HashMap works on the principle of Hashing . To understand Hashing , we should understand the three terms first i.e Hash Function , Hash Value and Bucket . What is Hash Function , Hash Value and Bucket ? hashCode() function which returns an integer value is the Hash function. The important point to note that , this method is present in Object class ( Mother of all class ) . This is the code for the hash function(also known as hashCode method) in Object Class : public native int hashCode(); The most important point to note from the above line : hashCode method return int value . So the Hash value is the int value returned by the hash function . So summarize the terms in the diagram below : how hash map works in java What is bucket ? A bucket is used to store key value pairs . A bucket can have multiple key-value pairs . In hash map, bucket used simple linked list to store objects .