Java Collections Interview Questions

The Java Collections Framework comes up in almost every Java interview, from fresher rounds to senior ones. This topic goes beyond the basics: how to choose between HashSet, LinkedHashSet and TreeSet or ArrayList and LinkedList, fail-fast and fail-safe iterators, avoiding ConcurrentModificationException, HashMap capacity, load factor and null keys, and how HashMap, Hashtable and ConcurrentHashMap differ.

It also covers List.of and unmodifiable lists, multi-field sorting with Comparator, the Java 8 Map methods, ArrayDeque, PriorityQueue and the top-K problem, TreeMap range queries, an LRU cache with LinkedHashMap, EnumMap, CopyOnWriteArrayList, BlockingQueue and WeakHashMap. Every answer has a complete program with its real output and a ready-to-use interview answer.

• What is the difference between HashSet, LinkedHashSet and TreeSet?
• When is ArrayList faster than LinkedList, and is LinkedList ever the better choice?
• What is the best way to iterate over a HashMap in Java?
• What is the difference between fail-fast and fail-safe iterators in Java?
• How do you avoid ConcurrentModificationException when removing elements from a list?
• Can a HashMap have a null key? How is it stored?
• What are the initial capacity and load factor of a HashMap, and what happens when it resizes?
• What is the difference between HashMap, Hashtable and ConcurrentHashMap?
• What is the difference between List.of(), Collections.unmodifiableList() and Arrays.asList()?
• How do you sort a list of objects by multiple fields using Comparator?
• What are the most useful methods of the Collections utility class?
• Which Map methods added in Java 8 should every developer know?
• Why is ArrayDeque preferred over Stack and LinkedList for stacks and queues?
• How does PriorityQueue work, and how do you use it to find the top K elements?
• How do you use TreeMap and NavigableMap methods like floorKey, ceilingKey, headMap and subMap?
• How do you implement an LRU cache using LinkedHashMap?
• What are EnumMap and EnumSet, and why are they faster than HashMap and HashSet?
• What is CopyOnWriteArrayList, and when should you use it?
• What is a BlockingQueue, and how do you use it for the producer–consumer problem?
• What is WeakHashMap, and when would you use it?