Java.Collections.How much additional memory is needed when calling ArrayList.add()?

✅ Normal case (no resize)

list.add(element);

And the internal array still has room, the memory cost is:

💾 Only the reference to the new element is added (e.g., 4 or 8 bytes depending on JVM architecture).

So:

• No new array allocation
• Just storing a pointer to the object
• Super efficient: O(1) time + tiny memory cost

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🚨 When resizing happens

When the internal array is full, calling add() triggers a resize, which means:

1. A new array is created with a larger capacity (usually 1.5× the old size).
2. All existing references are copied into the new array.
3. The new element is added.

🧠 Memory cost in this case:

• New array: needs memory for newCapacity references
• Old array: still exists temporarily until garbage collected
• Total temporary memory: roughly (oldCapacity + newCapacity) × reference size

For a large list, this can mean hundreds of KB or even MB of extra memory during a resize.

🔢 Example:

Say your ArrayList grows from 1,000,000 to 1,500,000 elements:

• Old array: 1,000,000 refs × 4 bytes = ~4 MB
• New array: 1,500,000 refs × 4 bytes = ~6 MB
• Peak temporary memory usage: ~10 MB

(plus memory for the actual objects pointed to, but that’s separate)

✅ Best practice:

If you know in advance how many elements you’ll add, pre-size the list:

ArrayList<Integer> list = new ArrayList<>(1_000_000);

This avoids multiple resizes and reduces memory churn.

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🧠 Summary:

Scenario	Additional memory
add() with room	🟢 Minimal (1 ref)
add() with resize	🔴 High (new array + temporary double memory)
Pre-sized ArrayList	🟡 Optimal (no resize)