How to order a HashMap by properties

Question

As you probably know already, when you put something into a HashMap, the order in which things are stored is random. I want to order my HashMap using Comparable, but I can't get it to work properly.

This so I have a map:

Map<MyKeyObject, List<MyValueObject>> myObjectMap = new HashMap<>();

The key of this map is a construction of multiple an id and a name (MyKeyObject), and I want to sort the map on the keys based on the id first, and if the id is identical then name.

Here is what I've tried:

public class MyKeyObject implements Comparable<MyKeyObject> {
  private Long id;
  private String name;

  public MyKeyObject(Long id, String name) {
        this.id = id;
        this.name = name;
    }

  public boolean equals(Long id, String name) {
        return this.id.equals(id) && this.name.equals(name)
    }

  @Override
  public int compareTo(MyKeyObject myKeyObject) {       
      if (this.id myKeyObject.getId() != 0) {
          return (this.Id - myKeyObject.getId() == 1) ? 1 : -1;
      } else {
          return (this.name().compareTo(myKeyObject.name()) == 1) ? 1 : -1;
      }
  }

  @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        MyKeyObject that = (MyKeyObject) o;
        return id.equals(that.id) &&
                name == that.name;
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, name);
    }
}

public class MyKeyObject implements Comparable<MyKeyObject> {
  Map<MyKeyObject, List<MyValueObject>> myObjectMap = new HashMap<>();

  //Here I have a lot of code that populates the HashMap

  myObjectMap.entrySet().stream().sorted(Map.Entry.comparingByKey());
}

To be honest I don't even think the compareTo method is hit, what am I doing wrong here?

Update: I know that there are types such as TreeMap, but it doesn't work for me. I've just given a very simple example here, my real code is much mode complex. Is it possible to make this work with Comparable like I've tried here?

Answer 1

I think what you are looking for is TreeMap instead of HashMap.

Here you have a simplified example of your use case.

import java.util.Comparator;
import java.util.Map;
import java.util.Objects;
import java.util.TreeMap;

public class TreeMapExample {
    public static void main(String[] args) {
        Map<MyKeyObject, Object> myTreeMap = new TreeMap<>();

        myTreeMap.put(new MyKeyObject(5L, "Jay"), null);
        myTreeMap.put(new MyKeyObject(5L, "Bob"), null);
        myTreeMap.put(new MyKeyObject(1L, "Alison"), null);
        myTreeMap.put(new MyKeyObject(3L, "Frey"), null);

        myTreeMap.entrySet()
                .forEach(myKeyObjectObjectEntry ->
                        System.out.println(String.format(
                                "Id= %s, Name=%s",
                                myKeyObjectObjectEntry.getKey().id,
                                myKeyObjectObjectEntry.getKey().name )));
    }

    public static class MyKeyObject implements Comparable<MyKeyObject> {
        private Long id;
        private String name;

        public MyKeyObject(Long id, String name) {
            this.id = id;
            this.name = name;
        }

        @Override
        public int compareTo(MyKeyObject myKeyObject) {
            return Comparator.comparing((MyKeyObject keyObject)->keyObject.id)
                    .thenComparing(keyObject->keyObject.name)
                    .compare(this, myKeyObject);
        }

    }
}

The output is:

Id=1, Name=Alison
Id=3, Name=Frey
Id=5, Name=Bob
Id=5, Name=Jay

You can make MyKeyObject Comparable or provider a Comparator to the TreeMap.