Java Collections interview questions

Java Collections interview questions

Q. 1. Difference Between List and Set in Java

- **List**: 

  - Ordered collection that allows duplicate elements.

  - Examples: `ArrayList`, `LinkedList`.

- **Set**: 

  - Unordered collection that does not allow duplicate elements.

  - Examples: `HashSet`, `TreeSet`.


Q. 2. Difference Between Collection and Collections in Java

- **Collection**: 

  - The root interface in the Java Collections Framework.

  - Provides methods to add, remove, and query elements.

- **Collections**: 

  - A utility class that provides static methods to operate on or return collections, such as sorting and searching.


Q. 3. Difference Between Array and ArrayList in Java

- **Array**: 

  - Fixed size, can store primitives and objects.

  - Faster access and more memory efficient.

- **ArrayList**: 

  - Resizable array, can only store objects.

  - Provides more functionality like dynamic resizing and various methods for manipulation.


Q. 4. Difference between ArrayList and LinkedList in Java

- **ArrayList**: 

  - Uses a dynamic array to store elements.

  - Better for random access and frequent reads.

- **LinkedList**: 

  - Uses a doubly linked list.

  - Better for frequent insertions and deletions.


Q. 5. Difference Between HashSet and LinkedHashSet in Java

- **HashSet**: 

  - Unordered collection, does not maintain insertion order.

  - Faster operations (O(1)).

- **LinkedHashSet**: 

  - Maintains insertion order.

  - Slightly slower than `HashSet`.


Q. 6. Difference Between HashSet and TreeSet in Java

- **HashSet**: 

  - Unordered collection, faster operations (O(1)).

- **TreeSet**: 

  - Ordered collection, elements are sorted.

  - Slower operations (O(log n)).


Q. 7. HashSet vs LinkedHashSet vs TreeSet in Java

- **HashSet**: 

  - Unordered, fastest access.

- **LinkedHashSet**: 

  - Maintains insertion order, slightly slower than `HashSet`.

- **TreeSet**: 

  - Maintains sorted order, slowest among the three.


Q. 8. Difference Between HashMap and HashTable in Java

- **HashMap**: 

  - Not synchronized, allows one null key and multiple null values.

  - Better performance, not thread-safe.

- **Hashtable**: 

  - Synchronized, does not allow null keys or values.

  - Thread-safe but slower.


Q. 9. Difference Between HashSet and HashMap in Java

- **HashSet**: 

  - Implements the Set interface.

  - Stores unique elements.

- **HashMap**: 

  - Implements the Map interface.

  - Stores key-value pairs, keys are unique.


Q. 10. HashMap vs LinkedHashMap in Java

- **HashMap**: 

  - Unordered, faster operations.

- **LinkedHashMap**: 

  - Maintains insertion order or access order.

  - Slightly slower than `HashMap`.


Q. 11. Difference between HashMap, LinkedHashMap, and TreeMap in Java

- **HashMap**: 

  - Unordered, fastest access.

- **LinkedHashMap**: 

  - Maintains insertion or access order, slightly slower than `HashMap`.

- **TreeMap**: 

  - Maintains sorted order based on keys, slowest among the three.


Q. 12. Collections vs Streams in Java

- **Collections**: 

  - Used for storing and manipulating groups of data.

  - Supports external iteration.

- **Streams**: 

  - Provides a functional approach to processing sequences of elements.

  - Supports internal iteration and functional programming.


Q. 13. How does the Size of the ArrayList Increase Dynamically?

- The `ArrayList` increases its size by 50% of its current size when it becomes full. This involves creating a new, larger array and copying the elements to it.


Q. 14. How To Remove Duplicate Elements From ArrayList In Java?

You can remove duplicates using a `Set`:



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