What is required for a key in a Hashtable?

• A. It can be null
• B. It must implement the hashCode method
• C. It can be of any data type
• D. It must be immutable

Answer: (B)

A key in a Hashtable must implement the hashCode method because the Hashtable relies on the hash code to determine how to store and retrieve entries efficiently. When a key-value pair is added to the Hashtable, the hash code for the key is computed, which helps in identifying the specific bucket where the key-value pair will be stored. When retrieving a value, the same hash code is calculated again to quickly locate the key and its corresponding value.

The requirement for implementing the hashCode method is crucial because it allows the Hashtable to maintain efficiency in lookups, insertions, and deletions, ensuring that operations can be performed in constant time on average. This characteristic is one of the fundamental aspects of using hash-based collections like a Hashtable.

While immutability of the key, being of any data type, and nullability might be aspects of other collections, they do not pertain directly to the functioning of a Hashtable as a key requires a specific hash code to work effectively within this structure.