The Complete Guide to Java Collections Interview Questions in 2023

One of the most important parts of the Java programming language that supports its basic ideas is the Collection Framework. This is very important to know about these basic ideas before you go for an interview if you want to become a Java Developer. This article will give you the Top 50 Java Collections Interview Questions and Answers that will help you make it through your interview with flying colors.

As a Java developer, having a strong grasp of collections is essential for performing well in coding interviews Collections provide powerful data structures and algorithms that are widely used across Java projects and frameworks In this comprehensive guide, we will explore the most common Java collections interview questions that you need to know in 2023.

Why Collections Matter for Interviews

The Java Collections Framework provides interfaces and implementation classes to represent and manipulate groups of objects as a single unit. This allows developers to store, retrieve, manipulate, and communicate aggregate data more efficiently.

Some key reasons why collections are heavily tested in interviews

  • Collections are used extensively in real-world Java applications. You need to select the right collection for the job.
  • Understanding the performance trade-offs between collections shows your experience with common algorithms and data structures.
  • Questions on collections test your knowledge of core Java APIs and OOP concepts like interfaces, inheritance, and generics.

Let’s now dive into the top collections interview questions and sample answers for each major topic

Arrays vs Collections

Q: What are the differences between arrays and collection classes in Java?

Arrays have a fixed size that you need to specify at initialization. Collections like ArrayList can grow and shrink dynamically as you add/remove elements.

Elements in arrays are accessed via an index. Collections have no notion of index and provide methods to access elements (like get(), iterator()).

Arrays can contain primitives (int[]) or objects (String[]). Generic collections only work with object references.

Arrays support basic operations like sorting, searching. Collections have a rich API with methods for easy insertion, deletion, filtering.

Q: When will you prefer array over ArrayList in Java?

If you know the size of data in advance and it doesn’t change, arrays are faster and more memory efficient.

For storing primitive data types like int, char, double. Collections can only store objects.

When you need speed and frequent element access by index. Get/set operations on array indexes are faster.

If your data is immutable once initialized, arrays are a safer choice since collections are mutable.

ArrayList and LinkedList

Q: What is the difference between ArrayList and LinkedList?

ArrayList is backed by a dynamically resizing array. It offers constant time index based access and is better for frequent get/set operations.

LinkedList stores elements in a doubly linked list. It can efficiently add and remove elements from any position but slow for random index access.

ArrayLists have better cache locality compared to LinkedLists where elements are scattered across multiple nodes.

Q: When will you choose LinkedList over ArrayList in Java?

When you frequently add/remove elements from the middle of the list. LinkedLists have constant time add/remove whereas ArrayLists need resizing and copying on inserts.

Memory overhead is less for LinkedLists as extra space for future growth is not allocated upfront like arrays.

For implementing stacks, queues which need efficient addition/removal from both ends.

HashSet and TreeSet

Q: What is the difference between HashSet and TreeSet?

HashSet stores unique elements in a hash table based on the element’s hashCode(). It provides constant time add, remove and contains. Order is not guaranteed.

TreeSet stores elements in a Red-Black tree structure, ordered by natural sorting or a custom comparator. Lookup time is O(log n).

HashSet allows null element but null cannot be added to TreeSet as it relies on comparisons.

Q: When will you use TreeSet over HashSet?

If you need to maintain ordering of elements – use natural ordering or a custom comparator.

For sorted data or if you need navigation methods like ceiling(), higher(), floor(), and lower() methods.

If your Set operations involve frequent range queries, intersections, unions etc.

When you want guaranteed worst case O(log n) lookup time vs O(1) average case for HashSet.

HashMap and TreeMap

Q: What is the difference between HashMap and TreeMap?

HashMap stores key-value pairs in a hash table. It provides O(1) lookup but ordering is not guaranteed. Allows one null key.

TreeMap stores elements in Red-Black tree structure, ordered by key’s natural sorting or custom comparator. Lookup time is O(log n).

HashMap is better for frequent get/put operations. TreeMap good for maintaining order and range operations.

Q: When will you choose TreeMap over HashMap?

When you need to maintain natural ordering of keys or custom ordering using comparator.

For sorted Map requirements or navigating entries in sorted order.

If your application needs to rely on consistent ordering of keys.

For range queries like getting submaps based on key range, lowerKey(), higherKey() etc.

Comparable and Comparator

Q: How can you sort a custom object in Java?

Implement Comparable interface to define natural ordering of objects of that class. Override compareTo() to compare two instances.

Use Comparator interface to define custom comparison logic for a class seperately from the main class. Pass comparator instance to sort().

Q: When will you implement Comparator over Comparable?

If multiple ways of comparison are needed for a class. Like Person can be sorted by name or age.

The original class is from a 3rd party library and you don’t have access to modify its source to make it Comparable.

To avoid mutating the original class just for supporting comparison logic. Keep your classes focused.

The comparison logic needs to access external mutable state – like sorting Person by another field.


Q: What are generics in Java and why were they introduced?

Generics allow defining classes, interfaces and methods that work for multiple data types without compromising type safety. This increased code reusability and avoids repetitive casting.

For example, with generics we can have an add() method that works for List<Integer>, List<String> or List<Person>.

Q: Can you use primitive types like int, char for generics?

No, generics in Java only work with reference object types. We cannot have something like List<int>.

However, the wrapper classes like Integer, Character can be used.

Collections Interview Questions – Misc

Q: How can you synchronize a HashMap?

  1. Use Collections.synchronizedMap() which returns a synchronized wrapper around a map.

  2. Use ConcurrentHashMap which is thread-safe without explicit synchronization.

Q: What are some differences between Array and ArrayList?

  1. Arrays have fixed size. ArrayLists are dynamic.
  2. Arrays can contain primitives or objects. ArrayLists only objects.
  3. ArrayLists have handy methods like add(), remove(), iterator() etc.

Q: How can you avoid ConcurrentModificationException when iterating a collection?

  1. Use iterator’s remove() method rather than Collection’s remove().
  2. Lock the collection before iterating and modify after.
  3. Use concurrent collections like ConcurrentHashMap.


This guide covered the most frequently asked collections interview questions for Java developers. Mastering these core topics will ensure you can efficiently store and manipulate data in real world apps.

Remember to analyze the problem at hand, choose the right collection, think about performance tradeoffs, and be thorough with the APIs. With diligent practice, you’ll be able to tackle any collections questions that come your way!

4 Differentiate between Iterator and Enumeration.

Iterator Enumeration
Collection element can be removed while traversing it Can only traverse through the Collection
Used to traverse most of the classes of the Java Collection framework Used to traverse the legacy classes such as Vector, HashTable, etc
Is fail-fast in nature Is fail-safe in nature
Is safe and secure Is not safe and secure
Provides methods like hasNext(), next() and remove() Provides methods like hasMoreElements() and nextElement()

2 What is the ConcurrentHashMap in Java and do you implement it?

ConcurrentHashMap is a Java class that implements ConcurrentMap as well as Serializable interfaces. This class is the enhanced version of HashMap as it doesn’t perform well in the multithreaded environment. It has a higher performance rate compared to the HashMap.

Below is a small example demonstrating the implementation of ConcurrentHashMap:

Java collections framework interview questions and Answers | MOST ASKED | Core Java | Code Decode

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