How to sort an ArrayList

Question

I have a List of doubles in java and I want to sort ArrayList in descending order   Input ArrayList is as below   List lt Double gt  testList   new ArrayList     testList add 0 5   testList add 0 2   testList add 0 9   testList add 0 1   testList add 0 1   testList add 0 1   testList add 0 54   testList add 0 71   testList add 0 71   testList add 0 71   testList add 0 92   testList add 0 12   testList add 0 65   testList add 0 34   testList add 0 62     The out put should be like this  0 92 0 9 0 71 0 71 0 71 0 65 0 62 0 54 0 5 0 34 0 2 0 12 0 1 0 1 0 1

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yearList   arrayListOf       for  year in 1950 until 2021            yearList add year            yearList reverse       val list  ArrayList lt String gt    arrayListOf        for  year in yearList            list add year toString

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Here is a short cheatsheet that covers typical cases      sort list sort naturalOrder        sort  reversed  list sort reverseOrder        sort by field list sort comparing Type  getField       sort by field  reversed  list sort comparing Type  getField  reversed        sort by int field list sort comparingInt Type  getIntField       sort by double field  reversed  list sort comparingDouble Type  getDoubleField  reversed        sort by nullable field  nulls last  list sort comparing Type  getNullableField  nullsLast naturalOrder          two-level sort list sort comparing Type  getField1  thenComparing Type  getField2

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Descending   Collections sort mArrayList  new Comparator lt CustomData gt           Override     public int compare CustomData lhs  CustomData rhs               -1 - less than  1 - greater than  0 - equal  all inversed for descending         return lhs customInt  gt  rhs customInt   -1    lhs customInt  lt  rhs customInt    1   0

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You can do like this   List lt String gt  yourList   new ArrayList lt String gt     Collections sort yourList  Collections reverseOrder       Collection has a default Comparator that can help you with that   Also  if you want to use some Java 8 new features  you can do like that   List lt String gt  yourList   new ArrayList lt String gt     yourList   yourList stream   sorted Collections reverseOrder    collect Collectors toList

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Sorting an List    import java util Collections        Sort Asc Order    Collections sort NamAryVar         Sort Dsc Order    Collections sort NamAryVar  Collections reverseOrder             Reverse the order of List    Collections reverse NamAryVar

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With Java8 there is a default sort method on the List interface that will allow you to sort the collection if you provide a Comparator   You can easily sort the example in the question as follows   testList sort  a  b  - gt  Double compare b  a      Note  the args in the lambda are swapped when passed in to Double compare to ensure the sort is descending

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For your example  this will do the magic in Java 8  List lt Double gt  testList   new ArrayList    testList sort Comparator naturalOrder       But if you want to sort by some of the fields of the object you are sorting  you can do it easily by   testList sort Comparator comparing ClassName  getFieldName      or    testList sort Comparator comparing ClassName  getFieldName  reversed       or   testList stream   sorted Comparator comparing ClassName  getFieldName  reversed    collect Collectors toList       Sources   https   docs oracle com javase 8 docs api java util Comparator html

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You can use like that   ArrayList lt Group gt  groupList   new ArrayList lt  gt     Collections sort groupList  Collections reverseOrder     Collections reverse groupList

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Collections sort allows you to pass an instance of a Comparator which defines the sorting logic  So instead of sorting the list in natural order and then reversing it  one can simply pass Collections reverseOrder   to sort in order to sort the list in reverse order      import java util Collections  Collections sort testList  Collections reverseOrder       As mentioned by  Marco13  apart from being more idiomatic  and possibly more efficient   using the reverse order comparator makes sure that the sort is stable  meaning that the order of elements will not be changed when they are equal according to the comparator  whereas reversing will change the order

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An alternative way to order a List is using the Collections framework  in this case using the SortedSet  the bean in the list should implement Comparable  so Double is ok   List lt Double gt  testList      SortedSet lt Double gt  sortedSet  new TreeSet lt Double gt     for Double number  testList       sortedSet add number     orderedList new ArrayList sortedSet    In general  to order by an attribute of a bean in the list put all the elements of the list in a SortedMap  using as a key the attribute  then get the values   from the SortedMap  the attribute should implement Comparable   List lt Bean gt  testList      SortedMap lt AttributeType Bean gt  sortedMap  new TreeMap lt AttributeType  Bean gt     for Bean bean   testList       sortedMap put bean getAttribute   bean     orderedList new ArrayList sortedMap values

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With Eclipse Collections you could create a primitive double list  sort it and then reverse it to put it in descending order   This approach would avoid boxing the doubles     MutableDoubleList doubleList       DoubleLists mutable with          0 5  0 2  0 9  0 1  0 1  0 1  0 54  0 71          0 71  0 71  0 92  0 12  0 65  0 34  0 62           sortThis   reverseThis    doubleList each System out  println     If you want a List lt Double gt   then the following would work   List lt Double gt  objectList       Lists mutable with          0 5  0 2  0 9  0 1  0 1  0 1  0 54  0 71          0 71  0 71  0 92  0 12  0 65  0 34  0 62           sortThis Collections reverseOrder     objectList forEach System out  println     If you want to keep the type as ArrayList lt Double gt   you can initialize and sort the list using the ArrayListIterate utility class as follows   ArrayList lt Double gt  arrayList       ArrayListIterate sortThis              new ArrayList lt  gt  objectList   Collections reverseOrder     arrayList forEach System out  println     Note  I am a committer for Eclipse Collections

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if you are using Java SE 8  then this might be of help     create a comparator object using a Lambda expression Comparator lt Double gt  compareDouble    d1  d2  - gt  d1 compareTo d2      Sort the Collection in this case  testList  in reverse order Collections sort testList  Collections reverseOrder compareDouble       print the sorted list using method reference only applicable in SE 8 testList forEach System out  println

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Using lambdas  Java8   and stripping it down to the barest of syntax  the JVM will infer plenty in this case   you get   Collections sort testList   a  b  - gt  b compareTo a      A more verbose version       Implement a reverse-order Comparator by lambda function Comparator lt Double gt  comp    Double a  Double b  - gt        return b compareTo a       Collections sort testList  comp     The use of a lambda is possible because the Comparator interface has only a single method to implement  so the VM can infer which method is implementing  Since the types of the params can be inferred  they don t need to be stated  i e   a  b  instead of  Double a  Double b   And since the lambda body has only a single line  and the method is expected to return a value  the return is inferred and the braces aren t necessary

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The following line should do the thick  testList sort Collections reverseOrder

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Here is sorted List alphabetically with syncronized  package com mnas technology automation utility  import java util ArrayList  import java util Collections  import java util Comparator  import java util Iterator  import java util List   import org apache log4j Logger           author manoj kumar     public class SynchronizedArrayList       static Logger log   Logger getLogger SynchronizedArrayList class getName           SuppressWarnings  unchecked       public static void main String   args             List lt Employee gt  synchronizedList   Collections synchronizedList new ArrayList lt Employee gt              synchronizedList add new Employee  Aditya             synchronizedList add new Employee  Siddharth             synchronizedList add new Employee  Manoj             Collections sort synchronizedList  new Comparator                 public int compare Object synchronizedListOne  Object synchronizedListTwo                      use instanceof to verify the references are indeed of the type in question                 return   Employee  synchronizedListOne  name                          compareTo   Employee  synchronizedListTwo  name                                    for  Employee sd   synchronizedList        log info  Sorted Synchronized Array List     sd name                       when iterating over a synchronized list  we need to synchronize access to the synchronized list         synchronized  synchronizedList                Iterator lt Employee gt  iterator   synchronizedList iterator                while  iterator hasNext                      log info  Sorted Synchronized Array List Items      iterator next   name                                     class Employee       String name       Employee String name            this name   name

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In JAVA 8 its much easy now   List lt String gt  alphaNumbers   Arrays asList  one    two    three    four    List lt String gt  alphaNumbersUpperCase   alphaNumbers stream        map String  toUpperCase       sorted        collect Collectors toList     System out println alphaNumbersUpperCase       FOUR  ONE  THREE  TWO    -- For reverse use this   sorted Comparator reverseOrder

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Use util method of java util Collections class  i e  Collections sort list    In fact  if you want to sort custom object you can use  Collections sort List lt T gt  list  Comparator lt   super T gt  c     see collections api

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You can use Collections sort list  to sort list if your list contains Comparable elements  Otherwise I would recommend you to implement that interface like here   public class Circle implements Comparable lt Circle gt       and of course provide your own realization of compareTo method like here    Override     public int compareTo Circle another            if  this getD   lt another getD                 return -1           else              return 1                    And then you can again use Colection sort list  as now list contains objects of Comparable type and can be sorted  Order depends on compareTo method  Check this https   docs oracle com javase tutorial collections interfaces order html for more detailed information

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For example I have a class Person  String name  int age    Constructor new Person name age    import java util Collections  import java util ArrayList  import java util Arrays    public void main String   args       Person ibrahima new Person  Timera  40       Person toto new Person  Toto  35       Person alex new Person  Alex  50       ArrayList lt Person gt  myList new ArrayList lt Person gt      Collections sort myList  new Comparator lt Person gt               Override         public int compare Person p1  Person p2                   return p1 age    compareTo p2 age        sort by age             return p1 name compareTo p2 name      if you want to short by name                       System out println myList toString            Person  name Alex  age 50   Person  name Timera  age 40   Person  name Toto  age 35       Collections reverse myList       System out println myList toString            Person  name Toto  age 35   Person  name Timera  age 40   Person  name Alex  age 50

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Collections sort testList   Collections reverse testList     That will do what you want  Remember to import Collections though   Here is the documentation for Collections

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If you have to sort object based on its id in the ArrayList   then use java8 stream   List lt Person gt  personList   new ArrayList lt  gt          List lt Person gt  personListSorted                   personList stream                      sorted Comparator comparing Person  getPersonId                      collect Collectors toList

[java] How to sort an ArrayList?

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