Why is it necessary to use immutable keys when working with HashMap?

In HashMap, keys must be immutable because the hash code of the key is used to determine the position of the element in the data structure. If the key changes after being placed in the HashMap, its hash code will also change, and then:

• HashMap will not be able to find the element by the key, as it will look in a different bucket.
• This will lead to data loss or inability to retrieve data correctly.

Therefore, it is common to use objects with immutable state as keys, such as Strings or wrapper classes over primitives. If mutable objects are used, it is necessary to ensure that the fields affecting hashCode() and equals() do not change after insertion.