Java Spring: Enum keys in (LinkedHash)Map are not ordered

My entity Mealplan has an mealsPerWeek property as a LinkedHashMap.

@Entity
public class Mealplan {

@Id
private int id;

@ManyToMany
private Map<Weekday, Meal> mealsPerWeek = new LinkedHashMap<>();

public Map<Weekday, Meal> getMealsPerWeek() {
    return mealsPerWeek;
}

}

The key of the map property is Weekdayand is an enumeration:

public enum Wochentag {
    Monday, Tuesday, Wednesday, Thursday, Friday
}

Now I expect that in my API the Meals get displayed in the correct order of the enum. But it is displayed pretty random:

{
id: 1,
mealsPerWeek: {
  Tuesday: {
   id: 3,
   name: "Salad",
   preis: 3,
   art: "vegan"
},
  Monday: {
   id: 4,
   name: "Linsensuppe",
   preis: 23.5,
   art: "vegan"
},

How do I order it in my REST API so it displays the keys in its correct order?

Edit: I insert objects via data.sql on every application startup and realised, that everytime the order is differnet.

Source Code: https://gitlab.com/joshua.olberg/java-spring-backend

The problem is that when loading the map (and generally, all collections) from the db, hibernate uses its own collection types.

https://docs.jboss.org/hibernate/orm/5.1/userguide/html_single/chapters/domain/collections.html

> Hibernate uses its own collection implementations which are enriched with lazy-loading, caching or state change detection semantics. For this reason, persistent collections must be declared as an interface type. The actual interface might be java.util.Collection, java.util.List, java.util.Set, java.util.Map, java.util.SortedSet, java.util.SortedMap or even other object types (meaning you will have to write an implementation of org.hibernate.usertype.UserCollectionType).

> As the following example demonstrates, it’s important to use the interface type and not the collection implementation, as declared in the entity mapping.

> Example 1. Hibernate uses its own collection implementations
>```
@Entity(name = "Person")
public static class Person {
@Id
private Long id;

> @ElementCollection
private List<String> phones = new ArrayList<>();

> public List<String> getPhones() {
return phones;
}
>}

> Person person = entityManager.find( Person.class, 1L );
//Throws java.lang.ClassCastException: org.hibernate.collection.internal.PersistentBag cannot be cast to java.util.ArrayList
ArrayList<String> phones = (ArrayList<String>) person.getPhones();
> ```

Therefore:

private Map<Weekday, Meal> mealsPerWeek = new LinkedHashMap<>();

will be replaced by PersistentMap.
That is why initializing a Map with EnumMap or TreeMap fails.

You want Hibernate to use PersistentSortedMap, and for that

• use a correct interface SortedMap
• add @SortNatural or @SortComparator

EnumMap may help you here. Iteration order is based on declared order in the Enum - https://docs.oracle.com/javase/7/docs/api/java/util/EnumMap.html