In Java, which of the following statements is true about the `Set` interface?

• A. A Set can contain duplicate elements
• B. A Set guarantees the order of elements
• C. A Set does not allow duplicate elements
• D. A Set can only store primitive data types

Answer: (C)

The statement that a Set does not allow duplicate elements is indeed accurate. The primary characteristic of the Set interface in Java is that it represents a collection of unique elements. When a new element is added to a Set, Java automatically checks for duplicates. If the element being added is already present in the Set, it will not be added again, ensuring that all elements within a Set collection remain distinct. This property is fundamental to the functionality of sets in mathematical terms as well, where sets are defined by their unique members.

Regarding the other choices, they incorrectly reflect the capabilities or behavior of the Set interface. A Set can indeed maintain unique elements, and this is the key feature that differentiates it from other collections like Lists. Also, while there are implementations of Set that maintain insertion order (like LinkedHashSet), not all Sets do; some implementations, such as HashSet, do not guarantee any order. Furthermore, Sets can store objects and not just primitive data types directly; primitive types must be wrapped in their corresponding wrapper classes (like Integer for int) to be stored in a Set.