ArrayList vs LinkedList in Java (Complete Guide for Interviews & Backend)

Most developers think they understand ArrayList and LinkedList… until an interviewer asks:

“When would you use one over the other?”

If you can’t confidently answer that — this guide will fix it.

Watch Full Video Explanation

What is a List in Java?

A List is an ordered collection that:

Maintains insertion order
Allows duplicate elements
Supports index-based access

List<Integer> list = new ArrayList<>();

ArrayList Deep Dive

1. Internal Working

ArrayList is backed by a dynamic array.

2. Contiguous Memory

[10] [20] [30] [40]

👉 Enables fast access → O(1)

3. Size vs Capacity

Size = 3
Capacity = 5

[10] [20] [30] [_] [_]

4. Resizing Mechanism

New array created
Capacity increases (~1.5x)
Elements copied

👉 Resizing cost = O(n)

5. Performance

Operation	Time Complexity
Access	O(1)
Insert (end)	O(1) amortized
Insert (middle)	O(n)
Delete	O(n)

⚠️ Limitation

Before: [10, 20, 30, 40]
Insert 25

After:  [10, 20, 25, 30, 40]

👉 Shifting required → O(n)

LinkedList Deep Dive

1. Structure

[10] → [20] → [30] → null

Each node contains:

Data
Pointer to next node

2. Key Advantage

No shifting required — only pointer updates

3. Performance

Operation	Time Complexity
Access	O(n)
Insert	O(1)*
Delete	O(1)*

*after traversal

Interview Problem — Insertion

Node curr = head;

while (curr.next != null && curr.next.data < key) {
    curr = curr.next;
}

Node newNode = new Node(key);
newNode.next = curr.next;
curr.next = newNode;

👉 Core idea = pointer manipulation

Edge Cases (CRITICAL)

Insert at head
Insert at end
Empty list
Middle insertion

👉 Every LinkedList problem = Logic + Edge Cases

Deletion in LinkedList

Node curr = head;

while (curr.next != null && curr.next.data != key) {
    curr = curr.next;
}

if (curr.next != null) {
    curr.next = curr.next.next;
}