NOTE
ArrayList
ArrayList's resizable-array model, complexity, capacity growth, iteration, and concurrency characteristics.
This is a historical learning note and may contain outdated or incomplete understanding.
ArrayList stores elements in a resizable array. Indexed reads are constant-time in the usual model; appending is amortized constant-time; insertion/removal in the middle usually shifts elements and is linear.
Capacity growth is an implementation detail. ensureCapacity can reduce reallocations when a large final size is known, but code should not depend on a specific growth factor.
ArrayList is not thread-safe. Concurrent structural mutation requires external synchronization or a collection designed for the access pattern. Its iterators are fail-fast on a best-effort basis, not a concurrency-safety mechanism.