Top 15 Interview Questions on Java Collection Framework

Prepare for success with the Top 15 Interview Questions on Java Collection Framework. Ace your interview with key concepts, examples, and expert tips!

 

1. What is the Java Collections Framework?

Answer:
The Java Collections Framework (JCF) is a unified architecture for representing and manipulating collections in Java. It provides interfaces (e.g., List, Set, Map), classes (e.g., ArrayList, HashSet, HashMap), and algorithms (e.g., sorting and searching). It simplifies programming by offering ready-to-use collection implementations and algorithms.

 

2. What are the main differences between List, Set, and Map?

Answer:

• List: An ordered collection of elements that allow duplicates (e.g., ArrayList, LinkedList).
• Set: Unordered collection of unique elements (e.g., HashSet, TreeSet).
• Map: Key-value pairs, where keys are unique and values can be duplicated (e.g., HashMap, TreeMap).

 

3. Explain the difference between ArrayList and LinkedList.

Answer:

• ArrayList:
  • Backed by a dynamic array.
  • Faster for random access (get() operation).
  • Slower for insertions and deletions in the middle because it involves shifting elements.
• LinkedList:
  • Backed by a doubly linked list.
  • Faster for insertions and deletions in the middle.
  • Slower for random access since traversal is required.

 

4. What is the difference between HashSet and TreeSet?

Answer:

• HashSet:
  • Backed by a HashMap.
  • Does not maintain any order of elements.
  • Provides constant time for basic operations like add, remove, and contain.
• TreeSet:
  • Backed by a TreeMap.
  • Maintains elements in sorted (natural) order or by a custom comparator.
  • Provides logarithmic time for add, remove, and contain operations.

 

5. How is HashMap implemented in Java?

Answer:

• HashMap uses a hash table.
• It stores key-value pairs, where keys are hashed using the hashCode() method.
• Collisions are handled using a linked list or balanced tree (from Java 8 onward).
• The bucket array dynamically resizes when the load factor exceeds 0.75.

 

6. What is the difference between HashMap and Hashtable?

Answer:

• HashMap:
  • Not synchronized (not thread-safe).
  • Allows one null key and multiple null values.
• Hashtable:
  • Synchronized (thread-safe).
  • Does not allow any null key or value.

 

7. How does ConcurrentHashMap work?

Answer:

• ConcurrentHashMap allows concurrent read and write operations.
• It achieves thread safety by dividing the map into segments and locking only the affected segment during write operations.
• From Java 8, it uses a combination of lock-free and lock-based mechanisms for better concurrency.

 

8. What is the difference between Iterator and ListIterator?

Answer:

• Iterator:
  • Can traverse a collection in a forward direction only.
  • Works for all collections (e.g., Set, List).
• ListIterator:
  • Can traverse a List in both forward and backward directions.
  • Supports adding and replacing elements during iteration.

 

9. What is the difference between fail-fast and fail-safe iterators?

Answer:

• Fail-fast:
  • Throws a ConcurrentModificationException if the collection is modified during iteration (e.g., ArrayList, HashMap iterators).
• Fail-safe:
  • Operates on a copy of the collection and does not throw an exception for modifications (e.g., CopyOnWriteArrayList, ConcurrentHashMap).

 

10. How can you make a collection thread-safe?

Answer:

• Use synchronized wrappers from Collections class (e.g., Collections.synchronizedList).
• Use concurrent collection classes from java.util.concurrent (e.g., ConcurrentHashMap, CopyOnWriteArrayList).

 

11. What is the difference between Comparable and Comparator?

Answer:

• Comparable:
  • Defines natural ordering using the compareTo() method.
  • Implemented by the class itself.
• Comparator:
  • Defines custom ordering using the compare() method.
  • Implemented as a separate class or lambda.

 

12. What is the priorityQueue in Java?

Answer:

• PriorityQueue is a queue implementation where elements are ordered based on their natural order or a custom comparator.
• The element with the highest priority is at the head of the queue.

 

13. How does LinkedHashMap differ from HashMap?

Answer:

• LinkedHashMap maintains the insertion order or access order of elements, while HashMap does not.
• Useful when you need a predictable iteration order.

 

14. What is a WeakHashMap?

Answer:

• A WeakHashMap is a Map implementation where keys are stored as weak references.
• If a key is no longer referenced elsewhere, it can be garbage collected, even if it is still present on the map.

 

15. Explain TreeMap and its use case.

Answer:

• TreeMap is a Map implementation that maintains keys in sorted order (natural or custom).
• Use it when you need a sorted Map for quick range queries or ordered iteration.

 

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