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 - gt 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  lt  list size    i         System out println  list      i 1       print list  i       And the print method   public static void print List lt List gt  list  int i        System out println list get i  getItem   getValue

User · Answer

Use list sort instead   list sort  o1  o2  - gt  o1 getItem   getValue   compareTo o2 getItem   getValue        and make it more succinct using Comparator comparing   list sort Comparator comparing o - gt  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

User · Answer

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 lt Integer gt  integerList   Arrays asList 1  4  3  4  5     Creating comparator on fly   integerList stream   sorted  i1  i2  - gt  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 lt Person gt  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  - gt    Long p1 getPersonId    compareTo p2 getPersonId              forEach person - gt  System out println person getName        Using Comparator comparingLong   method We have comparingDouble    comparingInt   methods too    personList stream   sorted Comparator comparingLong Person  getPersonId   forEach person - gt  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 - gt  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 - gt  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

User · Answer

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 - gt o1 getItem   getValue                                       compareTo o2 getItem   getValue                                         collect Collectors toList       You ve just missed to assign the result

User · Answer

Collection lt Map lt Item  Integer gt  gt  itemCollection   basket values    Iterator lt Map lt Item  Integer gt  gt  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 lt Map lt Item  Integer gt  gt        comparator is used to sort the Items based on the price        Override     public int compare Map lt Item  Integer gt  o1  Map lt Item  Integer gt  o2                System out println      compare method will be called                    Item item1   null          Item item2   null            Set lt Item gt  itemSet1   o1 keySet            Iterator lt Item gt  itemIterator1   itemSet1 iterator            if itemIterator1 hasNext                item1     itemIterator1 next                       Set lt Item gt  itemSet2   o2 keySet            Iterator lt Item gt  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

User · Answer

It seems to be working fine   List lt BigDecimal gt  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 lt BigDecimal gt  sortedList   list stream   sorted  o1  o2  - gt  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

User · Answer

This is a simple example    List lt String gt  citiesName   Arrays asList   Delhi   Mumbai   Chennai   Banglore   Kolkata    System out println  Cities     citiesName   List lt String gt  sortedByName   citiesName stream                    sorted  s1 s2 - gt 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

User · Answer

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 lt TestData gt  test   new ArrayList lt  gt             test add prepareTestData  quot Asmin quot  1            test add prepareTestData  quot saurav quot  4            test add prepareTestData  quot asmin quot  2            test add prepareTestData  quot Saurav quot  3             test forEach data- gt  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- gt  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

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

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