Collections Interview Questions and Answers for 10 years experience

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100 Collections Interview Questions & Answers (10 Years Experience)

  1. What are the key differences between a HashMap and a TreeMap in Java?

    • Answer: HashMap uses a hash function for key-based lookup, providing O(1) average time complexity for get, put, and remove operations. It does not maintain any specific order. TreeMap, on the other hand, uses a Red-Black tree for storing key-value pairs, providing O(log n) time complexity for the same operations. It maintains keys in a sorted order (natural ordering or based on a provided Comparator).

  2. Explain the concept of fail-fast iterators.

    • Answer: Fail-fast iterators are iterators that throw a `ConcurrentModificationException` if the underlying collection is structurally modified after the iterator is created, but before the iteration is complete. This helps detect concurrent modification issues, enhancing thread safety.

  3. How do you handle exceptions during collection processing?

    • Answer: Exceptions are handled using try-catch blocks. Specifically, for collections, I'd use try-catch blocks within loops iterating over the collection. Depending on the exception type and context, you might log the error, skip the problematic element, retry the operation, or terminate the processing. Consider using a checked exception approach for critical errors that require the calling method to handle the exception explicitly.

  4. Describe different ways to sort a list in Java.

    • Answer: Java offers several ways to sort a List: 1. Using `Collections.sort()`: This method uses merge sort for stable sorting. 2. Using a custom `Comparator`: This allows sorting based on specific criteria. 3. Using streams with `sorted()`: This provides a functional approach to sorting. The choice depends on the specific needs and complexity of the sorting criteria.

  5. What is the difference between ArrayList and LinkedList? When would you choose one over the other?

    • Answer: ArrayList uses a dynamic array internally, providing fast random access (O(1)) but slower insertion/deletion in the middle (O(n)). LinkedList uses a doubly-linked list, offering fast insertion/deletion (O(1)) but slower random access (O(n)). Choose ArrayList for frequent random access, and LinkedList for frequent insertions/deletions, especially in the middle of the list.

  6. Explain the concept of a Set in Java. What are the different types of Sets?

    • Answer: A Set is an unordered collection of unique elements. Java offers various Set implementations: HashSet (uses a hash table, unordered), LinkedHashSet (maintains insertion order), TreeSet (uses a tree structure, sorted).

  7. How do you remove duplicates from a list in Java?

    • Answer: Several approaches exist: 1. Using a Set: Convert the list to a Set to automatically remove duplicates, then convert back to a list if necessary. 2. Iterating and checking for duplicates: Iterate through the list, and remove elements that are already present in a separate collection. 3. Using streams: Stream the list, filter out duplicates, and collect to a new list.

  8. What is a Queue and how is it used?

    • Answer: A Queue is a linear collection that follows the FIFO (First-In, First-Out) principle. Elements are added to the rear (enqueue) and removed from the front (dequeue). It's commonly used in scenarios like task scheduling, buffering, and breadth-first search.

  9. What is a Stack and how is it used?

    • Answer: A Stack is a linear collection that follows the LIFO (Last-In, First-Out) principle. Elements are added and removed from the top. It's used in function calls (call stack), expression evaluation (reverse Polish notation), and undo/redo functionality.

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