TreeMap Comparator没有给出预期的订单？

Question

I am attempting to create a TreeMap with the following Format:

TreeMap<Kenel, List<Dog>> treeMapOfKennelsAndDogs;

Each entry is a Kennel object with an attached list of Dogs . I want to compare each Kennel by the oldest Dog in each and order accordingly.

However the order of the treeMap is not what I am expecting, it seems that when a Kennel has a Dog List of size 1 in the list then the ordering is incorrect.

Below is my code to find the oldest Dog related to each Kennel, and also my comparator logic.

@Entity
@Table(name = "KENNEL")
public class Kennel {

    //other fields 

    @OneToMany(cascade = CascadeType.MERGE, orphanRemoval = true)
    @JoinColumn(name = "KENNEL_ID", referencedColumnName = "ID", updatable = true, insertable = true)
    private List<Dog> dogList;

     public DateTime getOldestDogInList() {
        if (dogList.size() == 0) {
            return null; }

        Dog dog= dogList.get(0);
        for (int i = 1; i < dogList.size(); i++) {
            dog d = dogList.get(i);
            if (d.getCreated().isBefore(dog.getCreated())) {
                dog = d;
            }
        }
        return dog.getCreated();
    }

Comparator logic in service class:

    Comparator<Kennel> oldestDog = new Comparator<Kennel>() {
        @Override
        public int compare(Kennel k1, Kennel k2) {

            int result = k1.getOldestDogInList().compareTo(k2.getOldestDogInList());

            return result;
        }
    };

  TreeMap<Kennel, List<Dog>> treeMapOfKennelsAndDogs = new TreeMap<>(oldestDog);    

Is there an error in code that would cause the incorrect ordering in the TreeMap?

Answer 1

Comparator logic in service class:

TreeSet don't know about your service classes. It knows only about Generic type inside.

1st sollution:

Comparator logic should be in a class that will be sorted in TreeSet . You should implement Comparator and ovverade compare method in Kennel class.

2nd sollution:

Set Comparator when create TreeMap .

TreeMap<Kenel, List<Dog>> treemap = new TreeMap<Kenel, List<Dog>>(new Comparator<Kenel>() {
    public int compare(Kennel k1, Kennel k2) {
        int result = k1.getOldestDogInList().compareTo(k2.getOldestDogInList());
        return result;
    }
});

Related question: What is the difference between compare() and compareTo()?

Answer 2

1. Compare logic of Map<K, V> keys should not depend on V class.
2. Do you modify keys instances after inserting in Map ? If yes - it is a problem. Equality check is only on insetion*.
3. The ordering maintained by a tree map, like any sorted map, and whether or not an explicit comparator is provided, must be consistent with equals. (From javadoc)