Interview replay

Full round replay — ArrayList vs LinkedList

10 minintermediate0–8 yrs

Timed verbal replay with pass/fail criteria per follow-up.

How to run this

Answer out loud, timed. Do not read the entry first. Then compare against "The Answer" and "Interviewer's Next Move" and mark yourself.

The opener

“When would you use LinkedList instead of ArrayList?”

Budget: 45 seconds. Going long here is itself a fail signal.

Follow-ups

  1. 1. “LinkedList is better for inserting in the middle — agreed?”

    Testing: The central myth. Will they defend it or measure it?

    Scoring

    Pass: No. Both are O(n) at an index because add(i, e) walks to i first. Measured on JDK 21 ArrayList is ~23x faster: arraycopy moves contiguous bytes at memory bandwidth, LinkedList takes a cache miss per hop.

    Fail: Agrees, or hedges with 'in theory' and stops there.

  2. 2. “So when is LinkedList's O(1) insertion actually available?”

    Testing: Do they know the difference between holding a node and knowing an index?

    Scoring

    Pass: When you already hold the position — traversing with a ListIterator and calling its add/remove. Reaching a position by index never qualifies.

    Fail: Cannot name a case, or repeats 'in the middle'.

  3. 3. “Then use it for a queue?”

    Testing: ArrayDeque awareness — Java 6.

    Scoring

    Pass: No, ArrayDeque. Circular array, both ends are index arithmetic, no node per element. Its Javadoc says it is likely faster than LinkedList as a queue.

    Fail: Yes, LinkedList is the queue.

  4. 4. “Is iterating a LinkedList slow?”

    Testing: Separating iteration from indexed access.

    Scoring

    Pass: No — for-each uses an iterator and is competitive. An index loop calling get(i) is O(n^2), which is a bug rather than a property of the structure.

    Fail: Says iteration is slow, with no mention of index loops.

  5. 5. “Which uses more memory, and roughly by how much?”

    Testing: Whether they have a number or just a direction.

    Scoring

    Pass: LinkedList: a Node per element with item plus two pointers, ~24 bytes overhead each, against ~4 bytes per array slot. ArrayList wastes up to ~50% of its capacity after growth, still far less.

    Fail: 'LinkedList, because of pointers' with no magnitude.

  6. 6. “Does an ArrayList ever release memory when you remove elements?”

    Testing: Growth is amortised; shrinking is not automatic.

    Scoring

    Pass: No. Capacity only grows, around 1.5x per growth. trimToSize() is the only way back and it is not on the List interface.

    Fail: Assumes it shrinks.

  7. 7. “Difference between Arrays.asList and List.of?”

    Testing: A daily-use distinction most people get wrong.

    Scoring

    Pass: Arrays.asList is a fixed-size view over the array you passed — set() works and writes through, add() throws. List.of is immutable and rejects null elements with an NPE.

    Fail: Says both are immutable.

  8. 8. “What did Java 21 change here?”

    Testing: SequencedCollection, JEP 431.

    Scoring

    Pass: getFirst/getLast/addFirst/addLast/reversed on List, so ArrayList got the ergonomics LinkedList had. reversed() is a view, and the complexities did not change.

    Fail: Nothing / unaware.

Score yourself

8/8 — you can hold this at senior level 6-7 — solid; reread "Interviewer's Next Move" 4-5 — you know the Big-O table and not the constants; redo the Challenge 0-3 — run the benchmark in the Warm-up before rereading anything

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