Difference between HashSet and HashMap

Question

Apart from the fact that HashSet does not allow duplicate values  what is the difference between HashMap and HashSet   I mean implementation wise  It s a little bit vague because both use hash tables to store values

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It s really a shame that both their names start with Hash   That s the least important part of them   The important parts come after the Hash - the Set and Map  as others have pointed out   What they are  respectively  are a Set - an unordered collection - and a Map - a collection with keyed access   They happen to be implemented with hashes - that s where the names come from - but their essence is hidden behind that part of their names   Don t be confused by their names  they are deeply different things

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you pretty much answered your own question - hashset doesn t allow duplicate values  it would be trivial to build a hashset using a backing hashmap  and just a check to see if the value already exists   i guess the various java implementations either do that  or implement some custom code to do it more efficiently

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The main difference between them you can find as follows  HashSet  It does not allow duplicate keys  Even it is not synchronized  so this will have better performance  It allows a null key  HashSet can be used when you want to maintain a unique list  HashSet implements Set interface and it is backed by the hash table actually HashMap instance   HashSet stores objects  HashSet doesn   t allow duplicate elements but null values are allowed  This interface doesn   t guarantee that order will remain constant over time   HashMap  It allows duplicate keys  It is not synchronized  so this will have better performance  HashMap does not maintain insertion order  The order is defined by the Hash function  It is not Thread Safe It allows null for both key and value  It allows one null key and as many null values as you like  HashMap is a Hash table-based implementation of the Map interface  HashMap store object as key and value pair  HashMap does not allow duplicate keys but null keys and values are allowed  Ordering of the element is not guaranteed overtime

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Differences  with respect to heirarchy   HashSet implements Set  HashMap implements Map and stores a mapping of keys and values   A use of HashSet and HashMap with respect to database would help you understand the significance of each   HashSet  is generally used for storing unique collection objects  E g  It might be used as implementation class for storing many-to-one relation ship between  class Item and Class Bid where  Item has many Bids  HashMap  is used to map a key to value the value may be null or any Object  list of Object  which is object in itself

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HashSet   HashSet class implements the Set interface In HashSet  we store objects elements or values  e g  If we have a HashSet of string elements then it could depict a set of HashSet elements      Hello        Hi        Bye        Run     HashSet does not allow duplicate elements that mean you can not store duplicate values in HashSet    HashSet permits to have a single null value  HashSet is not synchronized which means they are not suitable for thread-safe operations until unless synchronized explicitly  similarity                         add      contains next     notes HashSet               O 1      O 1      O h n    h is the table     HashMap   HashMap class implements the Map interface HashMap is used for storing key  amp  value pairs  In short  it maintains the mapping of key  amp  value  The HashMap class is roughly equivalent to Hashtable  except that it is unsynchronized and permits nulls   This is how you could represent HashMap elements if it has integer key and value of String type  e g   1-    Hello     2-    Hi     3-    Bye     4-    Run     HashMap does not allow duplicate keys however it allows having duplicate values  HashMap permits single null key and any number of null values  HashMap is not synchronized which means they are not suitable for thread-safe operations until unless synchronized explicitly  similarity                          get      containsKey next     Notes  HashMap               O 1      O 1         O h n    h is the table     Please refer this article to find more information

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HashMap is a Map implementation  allowing duplicate values but not duplicate keys   For adding an object a Key Value pair is required  Null Keys and Null values are allowed  eg       The- 3 world- 5 is- 2 nice- 4    HashSet is a Set implementation which does not allow duplicates If you tried to add a duplicate object  a call to public boolean add Object o  method  then the set remains unchanged and returns false  eg       The world is nice

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HashMaps allow one null key and null values  They are not synchronized  which increases efficiency  If it is required  you can make them synchronized using Collections SynchronizedMap    Hashtables don t allow null keys and are synchronized

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As the names imply  a HashMap  is an associative Map  mapping from a key to a value   a HashSet is just a Set

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A HashMap is to add  get  remove      objects indexed by a custom key of any type  A HashSet is to add elements  remove elements and check if elements are present by comparing their hashes   So a HashMap contains the elements and a HashSet remembers their hashes

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Basically in HashMap  user has to provide both Key and Value  whereas in HashSet you provide only Value  the Key is derived automatically from Value by using hash function  So after having both Key and Value  HashSet can be stored as HashMap internally

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A HashSet is implemented in terms of a HashMap   It s a mapping between the key and a PRESENT object

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HashSet allows us to store objects in the set where as HashMap allows us to store objects on the basis of key and value  Every object or stored object will be having key

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The Hashset Internally implements HashMap  If you see the internal implementation the values inserted in HashSet are stored as keys in the HashMap and the value is a Dummy object of Object class  Difference between HashMap vs HashSet is -     HashMap contains key value pairs and each value can be accessed by key where as HashSet needs to be iterated everytime as there is no get method  HashMap implements Map interface and allows one null value as a key and multiple null values as values Where as HashSet implements Set interface  allows only one null value and no duplicated values  Remeber one null key is allowed in HashMap key hence one null value in HashSet as HashSet implemements HashMap internally     HashSet and HashMap does not maintain the order of insertion while iterating

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They are entirely different constructs   A HashMap is an implementation of Map   A Map maps keys to values   The key look up occurs using the hash   On the other hand  a HashSet is an implementation of Set   A Set is designed to match the mathematical model of a set   A HashSet does use a HashMap to back its implementation  as you noted   However  it implements an entirely different interface   When you are looking for what will be the best Collection for your purposes  this Tutorial is a good starting place   If you truly want to know what s going on  there s a book for that  too

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Differences between HashSet and HashMap in Java  HashSet internally uses HashMap to store objects when add String  method called it calls HahsMap put key value  method where key String object  amp  value new Object Dummy  so it maintain no duplicates because keys are nothing but Value Object   the Objects which are stored as key in Hashset HashMap should override hashcode  amp  equals contract    Keys which are used to access store value objects in HashMap should declared as Final because when it is modified Value object can t be located  amp  returns null

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HashMap  is a implementation of Map interface HashSet is an implementation of Set Interface  HashMap Stores data in form of  key value pair HashSet Store only objects  Put method is used to add element in map Add method is used to add element is Set  In hash map hashcode value is calculated using key object Here member object is used for calculating hashcode value which can be same for two objects so equal    method is used to check for equality if it returns false that means two objects are different   HashMap is faster than hashset because unique key is used to access object HashSet is slower than Hashmap

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Differences between HashSet and HashMap in Java  1  First and most significant difference between HashMap and HashSet is that HashMap is an implementation of Map interface while HashSet is an implementation of Set interface  which means HashMap is a key value based data-structure and HashSet guarantees uniqueness by not allowing duplicates In reality HashSet is a wrapper around HashMap in Java  if you look at the code of add E e  method of HashSet java you will see following code    public boolean add E e         return map put e  PRESENT   null      where its putting  Object into map as key and value is an final object PRESENT which is dummy   2  Second difference between HashMap and HashSet is that   we use add   method to put elements into Set but we use put   method to insert key and value into HashMap in Java   3  HashSet allows only one null key  but HashMap can allow one null key   multiple null values   That s all on difference between HashSet and HashMap in Java  In summary HashSet and HashMap are two different type of Collection one being Set and other being Map

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HashSet and HashMap both store  pairs   the difference lies that in HashMap you can specify a key while in HashSet the key comes from object s hash code

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A HashSet uses a HashMap internally to store its entries   Each entry in the internal HashMap is keyed by a single Object  so all entries hash into the same bucket   I don t recall what the internal HashMap uses to store its values  but it doesn t really matter since that internal container will never contain duplicate values   EDIT   To address Matthew s comment  he s right  I had it backwards   The internal HashMap is keyed with the Objects that make up the Set elements   The values of the HashMap are an Object that s just simply stored in the HashMap buckets

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HashSet is a set  e g   1 2 3 4 5   HashMap is a key -  value  key to value  map  e g   a -  1  b -  2  c -  2  d -  1   Notice in my example above that in the HashMap there must not be duplicate keys  but it may have duplicate values   In the HashSet  there must be no duplicate elements

[java] Difference between HashSet and HashMap?

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