- 当存储空间不足时，ArrayList默认增加原始容量的一半，而Vector默认增加为原来的一倍。
- Vector源码的方法中多有synchronized关键字，ArrayList中没有，所以Vector是线程安全的，但性能更低。所以，建议在单线程中才使用 ArrayList，而在多线程中可以选择 Vector 或者 CopyOnWriteArrayList。
- Vector可以设置一个指定的容量增量，ArrayList则不可以。
ArrayList扩容机制：（int newCapacity = oldCapacity + (oldCapacity >> 1);
//下面是ArrayList的扩容机制
//ArrayList的扩容机制提高了性能，如果每次只扩充一个，
//那么频繁的插入会导致频繁的拷贝，降低性能，而ArrayList的扩容机制避免了这种情况。/*** 如有必要，增加此ArrayList实例的容量，以确保它至少能容纳元素的数量* @param minCapacity 所需的最小容量*/public void ensureCapacity(int minCapacity) {int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)// any size if not default element table? 0// larger than default for default empty table. It's already// supposed to be at default size.: DEFAULT_CAPACITY;if (minCapacity > minExpand) {ensureExplicitCapacity(minCapacity);}}//得到最小扩容量private void ensureCapacityInternal(int minCapacity) {if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {// 获取默认的容量和传入参数的较大值minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);}ensureExplicitCapacity(minCapacity);}//判断是否需要扩容private void ensureExplicitCapacity(int minCapacity) {modCount++;// overflow-conscious codeif (minCapacity - elementData.length > 0)//调用grow方法进行扩容，调用此方法代表已经开始扩容了grow(minCapacity);}/*** 要分配的最大数组大小*/private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;/*** ArrayList扩容的核心方法。*/private void grow(int minCapacity) {// oldCapacity为旧容量，newCapacity为新容量int oldCapacity = elementData.length;//将oldCapacity 右移一位，其效果相当于oldCapacity /2，//我们知道位运算的速度远远快于整除运算，整句运算式的结果就是将新容量更新为旧容量的1.5倍，int newCapacity = oldCapacity + (oldCapacity >> 1);//然后检查新容量是否大于最小需要容量，若还是小于最小需要容量，那么就把最小需要容量当作数组的新容量，if (newCapacity - minCapacity < 0)newCapacity = minCapacity;//再检查新容量是否超出了ArrayList所定义的最大容量，//若超出了，则调用hugeCapacity()来比较minCapacity和 MAX_ARRAY_SIZE，//如果minCapacity大于MAX_ARRAY_SIZE，则新容量则为Interger.MAX_VALUE，否则，新容量大小则为 MAX_ARRAY_SIZE。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);}//比较minCapacity和 MAX_ARRAY_SIZEprivate static int hugeCapacity(int minCapacity) {if (minCapacity < 0) // overflowthrow new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :MAX_ARRAY_SIZE;}
Vector扩容机制：（int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
capacityIncrement : oldCapacity);）
public synchronized void ensureCapacity(int minCapacity) {if (minCapacity > 0) {modCount++;ensureCapacityHelper(minCapacity);}}
private void ensureCapacityHelper(int minCapacity) {// overflow-conscious codeif (minCapacity - elementData.length > 0)grow(minCapacity);}
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;private void grow(int minCapacity) {// overflow-conscious codeint oldCapacity = elementData.length;int newCapacity = oldCapacity + ((capacityIncrement > 0) ?capacityIncrement : oldCapacity);if (newCapacity - minCapacity < 0)newCapacity = minCapacity;if (newCapacity - MAX_ARRAY_SIZE > 0)newCapacity = hugeCapacity(minCapacity);elementData = Arrays.copyOf(elementData, newCapacity);}private static int hugeCapacity(int minCapacity) {if (minCapacity < 0) // overflowthrow new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :MAX_ARRAY_SIZE;}