Java Collections – Top 50 Interview Questions & Answers

1️⃣ What is the Java Collection Framework?

A unified architecture to store, retrieve, and manipulate groups of objects using interfaces like List, Set, and Map.

2️⃣ Why do we need collections?

To handle dynamic data, reduce coding effort, improve performance, and provide reusable data structures.

3️⃣ What are the main interfaces in the Collection Framework?

List
Set
Queue
Deque
Map (part of framework but not a child of Collection)

4️⃣ Difference between `Collection` and `Collections`?

Collection	Collections
Interface	Utility class
Represents data	Helper methods

5️⃣ Difference between `List`, `Set`, and `Map`?

Type	특징
List	Allows duplicates, ordered
Set	No duplicates
Map	Key–value pairs

6️⃣ What is an `ArrayList`?

A resizable array implementation of List.

7️⃣ Difference between `ArrayList` and `LinkedList`?

ArrayList	LinkedList
Fast read	Fast insert/delete
Uses array	Uses doubly linked list

8️⃣ What is `Vector`?

A legacy synchronized list (rarely used now).

9️⃣ Difference between `ArrayList` and `Vector`?

ArrayList	Vector
Not synchronized	Synchronized
Faster	Slower

🔟 What is a `Set`?

A collection that does not allow duplicates.

1️⃣1️⃣ Difference between `HashSet` and `TreeSet`?

HashSet	TreeSet
Unordered	Sorted
Faster	Slower

1️⃣2️⃣ What is `LinkedHashSet`?

Maintains insertion order while preventing duplicates.

1️⃣3️⃣ How does `HashSet` work internally?

Uses a HashMap internally to store elements as keys.

1️⃣4️⃣ What is a `Map`?

Stores data in key–value pairs.

1️⃣5️⃣ Difference between `HashMap` and `Hashtable`?

HashMap	Hashtable
Not synchronized	Synchronized
Allows null	No null

1️⃣6️⃣ Difference between `HashMap` and `LinkedHashMap`?

HashMap	LinkedHashMap
No order	Maintains insertion order

1️⃣7️⃣ Difference between `HashMap` and `TreeMap`?

HashMap	TreeMap
Unordered	Sorted
Faster	Slower

1️⃣8️⃣ How does `HashMap` work internally?

Uses hashing, buckets, and linked lists / red-black trees (Java 8+).

1️⃣9️⃣ What is load factor?

Determines when a HashMap resizes (default = 0.75).

2️⃣0️⃣ What happens when HashMap is full?

It resizes and rehashes entries.

2️⃣1️⃣ What is collision in HashMap?

When multiple keys produce the same hash index.

2️⃣2️⃣ How are collisions handled?

Using:

Linked list (Java 7)
Linked list → Red-Black Tree (Java 8+)

2️⃣3️⃣ What is `ConcurrentHashMap`?

A thread-safe, high-performance Map.

2️⃣4️⃣ Difference between `HashMap` and `ConcurrentHashMap`?

HashMap	ConcurrentHashMap
Not thread-safe	Thread-safe
Allows null	No null

2️⃣5️⃣ Why `ConcurrentHashMap` is faster than `Hashtable`?

Uses bucket-level locking instead of full synchronization.

2️⃣6️⃣ What is `Queue`?

A collection for holding elements before processing (FIFO).

2️⃣7️⃣ What is `PriorityQueue`?

Queue where elements are ordered by priority.

2️⃣8️⃣ Difference between `Queue` and `Deque`?

Queue	Deque
One-end operations	Both ends

2️⃣9️⃣ What is `ArrayDeque`?

A resizable array implementation of Deque.

3️⃣0️⃣ What is an Iterator?

Used to traverse a collection safely.

3️⃣1️⃣ Difference between `Iterator` and `ListIterator`?

Iterator	ListIterator
Forward only	Bidirectional
All collections	List only

3️⃣2️⃣ What is `fail-fast` iterator?

Throws ConcurrentModificationException if collection is modified.

3️⃣3️⃣ What is `fail-safe` iterator?

Works on a copy of the collection (no exception).

3️⃣4️⃣ Example of fail-safe collection?

CopyOnWriteArrayList

3️⃣5️⃣ What is `Collections.sort()`?

Utility method to sort a list.

3️⃣6️⃣ Difference between `Comparable` and `Comparator`?

Comparable	Comparator
Natural order	Custom order
`compareTo()`	`compare()`

3️⃣7️⃣ Can we sort a Map?

Directly ❌
Convert to:

List<Map.Entry>
or use TreeMap

3️⃣8️⃣ What is immutability in collections?

Unmodifiable collections cannot be changed.

3️⃣9️⃣ How to create immutable collections?


List.of("A", "B");

4️⃣0️⃣ What is `EnumMap`?

A Map optimized for enum keys.

4️⃣1️⃣ What is `WeakHashMap`?

Keys are weakly referenced → garbage collected when unused.

4️⃣2️⃣ What is `IdentityHashMap`?

Uses == instead of equals() for key comparison.

4️⃣3️⃣ What is `SynchronizedMap`?

Thread-safe wrapper using Collections.synchronizedMap().

4️⃣4️⃣ Difference between `synchronizedMap` and `ConcurrentHashMap`?

synchronizedMap	ConcurrentHashMap
Single lock	Segment/bucket locking
Slower	Faster

4️⃣5️⃣ What is `CopyOnWriteArrayList`?

Thread-safe list optimized for read-heavy scenarios.

4️⃣6️⃣ What is `NavigableMap`?

Provides navigation methods like lowerKey(), higherKey().

4️⃣7️⃣ What is `Spliterator`?

Supports parallel traversal of collections.

4️⃣8️⃣ What is Stream API’s relation to collections?

Streams process collections in a functional style.

4️⃣9️⃣ Why Map is not part of Collection interface?

Because Map works with key–value pairs, not single elements.

5️⃣0️⃣ When to use which collection?

Scenario	Use
Fast lookup	HashMap
Ordered data	LinkedHashMap
Sorted data	TreeMap
Thread safety	ConcurrentHashMap

🎯 Interview Final Tip

Most collection questions test internal working and trade-offs, not just definitions.