Foo.java

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;

public class Foo {
	private static class Person {
		private int age;
		private String name;

		Person(int age, String name) {
			this.age = age;
			this.name = name;
		}

		public int getAge() {
			return age;
		}

		public void setAge(int age) {
			this.age = age;
		}

		public String getName() {
			return name;
		}

		public void setName(String name) {
			this.name = name;
		}

		@Override
		public String toString() {
			return name + ": " + age;
		}
	}

	private static void sortByAgeA(List<Person> people) {
		people.sort(new Comparator<Person>() {
			@Override
			public int compare(Person a, Person b) {
				if (a.age == b.age) {
					return 0;
				}

				return a.age > b.age ? 1 : -1;
			}
		});
	}

	private static void sortByAgeB(List<Person> people) {
		people.sort(Comparator.comparingInt(person -> person.age));
	}

	public static void main(String[] args) {
		List<Person> people = Arrays.asList(
				new Person(20, "A"),
				new Person(18, "B"),
				new Person(22, "C")
		);
		sortByAgeA(people);
		System.out.println(Arrays.deepToString(people.toArray()));

		people = Arrays.asList(
				new Person(20, "A"),
				new Person(18, "B"),
				new Person(22, "C")
		);
		sortByAgeB(people);
		System.out.println(Arrays.deepToString(people.toArray()));
	}
}