protected String getExpressionSetSummary(Set<Integer> set) {

        if (set.contains(NO_SPEC)) {
            return "?";
        }
        if (set.contains(ALL_SPEC)) {
            return "*";
        }

        StringBuilder buf = new StringBuilder();

        Iterator<Integer> itr = set.iterator();
        boolean first = true;
        while (itr.hasNext()) {
            Integer iVal = itr.next();
            String val = iVal.toString();
            if (!first) {
                buf.append(",");
            }
            buf.append(val);
            first = false;
        }

        return buf.toString();
    }
Indicates whether the specified cron expression can be parsed into a
valid cron expression

@param cronExpression the expression to evaluate
@return a boolean indicating whether the given expression is a valid cron
        expression
server/core/src/main/java/com/ke/schedule/server/core/common/CronExpression.java