Comparator

External

• https://docs.oracle.com/javase/10/docs/api/java/util/Comparator.html

Internal

• Java 8 Lambda Expressions

Overview

@FunctionalInterface
public interface Comparator<T> {

    int compare​(T o1, T o2);

    ...
}

Comparator.comparing()

Factory methods that produces comparator lambdas, based on various criteria.

@FunctionalInterface
public interface Comparator<T> {

 static <T, U extends Comparable<? super U>> Comparator<T> comparing​(Function<? super T, ? extends U> keyExtractor);

 static <T, U> Comparator<T> comparing​(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator);

}

Concise Sorting

Java 8 List has a sort() method that accepts a java.util.Comparator.

java.util.Comparator is a functional interface, so we can provide a lambda expression instead.

java.util.Comparator comes with static methods that build Comparator instances, so if compare apples by weight, we can write this:

List<Apple> apples = ...
apples.sort(Comparator.comparing(a -> a.getWeight()));

The last form can be compacted even further using method references:

List<Apple> apples = ...
apples.sort(Comparator.comparing(Apple::getWeight));

Composition Methods

• reversed()
• thenComparing(Function)