Can you list the main types of contracts related to keys in the implementation of HashMap?

Main types of contracts (requirements) for keys in HashMap implementation in Java:

• hashCode(): the key must correctly implement the hashCode() method to ensure an even distribution across buckets.
• equals(): keys considered equal should return true when compared using the equals() method. This is important for correct search and replacement of values.
• immutability (preferably): keys should be made immutable so that their hashCode and equals do not change after being placed in the HashMap, otherwise it will lead to errors during search.

These contracts guarantee the correct operation of HashMap when storing and searching for elements by key.