Ace Java 2026: Rock Your 'Thinking in Java' Challenge!

The modern Java container library often uses the HashMap class as a replacement for the outdated Hashtable class.

Unlike the Hashtable class, HashMap is not synchronized which makes it more efficient. Additionally, HashMaps allow for null values and keys, while Hashtables do not.

ArrayList, Vector, and List are all different data structures that are not replacements for Hashtable in the modern Java container library.

They each have their own specific use cases and cannot be used interchangeably with Hashtable.

Therefore, the correct answer is C HashMap.