Which collection does NOT allow duplicates?

Question

BlackTom AI · Accepted Answer

A Set disallows duplicate elements by definition: its contract requires that no two elements are equal according to equals() (and consistent hashCode() for hash-based implementations).

1) Relevant concepts:
- Java Collections Framework: List, Set, ArrayList, Map.
- Set interface (HashSet, TreeSet, LinkedHashSet) enforces uniqueness of elements.
- List (and concrete ArrayList) are ordered, allow duplicates.
- Map stores key→value pairs: keys must be unique, values may repeat; Map is not a List/Set and has different semantics.

2) Step-by-step reasoning:
- If you add the same object twice to a Set, the second add is ignored (Set already contains an equal element).
- Example: create a HashSet, add "A", add "A" again → size remains 1.
- For ArrayList (a List implementation), add "A", add "A" again → size becomes 2 because List allows duplicates.
- For Map, adding the same key twice replaces the previous mapping (keys are unique), but Map can contain duplicate values across different keys; Map semantics are about unique keys, not “no duplicates” of values.
- List (interface) explicitly permits duplicate elements.

3) (No numeric calculation required; logical steps above serve as intermediate steps demonstrating behavior.)

4) Why "Set" is correct:
- The Set interface’s specification enforces that it contains no duplicate elements. Implementations check equality and prevent insertion of duplicates, so a Set does NOT allow duplicates.

5) Why other options are incorrect:
- a. Map: keys must be unique but values can duplicate; Map is not described as “no duplicates” for its entire content, and maps are key-based.
- b. ArrayList: a List implementation that allows duplicate elements and preserves insertion order.
- d. List: by contract, List permits duplicates (ordered collection).

6) Conclusion:
The correct answer is c. Set, which matches the provided answer.

Which collection does NOT allow duplicates?单项选择题

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