[java] Sorting a list with stream.sorted() in Java

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 sorting.

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());
}

This might help for people ending up here searching how to sort list alphabetically.

import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

public class SortService {
    public static void main(String[] args) {
        List<TestData> test = new ArrayList<>();
        test.add(prepareTestData("Asmin",1));
        test.add(prepareTestData("saurav",4));
        test.add(prepareTestData("asmin",2));
        test.add(prepareTestData("Saurav",3));

        test.forEach(data-> System.out.println(data));
        /** Output
         *  TestData(name=Asmin, id=1)
         *  TestData(name=saurav, id=4)
         *  TestData(name=asmin, id=2)
         *  TestData(name=Saurav, id=3)
         */

        test.sort(Comparator.comparing(TestData::getName,String::compareToIgnoreCase));
        test.forEach(data-> System.out.println(data));
        /**Sorted Output
         * TestData(name=Asmin, id=1)
         * TestData(name=asmin, id=2)
         * TestData(name=saurav, id=4)
         * TestData(name=Saurav, id=3)
         */
    }
    private static TestData prepareTestData(String name, int id){
        TestData testData= new TestData();
        testData.setId(id);
        testData.setName(name);
        return testData;
    }
}

@Getter
@Setter
@ToString
class TestData{
    private String name;
    private int id;
}

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?

This is a simple example :

List<String> citiesName = Arrays.asList( "Delhi","Mumbai","Chennai","Banglore","Kolkata");
System.out.println("Cities : "+citiesName);
List<String> sortedByName = citiesName.stream()
                .sorted((s1,s2)->s2.compareTo(s1))
                        .collect(Collectors.toList());
System.out.println("Sorted by Name : "+ sortedByName);

It may be possible that your IDE is not getting the jdk 1.8 or upper version to compile the code.

Set the Java version 1.8 for Your_Project > properties > Project Facets > Java version 1.8

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.

Collection<Map<Item, Integer>> itemCollection = basket.values();
Iterator<Map<Item, Integer>> itemIterator =   itemCollection.stream().sorted(new TestComparator()).collect(Collectors.toList()).iterator();



package com.ie.util;

import com.ie.item.Item;

import java.util.Comparator;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class TestComparator implements Comparator<Map<Item, Integer>> {

// comparator is used to sort the Items based on the price


    @Override
    public int compare(Map<Item, Integer> o1, Map<Item, Integer> o2) {


      //  System.out.println("*** compare method will be called *****");


        Item item1 = null;
        Item item2 = null;


        Set<Item> itemSet1 = o1.keySet();
        Iterator<Item> itemIterator1 = itemSet1.iterator();
        if(itemIterator1.hasNext()){
           item1 =   itemIterator1.next();
        }

        Set<Item> itemSet2 = o2.keySet();
        Iterator<Item> itemIterator2 = itemSet2.iterator();
        if(itemIterator2.hasNext()){
            item2 =   itemIterator2.next();
        }


        return -item1.getPrice().compareTo(item2.getPrice());


    }
}

**** this is helpful to sort the nested map objects like Map> here i sorted based on the Item object price .

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;
    }
}