How to map a HashMap that contains ArrayList

Question

I am working on a Java project on my spare time,
I started using JPA 2.1 and learning to use annotations.
I have a class named Schedule that I want to contain this field:

private HashMap<LocalDate, ArrayList<Lesson>> theLessons

Lesson is another entity in my Database, LocalDate is the Java class.
Is that possible?

I tried to do it combining @OneToMany with @MapKey like in How do you map a "Map" in hibernate using annotations? but with no effect.
I am thinking that maybe I will need to create other Entities to facilitate this, like maybe a ListOfLessons, that will contain the ArrayList<Lesson> in order to shorten the HashMap to :

private HashMap<LocalDate, ListOfLessons>

Any help welcome

how do you expect this to be stored in the database? what tables, columns? — Neil Stockton, Jun 14 '14 at 07:49
@Neil Stockton : Well that is the issue, I don't have much experience with designing DBs. I don't mind how it is saved, I just want to have this information — Dimitris, Jun 14 '14 at 11:29
You need to understand and draw out the relation between what a localDate, schedule and Lesson are. Why would a particular localDate have a list of lessons associated to it, and what is a localDate exactly? ListofLessons would have the LocalDate and a 1:m list of Lessons — Chris, Jun 14 '14 at 17:02

score 1 · Answer 1 · answered Jun 14 '14 at 12:48

1

Just create an intermediate "Entity" and have a Map (as you presumed) and you'll find that in any decent JPA docs.

answered Jun 14 '14 at 12:48

Neil Stockton

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Dimitris · Accepted Answer · 2014-06-16T12:16:26.420

What I did that appears to be working fine:

@Entity
public class Schedule implements Serializable {
....
    @OneToMany
    @MapKey(name = "theDate")
    private Map<LocalDate, ListOfLessons> allLessons = new HashMap<>();
....
}
@Entity
public class ListOfLessons implements Serializable {
...
    private LocalDate theDate;
    @OneToMany
    private List<Lesson> allLessons = new ArrayList<>();
...
}

Notice that ListOfLessons' field called theDate is used as a Map Key in class Schedule

How to map a HashMap that contains ArrayList

2 Answers2