Perfect Spring DTO (ours)

Example.java

package com.scottlogic.springdto;
import javax.validation.constraints.NotBlank;
import javax.validation.constraints.Positive;
import lombok.Value;

public class Example {
	public enum ProductDto{;
		public interface Id{
			/**
			 * The identifier for this product.
			 * IDs are unique amongst products but may be
			 * repeated across other entity types
			 */
			@Positive
			Long getId();
		}

		public interface Name{
			/**
			 * The product's name as should appear on the site.
			 * Any capitalisation here should be stored in the
			 * database but any filtering should be case-insensitive.
			 */
			@NotBlank
			String getName();
		}

		public interface Price{
			/**
			 * The amount that we sell a product for in GBP.
			 * This includes our margin but should not include
			 * any delivery fees or VAT.
			 */
			@Positive
			Double getPrice();
		}

		public interface Cost{
			/**
			 * The amount that it costs us to purchase this product
			 * For the amount we sell a product for, see {@link Price}.
			 * <b>This data is confidential</b>
			 */
			@Positive
			Double getCost();
		}

		public enum Request{;
			@Value public static class Create implements Name, Price, Cost{
				String name;
				Double price;
				Double cost;
			}
		}

		public enum Response{;
			@Value public static class Public implements Id, Name, Price{
				Long id;
				String name;
				Double price;
			}

			@Value public static class Private implements Id, Name, Price, Cost{
				Long id;
				String name;
				Double price;
				Double cost;
			}
		}

		private static <DTO extends Price & Cost> Double getMarkup(DTO dto){
			return (dto.getPrice() - dto.getCost()) / dto.getCost();
		}
	}
}

I might suggest that records would be an nicer fit removing the need for the static classes and the lombok annotations while maintaining immutability. For even more conciseness take a look at the @RecordInterface part of RecordBuilder here https://github.com/Randgalt/record-builder#RecordInterface-Example