Does adding a duplicate value to a HashSet HashMap replace the previous value

Question

Please consider the below piece of code   HashSet hs   new HashSet    hs add  hi    --  1  hs add  hi    --  2    hs size   will give 1 as HashSet doesn t allow duplicates so only one element will be stored   I want to know if we add the duplicate element  then does it replace the previous element or it simply doesn t add it     Also  what will happen usingHashMap for the same case

User · Answer

The docs are pretty clear on this  HashSet add doesn t replace      Adds the specified element to this set if it is not already present  More formally  adds the specified element e to this set if this set contains no element e2 such that  e  null   e2  null   e equals e2    If this set already contains the element  the call leaves the set unchanged and returns false     But HashMap put will replace      If the map previously contained a mapping for the key  the old value is replaced

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The first thing you need to know is that HashSet acts like a Set  which means you add your object directly to the HashSet and it cannot contain duplicates  You just add your value directly in HashSet    However  HashMap is a Map type  That means every time you add an entry  you add a key-value pair   In HashMap you can have duplicate values  but not duplicate keys  In HashMap the new entry will replace the old one  The most recent entry will be in the HashMap   Understanding Link between HashMap and HashSet   Remember  HashMap can not have duplicate keys  Behind the scene HashSet uses a HashMap   When you attempt to add any object into a HashSet  this entry is actually stored as a key in the HashMap - the same HashMap that is used behind the scene of HashSet  Since this underlying HashMap needs a key-value pair  a dummy value is generated for us   Now when you try to insert another duplicate object into the same HashSet  it will again attempt to be insert it as a key in the HashMap lying underneath  However  HashMap does not support duplicates  Hence  HashSet will still result in having only one value of that type  As a side note  for every duplicate key  since the value generated for our entry in HashSet is some random dummy value  the key is not replaced at all  it will be ignored as removing the key and adding back the same key  the dummy value is the same  would not make any sense at all   Summary   HashMap allows duplicate values  but not keys   HashSet cannot contains duplicates    To play with whether the addition of an object is successfully completed or not  you can check the boolean value returned when you call  add    and see if it returns true or false  If it returned true  it was inserted

User · Answer

Correct me if I m wrong but what you re getting at is that with strings   Hi      Hi  doesn t always come out true  because they re not necessarily the same object    The reason you re getting an answer of 1 though is because the JVM will reuse strings objects where possible   In this case the JVM is reusing the string object  and thus overwriting the item in the Hashmap Hashset     But you aren t guaranteed this behavior  because it could be a different string object that has the same value  Hi     The behavior you see is just because of the JVM s optimization

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HashMap basically contains Entry which subsequently contains Key Object  and Value Object  Internally HashSet are HashMap and HashMap do replace values as some of you already pointed  but does it really replaces the keys   No   and that is the trick here  HashMap keeps its value as key in the underlying HashMap and value is just a dummy object So if u try to reinsert same Value in HashMap Key in underlying Map  It just replaces the dummy value and not the Key Value for HashSet    Look at the below code for HashSet Class   public boolean   More      add E e        return map put e  PRESENT   null      Here e is the value for HashSet but key for underlying map and key is never replaced  Hope i am able to clear the confusion

User · Answer

You need to check put method in Hash map first as HashSet is backed up by HashMap   When you add duplicate value say a String  One  into HashSet  An entry   one   PRESENT  will get inserted into Hashmap for all the values added into set  the value will be  PRESENT  which if of type Object  Hashmap adds the entry into Map and returns the value  which is in this case  PRESENT  or null if Entry is not there  Hashset s add method then returns true if the returned value from Hashmap equals null otherwise false which means an entry already exists

User · Answer

To say it differently  When you insert a key-value-pair into a HashMap where the key already exists  in a sense hashvalue   gives the same value und equal   is true  but the two objects can still differ in several ways   the key isn t replaced but the value is overwritten  The key is just used to get the hashvalue   and find the value in the table with it  Since HashSet uses the keys of a HashMap and sets arbitrary values which don t really matter  to the user  as a result the Elements of the Set aren t replaced either

User · Answer

In the case of HashMap  it replaces the old value with the new one     In the case of HashSet  the item isn t inserted

User · Answer

It the case of HashSet  it does NOT replace it   From the docs   http   docs oracle com javase 6 docs api java util HashSet html add E    Adds the specified element to this set if it is not already present  More formally  adds the specified element e to this set if this set contains no element e2 such that  e  null   e2  null   e equals e2    If this set already contains the element  the call leaves the set unchanged and returns false

[java] Does adding a duplicate value to a HashSet/HashMap replace the previous value

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