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Collections Framework

General​

Q1: What is the Collections Framework in Java?
A: It provides interfaces and classes for storing and manipulating groups of objects, such as List, Set, Map, Queue, and Deque. It standardizes common data structures and algorithms.

Q2: What is the difference between the Collection interface and the Collections utility class?
A: Collection is the root interface for most collection types. Collections is a utility class with static methods for sorting, searching, and synchronizing collections.

Q3: What is fail-fast behavior in Java collections?
A: Fail-fast iterators throw ConcurrentModificationException if the collection is modified structurally while iterating, except through the iterator's own methods.

Q4: What is the difference between Iterator and ListIterator?
A: Iterator can traverse collections forward only. ListIterator works with lists, can traverse forwards/backwards, and can modify the list during iteration.

List​

Q5: Difference between ArrayList and LinkedList.
A: ArrayList is backed by a resizable array and provides fast random access. LinkedList is a doubly-linked list and offers faster insertions/deletions at the ends or middle.

Q6: What is the difference between ArrayList and Vector?
A: Vector is synchronized and thread-safe, while ArrayList is not. ArrayList is generally preferred for single-threaded use because it has less overhead.

Q7: When should you use ArrayList over LinkedList?
A: Use ArrayList when you need fast random access and fewer inserts/removals. Use LinkedList when you need many insertions/removals in the middle of the list.

Q8: What is the RandomAccess marker interface?
A: RandomAccess is a marker interface used by list implementations like ArrayList to indicate they support fast random access.

Q9: How can you safely remove elements while iterating a list?
A: Use the iterator's remove() method or ListIterator's remove() method. Avoid modifying the list directly while iterating with a standard iterator.

Set​

Q10: What is the difference between HashSet, LinkedHashSet, and TreeSet?
A: HashSet does not preserve order. LinkedHashSet preserves insertion order. TreeSet sorts elements naturally or by a comparator.

Q11: How does HashSet prevent duplicate elements?
A: HashSet uses the hash code and equals() methods. It stores elements in buckets and rejects duplicates when equals() returns true for matching hash codes.

Q12: What is EnumSet and when should you use it?
A: EnumSet is a high-performance set implementation for enum types. Use it when you need a compact set of enum values with fast iteration.

Q13: What is the difference between SortedSet and NavigableSet?
A: SortedSet maintains elements in sorted order. NavigableSet extends SortedSet with methods like lower(), higher(), floor(), and ceiling().

Map​

Q14: Explain HashMap.
A: HashMap is a Map implementation that stores key-value pairs. It uses hashing for fast retrieval. Keys must be unique; it allows one null key and multiple null values.

Q15: What is the difference between HashMap and Hashtable?
A: HashMap is not synchronized and allows null keys/values. Hashtable is synchronized and does not allow null keys or values.

Q16: When should you use ConcurrentHashMap?
A: Use ConcurrentHashMap in multi-threaded environments where many threads read and update the map concurrently. It provides thread-safe operations with better concurrency than synchronized maps.

Q17: What is LinkedHashMap accessOrder and when is it used?
A: LinkedHashMap can preserve insertion order or access order. accessOrder=true creates an LRU-style map useful for caches.

Q18: How do you iterate over a Map efficiently?
A: Iterate over entrySet() to access keys and values in one pass. Using keySet() or values() can be less efficient when both key and value are needed.

Q19: What happens if a map key's hashCode changes after insertion?
A: The map may not be able to locate the entry anymore because the hash code changes the bucket used for storage. Keys should be immutable while stored in hash-based maps.

Queue and Stack​

Q20: What is the Queue interface and which implementations are commonly used?
A: Queue represents a FIFO collection. Common implementations include LinkedList, ArrayDeque, and PriorityQueue.

Q21: What is the difference between PriorityQueue and ArrayDeque?
A: PriorityQueue orders elements by priority and does not guarantee FIFO order. ArrayDeque is a double-ended queue optimized for FIFO and LIFO operations.

Q22: When should you use Deque instead of Stack?
A: Deque is preferred over the legacy Stack class. Use implementations like ArrayDeque or LinkedList for stack behavior with push(), pop(), and peek().

Q23: What is the difference between poll, remove, peek, and element in Queue?
A: poll() and peek() return null when the queue is empty. remove() and element() throw an exception if empty.

Q24: How do you implement stack behavior using Deque?
A: Use Deque.push() to add, Deque.pop() to remove, and Deque.peek() to inspect the top element for LIFO behavior.

Tips​

  • Prefer ArrayList for random access and LinkedList for frequent middle insertions.
  • Use HashSet or LinkedHashSet when uniqueness is required. Use TreeSet for sorted sets.
  • Use HashMap for general-purpose maps, LinkedHashMap for predictable iteration order, and ConcurrentHashMap for thread-safe concurrent access.
  • Prefer ArrayDeque for stack/queue use instead of Stack or LinkedList when performance matters.