ArrayList - Concept Review: How Much Do You Remember? Q1. What is the default capacity of an ArrayList in Java? Q2. How does an ArrayList grow when the capacity is full? → It increases by 50% of its current capacity.
→ Formula: `new capacity = old + (old / 2)`
Q3. What is the difference between size() and capacity in an ArrayList? → `size()` = number of elements added
→ `capacity` = number of elements the internal array can hold
Q4. Where is an ArrayList stored in memory? → The reference (e.g., `al`) is stored in the stack
→ The actual data is stored in the heap
Q5. What happens when an ArrayList is full and a new element is added? → A new internal array is created with increased capacity,
old elements are copied into it, and the reference is updated.
Q6. Is ArrayList thread-safe? → No. It is not synchronized by default.
Q7. How to make an ArrayList thread-safe? → Use Collections.synchronizedList(list) or CopyOnWriteArrayList<>
Q8. What is the drawback of CopyOnWriteArrayList? → It is expensive — creates a new copy on every write operation.
Q9. Can an ArrayList hold primitive types like int? → No. It can only hold objects like `Integer` (wrapper class).
Q10. What will happen if you try to access an index greater than size? → IndexOutOfBoundsException
Q11. How to remove all elements from an ArrayList? Q12. Can ArrayList store null values? → Yes, multiple null`s are allowed
Q13. Does ArrayList maintain insertion order? Q14. How do you sort an ArrayList? → Collections.sort(list) for natural order
→ list.sort(Comparator) for custom order
Q15. Difference between remove(int) and remove(Object)? → remove(index) removes at given index
→ remove(object) removes first occurrence of that object
Q16. What does contains() vs indexOf() do? → contains(obj) returns true/false
→ indexOf(obj) returns index or `-1` if not found.
Q17. Can we clone an ArrayList? → Yes, using .clone() — it performs a shallow copy
Q18. How to make a read-only ArrayList? → Collections.unmodifiableList(list)
Q19. Is ArrayList fail-fast or fail-safe? → Fail-fast — throws ConcurrentModificationException if modified while iterating.
Q20. How does ArrayList differ from array in memory management? → Arrays are fixed-size; ArrayList grows dynamically & manages internal memory.
Q21. Can we manually set the capacity of an ArrayList?** → Yes, using new ArrayList<>(initialCapacity)