Hashcode and Equals for Hashset

Question

Please clarify my doubt in Hashset  Consider the following code   class Person       String name       Person String n                name n             public String getName                 return name                 Override     public boolean equals Object arg0             System out println  in equals             Person obj  Person arg0           System out println  1st   getName             System out println  2nd   obj getName              if this getName   equals obj getName                               return true                    return false               Override     public int hashCode              System out println  in hash code            System out println   value is   Integer valueOf name charAt 0             return Integer valueOf name charAt 0              in main I have the following code  Person obj1 new Person  bcd     Person obj2 new Person  cde     Person obj3 new Person  abc     Person obj4 new Person  abc      Now if I add these objects to hashset  Set lt Person gt  sset new HashSet lt Person gt      sset add obj1   sset add obj4   sset add obj2   sset add obj3     I am getting this output  in hash code                                                                       value is 98     in hash code    value is 97     in hash code     value is 99     in hash code     value is 97   in equals   1st abc      2nd abc   Question 1   why equals   function is called only once for checking obj3 and obj4   Why its not checked for rest of the objects    Question 2   If the answer is because they both have same hash code only then equals will be called  then why its not called for below code  sset add obj1   sset add obj4   sset add obj2   sset add obj4     output is     in hash code   value is 98   in hash code    value is 97    in hash code    value is 99    in hash code   value is 97    It s not going inside equals   method even though two same objects are added to hash set which has same hash code   Question 3  I iterated the above value and printed the contents but neither hashcode nor equals were called  when its really useful to override hashcode and equals method    Question 4   When will hashCode   and equals   be called

User · Answer

I think your questions will all be answered if you understand how Sets, and in particular HashSets work. A set is a collection of unique objects, with Java defining uniqueness in that it doesn't equal anything else (equals returns false).

The HashSet takes advantage of hashcodes to speed things up. It assumes that two objects that equal eachother will have the same hash code. However it does not assume that two objects with the same hash code mean they are equal. This is why when it detects a colliding hash code, it only compares with other objects (in your case one) in the set with the same hash code.

User · Answer

There s no need to call equals if hashCode differs  There s no need to call hashCode if  obj1    obj2   There s no need for hashCode and or equals just to iterate - you re not comparing objects When needed to distinguish in between objects

User · Answer

according jdk source code from javasourcecode org  HashSet use HashMap as its inside implementation  the code about put method of HashSet is below    public V put K key  V value            if  key    null              return putForNullKey value           int hash   hash key hashCode             int i   indexFor hash  table length           for  Entry lt K V gt  e   table i   e    null  e   e next                Object k              if  e hash    hash  amp  amp    k   e key     key    key equals k                      V oldValue   e value                  e value   value                  e recordAccess this                   return oldValue                                   modCount            addEntry hash  key  value  i           return null          The rule is firstly check the hash  then check the reference and then call equals method of the object will be putted in

User · Answer

You should read up on how to ensure that you ve implemented equals and hashCode properly  This is a good starting point  What issues should be considered when overriding equals and hashCode in Java

User · Answer

Because in 2nd case you adding same reference twice and HashSet have check against this in HashMap put   on which HashSet is based           if  e hash    hash  amp  amp    k   e key     key    key equals k                  V oldValue   e value              e value   value              e recordAccess this               return oldValue              As you can see  equals will be called only if hash of key being added equals to the key already present in set and references of these two are different

[java] Hashcode and Equals for Hashset

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