Data Structures & Collections Face-Off: ArrayList, LinkedList, Sets & Maps

Tricky Question 24: "What is the difference between ArrayList and LinkedList?"

• ArrayList: Backed by a dynamic array. Fast O(1) random access by index. Slower O(n) for insertions/deletions in the middle due to element shifting.

• LinkedList: Backed by a doubly linked list. Fast O(1) insertions/deletions at head/tail. Slow O(n) traversal for index-based access.

• HashSet vs TreeSet (Tricky Question 31): HashSet is unordered and uses hashing (O(1) average lookup). TreeSet is sorted and uses a Red-Black self-balancing tree (O(log n) lookup).

• HashMap vs Hashtable (Tricky Question 33): HashMap is not thread-safe and allows one null key and multiple null values. Hashtable is thread-safe (legacy synchronized) and does not permit any null keys or values.

Tricky Question 35: "What is the difference between fail-fast and fail-safe iterators?"

• Fail-Fast: Throws `ConcurrentModificationException` immediately if the collection is structurally modified while being iterated over (e.g. standard ArrayList iterator).

• Fail-Safe: Does NOT throw exceptions. It operates on a clone or snapshot of the collection (e.g. `CopyOnWriteArrayList`).

Tricky Question 29: "What is the difference between Comparable and Comparator?"

• Comparable: Defines natural ordering within the class itself by implementing `compareTo()`.

• Comparator: Defines external, customizable ordering outside the class by implementing `compare()`. Allows multiple sorting strategies (e.g., sort by age, sort by salary).