Java 8 sort on Class member's property

Question

Class declaration:

class Entity {
    String name;
    SubEntity subEntity; // subEntity has a method getAmount() which returns int
}

I understand with Java 8 we can sort like:

entities.sort(Comparator.comparing(Entity::name));

But is there a way I can sort it on sub-entities' properties, for eg:

entities.sort(Comparator.comparing(Entity::SubEntity::getAmount()));

P.S: All in for any one-liners.

Answer 1

Not by using a method reference, no - but it's easy to do with a lambda instead:

entities.sort(Comparator.comparing(entity -> entity.getSubEntity().getAmount()));

Fundamentally there's nothing magical about Comparator.comparing - it just accepts a Function<? super T,? extends U> keyExtractor parameter, so you need to work out some way of creating such a function. A method reference is one convenient way of creating a function, but a lambda expression is more flexible one.

Answer 2

Guys gave you good answers. It isn't supposed to be an improvement over their answers. I just want to provide an alternative idea.

entities.sort(Comparator.comparing(((Function<Entity, SubEntity>)Entity::getSubEntity).andThen(SubEntity::getAmount)));

I formed a key extractor by combining two functions Entity::getSubEntity and SubEntity::getAmount with Function#andThen. Both have been written as method references. The cast is required to determine the type of an instance and call andThen on that instance.

Answer 3

You can do that via a lambda as opposed to a method reference:

 entities.sort(Comparator.comparing(x -> x.getSubEntity().getAmount())

If you you have indeed an int as you say in your comments, then use :

 Comparator.comparingInt(...)