Set

tags: Collection, Data Structures

Set Interface

• Set Interface in Java is present in java.util package.
• It extends the Collection interface.
• It represents the unordered set of elements which doesn’t allow us to store the duplicate items. We can store at most one null value in Set.
• Set is implemented by HashSet, LinkedHashSet, and TreeSet.

What is the Set interface in Java?

• Set represents a group of unique elements.
• It does not allow duplicates and may or may not maintain insertion order based on the implementation.

What are the characteristics of a Set?

• No duplicate elements allowed
• At most one null element (implementation dependent)
• Order is not guaranteed (unless using LinkedHashSet or TreeSet)

Which classes implement the Set interface?

• HashSet
• LinkedHashSet
• TreeSet
• EnumSet
• CopyOnWriteArraySet

What is the difference between Set and List?

Feature	List	Set
Duplicates	Allowed	Not allowed
Order	Maintains insertion order	Unordered (except LinkedHashSet)
Indexing	Supports index-based access	Not supported

What is a HashSet?

• An implementation of Set backed by a HashMap
• Does not maintain insertion order
• Allows one null value
• Offers constant time performance for basic operations like add(), remove(), and contains()

What is a LinkedHashSet?

• Subclass of HashSet
• Maintains insertion order
• Slightly slower than HashSet due to extra memory usage

What is a TreeSet?

• Implements NavigableSet
• Stores elements in sorted (natural or custom) order
• Does not allow null
• Backed by a Red-Black tree

When should you use HashSet, LinkedHashSet, or TreeSet?

Use Case	Preferred Set Type
Fast access, no order needed	HashSet
Maintain insertion order	LinkedHashSet
Need sorted elements	TreeSet

What is the SortedSet Interface ?

• SortedSet is the alternate of Set interface that provides a total ordering on its elements.
• The elements of the SortedSet are arranged in the increasing (ascending) order.

How does Set ensure uniqueness?

Through equals() and hashCode() methods for HashSet/LinkedHashSet, and compareTo() or Comparator for TreeSet.

What happens if you add a duplicate to a Set?

The duplicate is ignored. The add() method returns false.

Can you store null in a Set?

Yes, but only one null is allowed in HashSet and LinkedHashSet.
TreeSet does not allow null if using natural ordering.

Explain the ClassCast Exception in TreeSet

If we add an object of the class that is not implementing the Comparable interface, the ClassCast Exception is raised.

It is because the TreeSet maintains the sorting order, and for doing the sorting the comparison of different objects that are being inserted in the TreeSet is must, which is accomplished by implementing the Comparable interface. So all the Objects used for TreeSet must implement Comparable interface.

How do you sort a Set?

• Convert it to a List and sort
• Use a TreeSet for automatic sorting

Set<String> sortedSet = new TreeSet<>(unsortedSet);

What is the time complexity of Set operations?

Operation	HashSet	LinkedHashSet	TreeSet
add	O(1)	O(1)	O(log n)
remove	O(1)	O(1)	O(log n)
contains	O(1)	O(1)	O(log n)

What is EnumSet?

• Specialized Set implementation for use with enum types only
• Very fast and memory-efficient
• Cannot store null

enum Day { MON, TUE, WED }
Set<Day> days = EnumSet.of(Day.MON, Day.WED);

What is CopyOnWriteArraySet?

• A thread-safe Set backed by a CopyOnWriteArrayList
• Ideal for frequent iteration and less frequent updates
• Used in concurrent applications

Can a Set be synchronized?

Yes:

Set<String> syncSet = Collections.synchronizedSet(new HashSet<>());

How to remove duplicates from a List using Set?

List<String> list = Arrays.asList("A", "B", "A");
Set<String> set = new HashSet<>(list);

What is the output if we insert elements with the same hashcode into a HashSet?

• Elements with the same hashcode are stored in the same bucket (linked list or tree)
• Uniqueness is still checked using equals()

How to compare two Sets for equality?

Use .equals() method. It returns true if both sets contain the same elements, regardless of order.

How do you convert a Set to an array?

String[] array = set.toArray(new String[0]);

How do you remove elements conditionally in a Set?

set.removeIf(s -> s.length() > 3);

What does retainAll() do in Set?

It performs set intersection — keeps only elements also present in the given collection.

set1.retainAll(set2);