List
特点：1.有序。2.可重复。
ArrayList:
底层是数组，数组是有下标的。
会自动扩容，底层默认初始化容量是10，扩大之后的容量预设是原来容量的一半（jdk 1.8)。以前好像是原容量的1.5倍
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);
}
Vector：
底层也是数组，扩容可以通过构造函数设置。默认初始化容量也是10，扩容一倍原来容量(在没有设置capacityIncrement的情况下）
public Vector(int initialCapacity, int capacityIncrement) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
this.capacityIncrement = capacityIncrement;
}
/**
* Constructs an empty vector with the specified initial capacity and
* with its capacity increment equal to zero.
*
* @param initialCapacity the initial capacity of the vector
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public Vector(int initialCapacity) {
this(initialCapacity, 0);
}
/**
* Constructs an empty vector so that its internal data array
* has size {@code 10} and its standard capacity increment is
* zero.
*/
public Vector() {
this(10);
}
private void grow(int minCapacity) {
// overflow-conscious code
int 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);
}
如何优化ArrayList和Vector?
尽量减少扩容操作，因为每次扩容都要拷贝数组（Arrays.copyof)，拷贝数组很消耗内存，所以一般推荐在创建集合的时候指定初始化容量。
对比ArrayList，LinkedList，Vector？
Vector是线程安全的，效率很低，所以现在很少使用。
ArrayList适合查询，不适合频繁随机的增删元素。
LinkedList适合频繁的增删元素，不适合查询。