Comparator
Allows to define custom ordering of objects. For example, while sorting.
Definition
@FunctionalInterface
public interface Comparator<T> {
int compare(T o1, T o2);
}
Usage
class AgeComparator implements Comparator<Person> {
@Override
public int compare(Person p1, Person p2) {
return p1.getAge() - p2.getAge();
}
}
public class ComparatorExample {
public static void main(String[] args) {
List<Person> people = new ArrayList<>();
Collections.sort(people, new AgeComparator());
}
}
Collections.sort(people, (p1, p2) -> p1.getAge() - p2.getAge());
Default methods
In addition to the abstract method compare, it also provides a bunch of utility methods to easily define a Comparator.
For example,
List<Person> sortedByAge = people.stream()
.sorted(Comparator.comparing(Person::getAge))
.collect(Collectors.toList());
Here's a list of all such methods:
- static comparing(Function keyExtractor): Like the example above
- static comparing(Function, Comparator): Like the example above but with Comparator for the given key.
Similar methods but for specific types:
- static comparingDouble
- static comparingInt
- static comparingLong
Lexicographic order sorting (to be used after the above methods):
- thenComparing
- thenComparingDouble
- thenComparingInt
- thenComparingLong
Methods for null friendly Comparator
- nullsFirst(Comparator)
- nullsLast(Comparator)
To modify ordering or comparator:
- static naturalOrder: returns Comparator which compares Comparable objects in natural order
- static reverseOrder()
- reversed()
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