Java 8: Sorting values in collections

Having realised that Java 8 is due for its GA release within the next few weeks I thought it was about time I had a look at it and over the last week have been reading Venkat Subramaniam’s book.

I’m up to chapter 3 which covers sorting a collection of people. The Person class is defined roughly like so:
 
 
 
 
 

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

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

    @Override
    public String toString() {
        return String.format("Person{name='%s', age=%d}", name, age);
    }
}

In the first example we take a list of people and then sort them in ascending age order:

List<Person> people = Arrays.asList(new Person("Paul", 24), new Person("Mark", 30), new Person("Will", 28));
people.stream().sorted((p1, p2) -> p1.age - p2.age).forEach(System.out::println);
Person{name='Paul', age=24}
Person{name='Will', age=28}
Person{name='Mark', age=30}

If we were to write a function to do the same thing in Java 7 it’d look like this:

Collections.sort(people, new Comparator<Person>() {
    @Override
    public int compare(Person o1, Person o2) {
        return o1.age - o2.age;
    }
});

for (Person person : people) {
    System.out.println(person);
}

Java 8 has reduced the amount of code we have to write although it’s still more complicated than what we could do in Ruby:

> people = [ {:name => "Paul", :age => 24}, {:name => "Mark", :age => 30}, {:name => "Will", :age => 28}]
> people.sort_by { |p| p[:age] }
=> [{:name=>"Paul", :age=>24}, {:name=>"Will", :age=>28}, {:name=>"Mark", :age=>30}]

A few pages later Venkat shows how you can get close to this by using the Comparator#comparing function:

Function<Person, Integer> byAge = p -> p.age ;
people.stream().sorted(comparing(byAge)).forEach(System.out::println);

I thought I could make this simpler by inlining the ‘byAge’ lambda like this:

people.stream().sorted(comparing(p -> p.age)).forEach(System.out::println);

This seems to compile and run correctly although IntelliJ 13.0 suggests there is a ‘cyclic inference‘ problem. IntelliJ is happy if we explicitly cast the lambda like this:

people.stream().sorted(comparing((Function<Person, Integer>) p -> p.age)).forEach(System.out::println);

IntelliJ also seems happy if we explicitly type ‘p’ in the lambda, so I think I’ll go with that for the moment:

people.stream().sorted(comparing((Person p) -> p.age)).forEach(System.out::println);

 

Reference: Java 8: Sorting values in collections from our JCG partner Mark Needham at the Mark Needham Blog blog.

Do you want to know how to develop your skillset to become a Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you our best selling eBooks for FREE!

1. JPA Mini Book

2. JVM Troubleshooting Guide

3. JUnit Tutorial for Unit Testing

4. Java Annotations Tutorial

5. Java Interview Questions

6. Spring Interview Questions

7. Android UI Design

and many more ....

One Response to "Java 8: Sorting values in collections"

  1. Why says:

    Compared to Scala.

    case class Person(name: String, age: Int)
    val people = List(Person(“Paul”, 24), Person(“Mark”, 30), Person(“Will”, 28))
    people.sortBy(_.age).foreach(prinln)

    Java syntax is way to verbose and repetitive.

Leave a Reply


seven × 6 =



Java Code Geeks and all content copyright © 2010-2015, Exelixis Media Ltd | Terms of Use | Privacy Policy | Contact
All trademarks and registered trademarks appearing on Java Code Geeks are the property of their respective owners.
Java is a trademark or registered trademark of Oracle Corporation in the United States and other countries.
Java Code Geeks is not connected to Oracle Corporation and is not sponsored by Oracle Corporation.
Do you want to know how to develop your skillset and become a ...
Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you our best selling eBooks for FREE!

Get ready to Rock!
To download the books, please verify your email address by following the instructions found on the email we just sent you.

THANK YOU!

Close