What is the difference between Set and List in Java?

Set vs List

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Quick answer: List and Set are both sub-interfaces of Collection but with fundamentally different contracts around ordering and uniqueness.

What a Strong Answer Covers

List allows duplicates
Set no duplicates
List maintains insertion order
List has index access
HashSet unordered
LinkedHashSet insertion order
TreeSet sorted

Senior-Level Answer

List and Set are both sub-interfaces of Collection but with fundamentally different contracts around ordering and uniqueness.

A List is an ordered sequence. It maintains insertion order (or a deliberately sorted order), allows duplicate elements, and provides index-based access via get(int index). The primary implementations are ArrayList (random access, backed by an array, O(1) get) and LinkedList (sequential access, O(n) get, but O(1) insert/remove at a known position).

A Set models a mathematical set: no duplicate elements are permitted. The equals() and hashCode() contract determines what counts as a duplicate. HashSet offers O(1) average add/contains/remove but no ordering guarantee. LinkedHashSet preserves insertion order with O(1) operations. TreeSet maintains elements in natural or comparator-defined sorted order with O(log n) operations, implementing the NavigableSet interface.

The practical decision is: do you need uniqueness or do you need indexed positional access? If you need to store and retrieve by position, or duplicates are meaningful, use a List. If the semantics of your data require uniqueness (email addresses, active user IDs, feature flags), use a Set — it enforces the constraint automatically rather than relying on manual checks.

Performance difference for membership testing is significant. contains() on an ArrayList is O(n) because it scans every element. contains() on a HashSet is O(1) average. For large collections where membership checks are frequent, HashSet dramatically outperforms ArrayList.

Note that iterating a HashSet in a specific order is unreliable across JVM runs (hash seed randomization). If deterministic iteration order matters, prefer LinkedHashSet or TreeSet.

Key Differences

Aspect	List	Set
Duplicates	Allowed	Not allowed
Order	Insertion order preserved	Depends on implementation (none/insertion/sorted)
Index access	Yes — get(int index)	No
contains() complexity	O(n) for ArrayList	O(1) avg for HashSet
Primary implementations	ArrayList, LinkedList	HashSet, LinkedHashSet, TreeSet
Use case	Ordered data, positional access, duplicates meaningful	Unique elements, fast membership testing

What Separates a 2/3 from a 3/3

2/3 — Passing but Incomplete

Correctly states List allows duplicates with ordered access and Set enforces uniqueness, and names at least one implementation of each.

3/3 — Strong Answer

Adds contains() performance comparison, distinguishes HashSet vs LinkedHashSet vs TreeSet ordering guarantees, and gives a concrete use-case rationale for choosing one over the other.

Common Mistakes

Saying Set has 'no order' without qualifying that LinkedHashSet preserves insertion order and TreeSet maintains sorted order.
Not knowing the O(n) vs O(1) contains() difference between ArrayList and HashSet, which is often the reason to choose Set.
Forgetting that Set relies on correct equals() and hashCode() implementations — objects with broken contracts will misbehave in Sets.

Follow-Up Questions

What happens when you add an object to a HashSet whose hashCode is always the same constant? — All objects hash to the same bucket, turning the HashSet into a linked list with O(n) performance for all operations.
When would you use TreeSet over HashSet? — When you need elements in sorted order or need range-view operations like headSet(), tailSet(), or subSet() from NavigableSet.
Can a List contain null elements? Can a Set? — ArrayList allows multiple nulls. HashSet and LinkedHashSet allow one null. TreeSet does not allow null because it calls compareTo, which throws NPE.

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