Easiest way to convert a List to a Set in Java

Question

What is the easiest way to convert a List to a Set in Java

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Let s not forget our relatively new friend  java-8 stream API  If you need to preprocess list before converting it to a set  it s better to have something like   list stream    lt here goes some preprocessing gt  collect Collectors toSet

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For Java 8 it s very easy   List  lt  UserEntity  gt  vList  new ArrayList lt  gt      vList  service       Set lt UserEntity gt  vSet  vList stream   collect Collectors toSet

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You can convert List lt  gt  to Set lt  gt   Set lt T gt  set new HashSet lt T gt        Added dependency - gt  If list is null then it will throw NullPointerExcetion   Set lt T gt  set  if list    null       set   new HashSet lt T gt  list

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I agree with sepp2k  but there are some other details that might matter   new HashSet lt Foo gt  myList     will give you an unsorted set which doesn t have duplicates  In this case  duplication is identified using the  equals   method on your objects  This is done in combination with the  hashCode   method   For more on equality look here   An alternative that gives a sorted set is   new TreeSet lt Foo gt  myList     This works if Foo implements Comparable  If it doesn t then you may want to use a comparator   Set lt Foo gt  lSet   new TreeSet lt Foo gt  someComparator   lSet addAll myList     This depends on either compareTo    from the comparable interface  or compare    from the comparator  to ensure uniqueness  So  if you just care about uniqueness  use the HashSet  If you re after sorting  then consider the TreeSet   Remember  Optimize later   If time efficiency matters use a HashSet if space efficiency matters  look at TreeSet  Note that more efficient implementations of Set and Map are available through Trove  and other locations

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With Java 10  you could now use Set copyOf to easily convert a List lt E gt  to an unmodifiable Set lt E gt    Example   var set   Set copyOf list     Keep in mind that this is an unordered operation  and null elements are not permitted  as it will throw a NullPointerException   If you wish for it to be modifiable  then simply pass it into the constructor a Set implementation

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If you use the Guava library   Set lt Foo gt  set   Sets newHashSet list     or  better   Set lt Foo gt  set   ImmutableSet copyOf list

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Using java 8 you can use stream   List lt Integer gt  mylist   Arrays asList 100  101  102   Set lt Integer gt  myset   mylist stream   collect Collectors toSet

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There are various ways to get a Set as       List lt Integer gt  sourceList   new ArrayList        sourceList add 1       sourceList add 2       sourceList add 3       sourceList add 4           Using Core Java     Set lt Integer gt  set1   new HashSet lt  gt  sourceList      needs null-check if sourceList can be null          Java 8     Set lt Integer gt  set2   sourceList stream   collect Collectors toSet         Set lt Integer gt  set3   sourceList stream   collect Collectors toCollection HashSet  new           Guava     Set lt Integer gt  set4   Sets newHashSet sourceList           Apache commons     Set lt Integer gt  set5   new HashSet lt  gt  4       CollectionUtils addAll set5  sourceList     When we use Collectors toSet   it returns a set and as per the doc There are no guarantees on the type  mutability  serializability  or thread-safety of the Set returned  If we want to get a HashSet then we can use the other alternative to get a set  check set3

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If you use Eclipse Collections   MutableSet lt Integer gt  mSet   Lists mutable with 1  2  3  toSet    MutableIntSet mIntSet   IntLists mutable with 1  2  3  toSet      The MutableSet interface extends java util Set whereas the MutableIntSet interface does not  You can also convert any Iterable to a Set using the Sets factory class   Set lt Integer gt  set   Sets mutable withAll List of 1  2  3      There is more explanation of the mutable factories available in Eclipse Collections here   If you want an ImmutableSet from a List  you can use the Sets factory as follows   ImmutableSet lt Integer gt  immutableSet   Sets immutable withAll List of 1  2  3     Note  I am a committer for Eclipse Collections

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A more Java 8 resilient solution with Optional ofNullable  Set lt Foo gt  mySet   Optional ofNullable myList  map HashSet  new  orElse null

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Java- addAll  set addAll aList     Java- new Object  new HashSet list    Java-8  list stream   collect Collectors toSet       Using Guva    Sets newHashSet list    Apache Commons  CollectionUtils addAll targetSet  sourceList     Java 10  var set   Set copyOf list

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Remember that  converting from List to Set will remove duplicates from collection because List supports duplicates but Set does not support duplicates in Java   Direct Conversion   The most common and simple way to convert a List to a Set     Creating a list of strings List lt String gt  list   Arrays asList  One    Two    Three    Four        Converting a list to set Set lt String gt  set   new HashSet lt  gt  list     Apache Commons Collections   You may also use the Commons Collections API to convert a List to a Set  -     Creating a list of strings List lt String gt  list   Arrays asList  One    Two    Three    Four        Creating a set with the same number of members in the list  Set lt String gt  set   new HashSet lt  gt  4       Adds all of the elements in the list to the target set CollectionUtils addAll set  list     Using Stream   Another way is to convert given list to stream  then stream to set  -     Creating a list of strings  List lt String gt  list   Arrays asList  One    Two    Three    Four         Converting to set using stream  Set lt String gt  set   list stream   collect Collectors toSet

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The best way to use constructor   Set s  new HashSet list     In java 8 you can also use stream api    Set s  list stream   collect Collectors toSet

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Set lt Foo gt  foo   new HashSet lt Foo gt  myList

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I would perform a Null check before converting to set    if myList    null   Set lt Foo gt  foo   new HashSet lt Foo gt  myList

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Set lt E gt  alphaSet    new HashSet lt E gt   lt your List gt      or complete example  import java util ArrayList  import java util HashSet  import java util List  import java util Set   public class ListToSet       public static void main String   args                List lt String gt  alphaList   new ArrayList lt String gt             alphaList add  A            alphaList add  B            alphaList add  C            alphaList add  A            alphaList add  B            System out println  List values                  for  String alpha   alphaList                        System out println alpha                     Set lt String gt  alphaSet   new HashSet lt String gt  alphaList           System out println   nSet values                  for  String alpha   alphaSet                        System out println alpha

[java] Easiest way to convert a List to a Set in Java

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