Why there is no ConcurrentHashSet against ConcurrentHashMap

Question

HashSet is based on HashMap   If we look at HashSet lt E gt  implementation  everything is been managed under HashMap lt E Object gt     lt E gt  is used as a key of HashMap   And we know that HashMap is not thread safe  That is why we have ConcurrentHashMap in Java   Based on this  I am confused that why we don t have a ConcurrentHashSet which should be based on the ConcurrentHashMap   Is there anything else that I am missing  I need to use Set in a multi-threaded environment   Also  If I want to create my own ConcurrentHashSet can I achieve it by just replacing the HashMap to ConcurrentHashMap and leaving the rest as is

User · Answer

Set lt String gt  mySet   Collections newSetFromMap new ConcurrentHashMap lt String  Boolean gt

User · Answer

You can use guava s Sets newSetFromMap map  to get one  Java 6 also has that method in java util Collections

User · Answer

With Guava 15 you can also simply use   Set s   Sets newConcurrentHashSet

User · Answer

It looks like Java provides a concurrent Set implementation with its ConcurrentSkipListSet   A SkipList Set is just a special kind of set implementation   It still implements the Serializable  Cloneable  Iterable  Collection  NavigableSet  Set  SortedSet interfaces   This might work for you if you only need the Set interface

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import java util AbstractSet  import java util Iterator  import java util Set  import java util concurrent ConcurrentHashMap  import java util concurrent ConcurrentMap    public class ConcurrentHashSet lt E gt  extends AbstractSet lt E gt  implements Set lt E gt      private final ConcurrentMap lt E  Object gt  theMap      private static final Object dummy   new Object        public ConcurrentHashSet          theMap   new ConcurrentHashMap lt E  Object gt               Override    public int size           return theMap size              Override    public Iterator lt E gt  iterator          return theMap keySet   iterator              Override    public boolean isEmpty          return theMap isEmpty              Override    public boolean add final E o         return theMap put o  ConcurrentHashSet dummy     null            Override    public boolean contains final Object o         return theMap containsKey o             Override    public void clear          theMap clear              Override    public boolean remove final Object o         return theMap remove o     ConcurrentHashSet dummy           public boolean addIfAbsent final E o         Object obj   theMap putIfAbsent o  ConcurrentHashSet dummy         return obj    null

User · Answer

As pointed by this the best way to obtain a concurrency-able HashSet is by means of Collections synchronizedSet    Set s   Collections synchronizedSet new HashSet          This worked for me and I haven t seen anybody really pointing to it   EDIT This is less efficient than the currently aproved solution  as Eugene points out  since it just wraps your set into a synchronized decorator  while a ConcurrentHashMap actually implements low-level concurrency and it can back your Set just as fine  So thanks to Mr  Stepanenkov for making that clear   http   docs oracle com javase 8 docs api java util Collections html synchronizedSet-java util Set-

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Like Ray Toal mentioned it is as easy as   Set lt String gt  myConcurrentSet   ConcurrentHashMap newKeySet

User · Answer

Why not use  CopyOnWriteArraySet from java util concurrent

User · Answer

There s no built in type for ConcurrentHashSet because you can always derive a set from a map  Since there are many types of maps  you use a method to produce a set from a given map  or map class    Prior to Java 8  you produce a concurrent hash set backed by a concurrent hash map  by using Collections newSetFromMap map   In Java 8  pointed out by  Matt   you can get a concurrent hash set view via ConcurrentHashMap newKeySet    This is a bit simpler than the old newSetFromMap which required you to pass in an empty map object  But it is specific to ConcurrentHashMap   Anyway  the Java designers could have created a new set interface every time a new map interface was created  but that pattern would be impossible to enforce when third parties create their own maps  It is better to have the static methods that derive new sets  that approach always works  even when you create your own map implementations

[java] Why there is no ConcurrentHashSet against ConcurrentHashMap

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