Why is it important for keys in a hash table to be immutable data types?

Keys in a hash table (for example, in HashMap in Java) must be immutable because the hash table uses the hash code of the key for quick lookup.

If the key changes after being placed in the table, its hash code may change, and the object will become inaccessible for lookup — the hash table will not be able to find the element by the new hash.

For example, if a mutable object (like ArrayList) is used as a key and its contents are changed after adding it to a HashMap, attempting to retrieve the value by this key will return null.

Therefore, immutable data types (such as String, Integer, or custom classes with overridden hashCode() and equals() that do not change after creation) are usually used.

This ensures the correct operation of the hash table and prevents errors during search and deletion of elements.