How HashMap Works in Java?

HashMap in Java
HashMap store Key/Value pairs just like HashTable. It is faster than HashTable and accept null values and it is not synchronized. These are the basic fundementals we know about HashMap. However, let us try to go through how Hashmap works in Java.

HashMap works on principle of hashing, we have put(key, value) and get(key) method for storing and retrieving Objects from HashMap. When we pass Key and Value object to put() method on Java HashMap, HashMap implementation calls hashCode() method on Key object and applies returned hashcode into its own hashing function to find a bucket location for storing Entry object, important point to mention is that HashMap in Java stores both key and value object as Map.Entry in bucket. Then, there is another important concept of Collision detection and Collision resolution in Java HashMap i.e What will happen when two objects have same hashcode?
According to the equals() and hashCode() contract, we know that for two objects to be equal, they need to have same hashcode. However, the oppositeis not true. i.e Two objects with same hashcode need not necessarily be equal. Here, when 2 objects with same hashcode is returned, this condition is called collision detection.

To resolve this collision, we should know that HashMap uses LinkedList to store object i.e the Map.Entry Object which comprises of Key and Value will be stored in a LinkedList. When an object is tried to be retrieved by calling the get(Object key) method, the linked list is traversed and if 2 objects are returned which has same hashCode, then it calls the key.equals() method to find the correct node in the linked list and return its associated value. So, it is very important that we maintain unique/unmodifiable keys.
Therefore, it is nice to remember and follow that using immutable, final object with proper equals() and hashcode() implementation would act as perfect Java HashMap keys and improve performance of Java HashMap by reducing collision. Immutability also allows caching there hashcode of different keys which makes overall retrieval process very fast and suggest that String and various wrapper classes e.g. Integer very good keys in Java HashMap.