Question

I try to store the “modules” data in a specific order inside a map. I use TreeMap to realise this.
When I read the data i get it in exactly the wrong order. It starts with key value 9 and stops a 0. ??? Does JPA do s.t. else then “putting” the values into the TreeMap?

@OneToMany(
    mappedBy="prditem", 
    fetch=FetchType.LAZY, 
    cascade=CascadeType.ALL,
    orphanRemoval=true,
    targetEntity=PubModule.class
)
@JoinColumn(name="prditmID")
@MapKey(name="order")
private Map<Integer, PubModule> modules = new PubModulesMap();

---

public class PubModulesMap extends TreeMap<Integer,PubModule> {

    @Override
public PubModule put(Integer key, PubModule value) {
    PubModule old = (PubModule)this.get(key);
        if (old!=null) {
            old.setPrditem(value.getPrditem());
    ....
}

---

Iterator <PubModule> it = list.values().iterator();
while (it.hasNext()) {
    PubModule pm = it.next();
    System.out.println(pm.getOrder());
}

9
8
7
6
5
4
3
2
1
0

Answer 1

The reason for unexpected order is that, JPA does not understand that the map should be sorted.

Generic solution would be to create a getter method for your map:

public Map<Integer, PubModule> getModules() {
   if (modules == null) {
      modules = new PubModulesMap();
   } else if (!(modules instanceof TreeMap)) {
      modules = new TreeMap(modules); // Or new PubModulesMap(modules)
   }
   return modules;
}

If you’re using hibernate, you should add @Sort(type = SortType.NATURAL) annotation.