private String executeCmd(String cmd, String author) {
        if(cmd.equals("counterget")) {
            if(counters.containsKey(author))
                return author + "'s counter value is " + counters.get(author);
            else
                return author + " has no counter";
        }

        int num = 0;
        int currentVal = 0;
        try {
            num = getNum(cmd);
        }
        catch(NumberFormatException ex) {
            return null;
        }

        if(counters.containsKey(author))
            currentVal = counters.get(author);

        currentVal += num;
        counters.put(author, (Integer)currentVal);
        return null;
    }