s = 0;
        int e = source.indexOf(searchString, s);
        if (e == -1) {
            return source;
        }
        StringBuffer sb = new StringBuffer(length);
        while (e != -1) {
            sb.append(source.substring(s, e));
            sb.append(replaceString);
            s = e + sl;
            e = source.indexOf(searchString, s);
        }
        e = len;
        sb.append(source.substring(s, e));
        return sb.toString();
    }