Sorting a list with stream.sorted() in Java
JavaListSortingJava StreamJava Problem Overview
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());
}
Java Solutions
Solution 1 - Java
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 - Java
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
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 - Java
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 - Java
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());
Solution 6 - Java
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
Solution 7 - Java
> 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;
}
Solution 8 - Java
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