ArrayList源碼

ArrayList內部由數組構成，初始容量0

private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData; 
private int size;

擴容爲1.5倍

int newCapacity = oldCapacity + (oldCapacity >> 1);

內部採用數組操作

elementData = Arrays.copyOf(elementData, newCapacity);

非線程安全