public String toString() {
            if (min == null && max == null)
                return "[...]";
            else if (min == null)
                return "[...," + max + "]";
            else if (max == null)
                return "[" + min + ",...]";
            else
                return "[" + min + ",...," + max + "]";
        }
A numeric range that checks both the upper (inclusive) and lower bound
clients/src/main/java/org/apache/kafka/common/config/ConfigDef.java