public String findIndefiniteField() {
        if (principal() == null)
            return "Principal is NULL";
        if (host() == null)
            return "Host is NULL";
        if (operation() == AclOperation.ANY)
            return "Operation is ANY";
        if (operation() == AclOperation.UNKNOWN)
            return "Operation is UNKNOWN";
        if (permissionType() == AclPermissionType.ANY)
            return "Permission type is ANY";
        if (permissionType() == AclPermissionType.UNKNOWN)
            return "Permission type is UNKNOWN";
        return null;
    }
Returns a string describing an ANY or UNKNOWN field, or null if there is
no such field.
clients/src/main/java/org/apache/kafka/common/acl/AccessControlEntryData.java