Query.cls

public class OrderBy {

        @TestVisible
        private String orderDirection;

        private String fieldName;

        public OrderBy(String orderDirection, String fieldName) {
            this.orderDirection = orderDirection;
            this.fieldName = fieldName;
        }

        public OrderBy(String orderDirection, Schema.SObjectField field) {
            this(orderDirection, field.getDescribe().getName());
        }

        public OrderBy(Schema.SObjectField field) {
            this('ASC', field);
        }

        public String generateString() {
            return String.format(
                '{0} {1}',
                new List<String> {
                    this.fieldName,
                    this.orderDirection
                }
            );
        }
    }