Java Array Expansion: The Automatic Expansion Principle of ArrayList, a Must-Know for Beginners

ArrayList is a dynamic array in Java that solves the problem of fixed-length arrays. Its core mechanism is automatic resizing: when adding elements, if the current number of elements (size) equals the length of the internal array (elementData), a resizing is triggered. During resizing, the minimum capacity is calculated as (size + 1). The initial capacity for the no-argument constructor is 10 by default. For other cases, the new capacity is 1.5 times the original capacity (e.g., 10 → 15, 15 → 22). The original elements are then copied to the new array. Setting the resizing factor to 1.5 balances the performance overhead of frequent resizing and memory waste. Understanding this principle helps avoid array index out of bounds errors. Pre-estimating the number of elements and setting an initial capacity (e.g., `new ArrayList(100)`) can reduce resizing operations and improve efficiency. It should be noted that resizing requires array copying and is not unlimited in capacity.