Can we use object as a key in hashmap in Java?

Question

How to use an object as a key in a hashmap. If you use an object as key do you need to override equals and hashcode methods for that object?

Answer 1

A simple thumb rule is to use immutable objects as keys in a HashMap.

because:

If it were mutable, then the hashcode() value or equals() condition might change, and you would never be able to retrieve the key from your HashMap.

More precisely, class fields that are used to compute equals() and hashcode() should be immutable!

Now, suppose you create your own class:

• To compare two objects of your class you will have to override equals()
• To use it as a key in any Hash based Data structure you will have to override hashcode() (again, keeping immutability in mind)

Remember that if two objects are equal(), then their hashcode() should be equal as well!

Answer 2

hashCode() -HashMap provides put(key, value) for storing and get(key) method for retrieving Values from HashMap. When put() method is used to store (Key, Value) pair, HashMap implementation calls hashcode on Key object to calculate a hash that is used to find a bucket where Entry object will be stored. When get() method is used to retrieve value, again key object is used to calculate a hash which is used then to find a bucket where that particular key is stored.

equals() - equals() method is used to compare objects for equality. In case of HashMap key object is used for comparison, also using equals() method Map knows how to handle hashing collision (hashing collision means more than one key having the same hash value, thus assigned to the same bucket. In that case objects are stored in a linked list, refer figure for more clarity. Where hashCode method helps in finding the bucket where that key is stored, equals method helps in finding the right key as there may be more than one key-value pair stored in a single bucket.