Java Collections Framework: Which statement accurately describes java.util.HashSet?

Introduction / Context:
HashSet is a fundamental collection type in Java that stores unique elements backed by a hash table. Understanding its characteristics—uniqueness, ordering, and mutability—is essential for correct API usage.

Given Data / Assumptions:

• We refer to the standard java.util.HashSet behavior.
• Default settings are assumed (no explicit LinkedHashSet or TreeSet).

Concept / Approach:
A HashSet contains no duplicate elements; it uses hash codes to place elements in buckets. It does not maintain any specific iteration order (in contrast to LinkedHashSet which preserves insertion order). It is mutable by default. It does not provide key-based access like Map types; it stores elements, not key–value pairs.

Step-by-Step Solution:

Verification / Alternative check:
Javadoc for HashSet states it makes no guarantees as to the iteration order and permits null elements (at most one).

Why Other Options Are Wrong:

• Ordered: not guaranteed by HashSet.
• Immutable: false—HashSet is mutable.
• Access by unique key: describes maps, not sets.
• Preserves insertion order: that is LinkedHashSet, not HashSet.

Common Pitfalls:
Assuming iteration order remains stable; resizing or different JVMs can produce different orders. Use LinkedHashSet if stable insertion order is required.

Final Answer:
The elements in the collection are guaranteed to be unique.