Sorting a list with stream.sorted() in Java

Question

I'm interested in sorting a list from a stream. This is the code I'm using:

list.stream()
    .sorted((o1, o2)->o1.getItem().getValue().compareTo(o2.getItem().getValue()))
    .collect(Collectors.toList());

Am I missing something? The list is not sorted afterward.

It should sort the lists according to the item with the lowest value.

for (int i = 0; i < list.size(); i++)
{
   System.out.println("list " + (i+1));
   print(list, i);
}

And the print method:

public static void print(List<List> list, int i)
{
    System.out.println(list.get(i).getItem().getValue());
}

Solution 1

This is not like Collections.sort() where the parameter reference gets sorted. In this case you just get a sorted stream that you need to collect and assign to another variable eventually:

List result = list.stream().sorted((o1, o2)->o1.getItem().getValue().
                                   compareTo(o2.getItem().getValue())).
                                   collect(Collectors.toList());

You've just missed to assign the result

Solution 2

Use list.sort instead:

list.sort((o1, o2) -> o1.getItem().getValue().compareTo(o2.getItem().getValue()));

and make it more succinct using Comparator.comparing:

list.sort(Comparator.comparing(o -> o.getItem().getValue()));

After either of these, list itself will be sorted.

Your issue is that list.stream.sorted returns the sorted data, it doesn't sort in place as you're expecting.

Solution 3

Java 8 provides different utility api methods to help us sort the streams better.

If your list is a list of Integers(or Double, Long, String etc.,) then you can simply sort the list with default comparators provided by java.

List<Integer> integerList = Arrays.asList(1, 4, 3, 4, 5);

Creating comparator on fly:

integerList.stream().sorted((i1, i2) -> i1.compareTo(i2)).forEach(System.out::println);

With default comparator provided by java 8 when no argument passed to sorted():

integerList.stream().sorted().forEach(System.out::println); //Natural order

If you want to sort the same list in reverse order:

 integerList.stream().sorted(Comparator.reverseOrder()).forEach(System.out::println); // Reverse Order

If your list is a list of user defined objects, then:

List<Person> personList = Arrays.asList(new Person(1000, "First", 25, 30000),
        new Person(2000, "Second", 30, 45000),
        new Person(3000, "Third", 35, 25000));

Creating comparator on fly:

personList.stream().sorted((p1, p2) -> ((Long)p1.getPersonId()).compareTo(p2.getPersonId()))
        .forEach(person -> System.out.println(person.getName()));

Using Comparator.comparingLong() method(We have comparingDouble(), comparingInt() methods too):

personList.stream().sorted(Comparator.comparingLong(Person::getPersonId)).forEach(person -> System.out.println(person.getName()));

Using Comparator.comparing() method(Generic method which compares based on the getter method provided):

personList.stream().sorted(Comparator.comparing(Person::getPersonId)).forEach(person -> System.out.println(person.getName()));

We can do chaining too using thenComparing() method:

personList.stream().sorted(Comparator.comparing(Person::getPersonId).thenComparing(Person::getAge)).forEach(person -> System.out.println(person.getName())); //Sorting by person id and then by age.

Person class

public class Person {
    private long personId;
    private String name;
    private int age;
    private double salary;

    public long getPersonId() {
        return personId;
    }

    public void setPersonId(long personId) {
        this.personId = personId;
    }

    public Person(long personId, String name, int age, double salary) {
        this.personId = personId;
        this.name = name;
        this.age = age;

        this.salary = salary;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

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

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }
}

Solution 4

It seems to be working fine:

List<BigDecimal> list = Arrays.asList(new BigDecimal("24.455"), new BigDecimal("23.455"), new BigDecimal("28.455"), new BigDecimal("20.455"));
System.out.println("Unsorted list: " + list);
final List<BigDecimal> sortedList = list.stream().sorted((o1, o2) -> o1.compareTo(o2)).collect(Collectors.toList());
System.out.println("Sorted list: " + sortedList);

Example Input/Output

Unsorted list: [24.455, 23.455, 28.455, 20.455]
Sorted list: [20.455, 23.455, 24.455, 28.455]

Are you sure you are not verifying list instead of sortedList [in above example] i.e. you are storing the result of stream() in a new List object and verifying that object?

Solution 5

sorting Integer using streamAPI

arr.stream()
.sorted((item1,item2)-> Integer.compare(item1.price, item2.price))
.forEach(item-> item.show());
//asc
System.out.println("--------------------");
//desc
arr.stream()
.sorted((item1,item2)-> item1.price<item2.price?1:-1)
.forEach(item->item.show());

View more solutions

Comments

[Neuron]

list.sort(Comparator.comparing(o -> o.getItem().getValue())); was new to me. Great!

[River]

I feel like this answer is way too detailed for the question, and yet doesn't address the question at all. Consider using this answer in a self-answered Q/A instead.

[kakabali]

I also feel that this is the best and correct answer. using Comparator.comparing* is the way better and more JDK8 oriented approach

[jmizv]

Looks like you don't take advantage of any stream related code.

[Gaurav]

works flawlessly!