Sorting HashMap objects on their properties rather values [duplicate]

Question

This is not my real code I have just simulated in order to understand what to do next.

I have class Person with properties age, height weight.
Now In my class Group
I create two four objects

Person programmer, student, clerk, tech;

I have HashMap rollCall

Map<Person, Integer> rollCall = new HashMap<Person, Integer>();

to add all these using Person and number of Persons as type Integer

rollCall.put(programmer, 1);
rollCall.put(clerk, 2);
rollCall.put(student, 1);
rollCall.put(tech, 3);

I have seen alot of people sorting HashMap using TreeMap on value I want to sort on a property of Person rather on value. I want to sort all these people on their age (i.e. programmer.getAge();). I am not sure if I will use comprator which works only on collection not map. . Please help ... .

Answer 1

You can get a Map<Person,Integer> which iterates by age increasing or decreasing order by using a custom comparator:

Map<Person, Integer> rollCall = new TreeMap<Person, Integer>(
  new Comparator<Person>() {
    @Override public int compare(Person p1, Person p2) {
      return p1.getAge() - p2.getAge(); // Acending.
      // or  p2.getAge() - p1.getAge(); // Descending.
    }
  }
);

When you add Persons to the collection they will be inserted in order by age.

Answer 2

You need to be able to compare your Person objects. If there is a canonical way to compare them, let them implement Comparable<Person> (i.e. give them a compareTo(Person) method.

If this is done, you can use the persons as keys for a SortedMap (like TreeMap).

If there are multiple ways two persons could be compared, implement a Comparator<Person> as a separate object.

Then give this comparator to the SortedMap on construction.

This will not sort your HashMap (a HashMap has always a seemingly random order), but give you another sorted datastructure instead.