Finding the median value from a List of objects using Java 8

Question

I have two classes that are structured like this:

public class Company {
     private List<Person> person;
     ...
     public List<Person> getPerson() {
          return person;
     }
     ...
}

public class Person {
     private Double age;
     ...
     public Double getAge() {
          return age;
     }
     ...
}

Basically the Company class has a List of Person objects, and each Person object can get an Age value.

If I get the List of the Person objects, is there a good way to use Java 8 to find the median Age value among all the Person objects (Stream doesn't support median but is there anything else)?

Double medianAge;
if(!company.getPerson().isEmpty) {
     medianAge = company.getPerson() //How to do this in Java 8?
}

Answer 1

You may use

List<Person> list = company.getPerson();
DoubleStream sortedAges = list.stream().mapToDouble(Person::getAge).sorted();
double median = list.size()%2 == 0?
    sortedAges.skip(list.size()/2-1).limit(2).average().getAsDouble():        
    sortedAges.skip(list.size()/2).findFirst().getAsDouble();

The advantage of this approach is that it doesn’t modify the list and hence also doesn’t rely on its mutability. However, it’s not necessarily the simplest solution.

If you have the option of modifying the list, you can use

List<Person> list = company.getPerson();
list.sort(Comparator.comparingDouble(Person::getAge));
double median = list.get(list.size()/2).getAge();
if(list.size()%2 == 0) median = (median + list.get(list.size()/2-1).getAge()) / 2;

instead.

Answer 2

Obligatory Guava way.

import java.util.List;
import java.util.stream.Collectors;

import com.google.common.math.Quantiles;

...

List<Person> people = company.getPerson();
List<Double> ages = people.stream().map(Person::getAge).collect(Collectors.toList());
double median = Quantiles.median().compute(ages);

Though as of Guava 28.1 Quantiles is still annotated as @Beta.

Answer 3

Here's a simplified version of @Holger's answer that also works for IntStream and LongStream, and avoids NoSuchElementException in the case of an empty stream:

int size = someList.size();

//replace 'XXX' with 'Int', 'Long', or 'Double' as desired
return someList.stream().mapToXXX(...).sorted()
    .skip((size-1)/2).limit(2-size%2).average().orElse(Double.NaN);

This will return NaN if the list is empty rather than throwing NoSuchElementException. If you'd prefer to throw a NoSuchElementException instead, simply replace .orElse(Double.NaN) with .getAsDouble().

Answer 4

You can use lambda expression reduction to get the median of list. Something like this

Integer median = Person .stream() .map(Person::getAge) .filter(n -> n.length()/2);