Java.Collections.You need to add 1 million elements, what structure do you use?

When you need to add 1 million elements, the best choice depends on where and how you’re adding them.

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🧪 Scenario: Adding 1 million elements to the end

✅ Use ArrayList

Why?

• Appending to the end is O(1) amortized.
• Resizing happens occasionally, but it’s fast overall.
• Excellent cache performance due to contiguous memory.
• Much faster iteration and access than LinkedList.

List<Integer> list = new ArrayList<>(1_000_000); // Pre-size to avoid resizing
for (int i = 0; i < 1_000_000; i++) {
    list.add(i);
}

This is the most efficient way—fast and memory-friendly.

🧪 Scenario: Adding 1 million elements to the beginning

❌ ArrayList is a poor choice here — every insertion shifts all existing elements → O(n²) total time.

✅ Use LinkedList if you must insert at the beginning:

List<Integer> list = new LinkedList<>();
for (int i = 0; i < 1_000_000; i++) {
    list.add(0, i); // Fast in LinkedList, terrible in ArrayList
}

But note: iterating through a LinkedList of a million elements is slower and uses more memory.

🧠 Bonus Pro Tip:

If you’re doing lots of insertions at both ends and performance matters, consider:

• ✅ Deque (ArrayDeque or LinkedList) — optimized for head & tail operations.
• ✅ ArrayList with batch add at the end.

🔚 Summary:

Task	Best Structure
Add 1M to the end	✅ ArrayList
Add 1M to the beginning	✅ LinkedList
Add 1M randomly in middle	❌ Neither great — consider data structure rethink (e.g., TreeList, Deque)