Data structure type 1 : Array of linked lists Until the version of Java 7, Hash Maps were implemented using a hash table (array) with chaining: each position in the array contains a linked list to store all the entries whose keys share the same hashcode. Details about this class and its methods can be found here https://docs.oracle.com/iavase/7/docs/api/iava/util/HashMap.html index (size -1) index 0 Entry A Entry B Entry C index 1 null EntryZArray of linked lists (or buckets/bins) null Nullable linked lists null Data structure type 1 : Array of linked lists Until the version of Java 7, Hash Maps were implemented using a hash table (array) with chaining: each position in the array contains a linked list to store all the entries whose keys share the same hashcode. Details about this class and its methods can be found here https://docs.oracle.com/iavase/7/docs/api/iava/util/HashMap.html index (size -1) index 0 Entry A Entry B Entry C index 1 null EntryZArray of linked lists (or buckets/bins) null Nullable linked lists null