Collections Interview Questions and Answers – From Basics to Deep Internals

Java Collections is one of the most frequently asked topics in technical interviews—from junior developer roles to senior system design rounds. Understanding how different collections behave under the hood, when to use what, and how to optimize for memory and performance can give you a competitive edge in interviews.

In this comprehensive guide, we’ll cover Java Collections Interview Questions ranging from basic to advanced, with explanations, best practices, and real-world analogies.

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Core Concepts

Java Collections Framework provides key data structures for storing, manipulating, and retrieving data. Core interfaces include:

• List: Ordered collection (e.g., ArrayList, LinkedList)
• Set: Unique elements (e.g., HashSet, TreeSet)
• Map: Key-value pairs (e.g., HashMap, TreeMap)
• Queue and Deque: FIFO/LIFO ordering

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Top Basic Interview Questions

1. What is the difference between List, Set, and Map?

Feature	List	Set	Map
Duplicates	Allowed	Not Allowed	Only keys disallow duplicates
Order	Maintained (ArrayList)	Not guaranteed (HashSet)	Keys are unordered unless LinkedHashMap or TreeMap

2. What is the default capacity of an ArrayList?

• 10 in Java 8+

3. How does HashSet work internally?

• Backed by a HashMap with dummy values.

4. What is the load factor in HashMap?

• Default: 0.75. Controls when the map resizes.

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Intermediate and Real-World Scenarios

1. How would you prevent ConcurrentModificationException?

• Use an Iterator and iterator.remove() OR use concurrent collections like CopyOnWriteArrayList.

2. How would you implement LRU Cache?

• Use LinkedHashMap with access-order and override removeEldestEntry.

3. How do you sort a list of custom objects?

Collections.sort(list, Comparator.comparing(Employee::getName));

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Advanced Internals and Performance Questions

1. How does HashMap handle collisions?

• Uses chaining with linked lists, and from Java 8 onwards, switches to balanced trees (red-black trees) if bucket size exceeds threshold.

2. What happens if two keys have the same hashcode?

• The equals() method is used to resolve collision.

3. Why is ConcurrentHashMap preferred over synchronized HashMap?

• It uses bucket-level locking or segment-based locking to improve concurrency.

4. How does ArrayList grow?

• Increases capacity by 50% on every resize.

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Comparisons: HashMap vs TreeMap, List vs Set

Feature	HashMap	TreeMap
Ordering	Unordered	Sorted by keys
Null keys	Allowed (1)	Not allowed
Performance	O(1) for get/put	O(log n)

Feature	List	Set
Duplicates	Allowed	Not allowed
Access	Index-based	Value-based

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Version-specific Enhancements

Java 8

• Streams API
• Collectors, Optional, Map.computeIfAbsent()

Java 9

• Factory methods like List.of(), Set.of(), Map.of()

Java 10+

• var type inference

Java 21

• Runtime performance improvements for collections
• Structured concurrency (related to parallel stream use)

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Functional Programming in Interviews

Sample question:

How do you group a list of employees by department using Streams?

Map<String, List<Employee>> grouped = employees.stream()
    .collect(Collectors.groupingBy(Employee::getDepartment));

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Best Practices for Interview Answers

• Use real-world analogies: e.g., “A HashMap is like a set of drawers labeled by key.”
• Know trade-offs: ArrayList vs LinkedList, HashMap vs TreeMap
• Emphasize immutability in multi-threaded contexts

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📌 What's New in Java Versions?

• Java 8
  • Stream API, Collectors, Optional, Lambdas
• Java 9
  • List.of(), Map.of(), immutable factory methods
• Java 10
  • var type inference
• Java 21
  • Memory optimizations for HashMap, structured concurrency

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Conclusion and Key Takeaways

Mastering Java Collections is essential for cracking interviews. Whether you're asked about data structure selection, memory models, or real-world scenarios—knowing your collections deeply can help you stand out.

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FAQ

1. Is HashMap ordered?

No. Use LinkedHashMap or TreeMap if ordering is important.

2. Can a Set contain null?

Yes. HashSet allows one null. TreeSet throws exception.

3. What’s the difference between ArrayList and LinkedList?

ArrayList is faster for access, LinkedList for insert/remove.

4. Can you use null keys in HashMap?

Yes, only one null key is allowed.

5. How do you prevent resizing in HashMap?

Pre-size with initialCapacity = expectedSize / loadFactor + 1.

6. When to use HashSet vs TreeSet?

HashSet for performance; TreeSet when sorting is required.

7. Does ConcurrentHashMap allow nulls?

No. Neither null keys nor null values.

8. What is fail-fast behavior?

Modifying a collection while iterating throws ConcurrentModificationException.

9. Can you sort a Map by values?

Yes, by converting entrySet() to a list and using a comparator.

10. What is the time complexity of HashMap.get()?

Average case O(1), worst case O(log n) with tree bins (Java 8+).