What are the requirements for keys and values in HashMap?

In Java, HashMap requires that keys correctly implement the hashCode() and equals() methods. This is necessary for proper distribution of elements across buckets and for correct key lookup.

Requirements for keys:

• The hashCode() method must be stable and return the same value for the same object throughout its lifetime in the HashMap.
• The equals() method must correctly compare objects for equality.
• Keys should be immutable if used as keys, to avoid violating the hashCode/equals contract.

Values in a HashMap can be any objects, including null. Unlike keys, values do not participate in hash code calculation or key comparison, so there are no special requirements for them.

Example:

Map<String, Integer> map = new HashMap<>();
map.put("key", 123); // key is a String that correctly implements hashCode and equals