How does HashMap works in Java

• Array created with default capacity of 16
• Then getting hash code of the key
• It rehashes the hash code to prevent against a bad hashing function from the key that would put all data in the same index (bucket) of the inner array
• It takes the rehashed hash hashcode and bit-masks it with the length (minus 1) of the array. This operation assures that the index can’t be greater than the size of the array. You can see it as a very computationally optimized modulo function.

• Finding appreciate array index according to hash code and saving in bucket associated with this index
• In Java 8 , if bucket size more than 8 automatically converting that bucket from linked list to read black tree
• Can auto size the map according to load factor. Initial arrays size is 16 and load factor is 0.75
• HashMap is not thread safe but HashTable is thread safe but locks whole data structure during concurrent access.  On the other hand , ConcurrentHashMap is locking only bucket
• Mostly Integer and String used as map key because they immutable and provide string hash code function
• If you have too many data to put on Map , it is advisable to create map with approximate high initial capacity. Because there is additional overhead of shrinking the map

Source : http://coding-geek.com/how-does-a-hashmap-work-in-java/

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