ArrayList的扩容方式和扩容时机

初始化

ArrayList的底层是⼀个动态数组，ArrayList⾸先会对传进来的初始化参数initalCapacity进⾏判断如果参数等于0，则将数组初始化为⼀个空数组，

如果不等于0，将数组初始化为⼀个容量为10的数组。 扩容时机

当数组的⼤⼩⼤于初始容量的时候(⽐如初始为10，当添加第11个元素的时候)，就会进⾏扩容，新的容量为旧的容量的1.5倍。扩容⽅式

扩容的时候，会以新的容量建⼀个原数组的拷贝，修改原数组，指向这个新数组，原数组被抛弃，会被GC回收。//ArraList初始化容量判断

public void ensureCapacity(int minCapacity) {

int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) 0 : DEFAULT_CAPACITY; if (minCapacity > minExpand) {

ensureExplicitCapacity(minCapacity); } }

//添加元素的⽅法

public boolean add(E e) {

ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; }

//判断是否需要扩容

private void ensureExplicitCapacity(int minCapacity) { modCount++;

// overflow-conscious code

if (minCapacity - elementData.length > 0) grow(minCapacity); }

//扩容的机制

private void grow(int minCapacity) { // overflow-conscious code

int oldCapacity = elementData.length;

int newCapacity = oldCapacity + (oldCapacity >> 1); if (newCapacity - minCapacity < 0) newCapacity = minCapacity;

if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity);

// minCapacity is usually close to size, so this is a win:

elementData = Arrays.copyOf(elementData, newCapacity); }