1：Vector简介
Java中Vector类是允许不同类型元素共存的变长数组，Java.util.Vector提供了向量（Vector）类以实现类似动态数组的功能。在Java语言中是没有指针概念的，但如果能正确灵活地使用指针又确实可以大大提高程序的质量，比如在C、C++中所谓“动态数组”一般都由指针来实现。为了弥补这点缺陷，Java提供了丰富的类库来方便编程者使用，Vector类便是其中之一。事实上，灵活使用数组也可完成向量类的功能，向量类中提供的大量方法也大大方便了用户的使用。
Vector 作为一款古老的集合工具（since JDK1.0），从 JDK1.2 开始被改造成 List 的实现加入集合框架。它的实现基本与ArrayList相同，主要区别在于它是一个线程安全的集合。
2：属性和构造方法
/**
* The array buffer into which the components of the vector are
* stored. The capacity of the vector is the length of this array buffer,
* and is at least large enough to contain all the vector's elements.
*
* <p>Any array elements following the last element in the Vector are null.
*
* @serial
* 存放元素的数组
*/
protected Object[] elementData;
/**
* The number of valid components in this {@code Vector} object.
* Components {@code elementData[0]} through
* {@code elementData[elementCount-1]} are the actual items.
*
* @serial
* 数组中实际元素数量
*/
protected int elementCount;
/**
* The amount by which the capacity of the vector is automatically
* incremented when its size becomes greater than its capacity. If
* the capacity increment is less than or equal to zero, the capacity
* of the vector is doubled each time it needs to grow.
*
* @serial
* 每次扩容的大小，如果为 0，每次扩容为原来的两倍
*/
protected int capacityIncrement;
/**
* Constructs an empty vector with the specified initial capacity and
* capacity increment.
*
* @param initialCapacity the initial capacity of the vector
* @param capacityIncrement the amount by which the capacity is
* increased when the vector overflows
* @throws IllegalArgumentException if the specified initial capacity
* is negative
* 初始化容器，并指定初始容量和扩容大小
*/
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
* 初始化容器并指定初始容量，扩容大小默认为0(即每次扩容为原来的两倍)
*/
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.
* 初始化容器并指定初始容量10，扩容大小默认为0(即每次扩容为原来的两倍)
*/
public Vector() {
this(10);
}
/**
* Constructs a vector containing the elements of the specified
* collection, in the order they are returned by the collection's
* iterator.
*
* @param c the collection whose elements are to be placed into this
* vector
* @throws NullPointerException if the specified collection is null
* @since 1.2
* 以一个集合为默认元素，初始化容器
*/
public Vector(Collection<? extends E> c) {
elementData = c.toArray();
elementCount = elementData.length;
// c.toArray might (incorrectly) not return Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
}3：增删查改
增加和删除操作才算作结构的改变，才会记录修改次数。修改时不记录修改次数。
3.1：增加元素
/**
* Appends the specified element to the end of this Vector.
*
* @param e element to be appended to this Vector
* @return {@code true} (as specified by {@link Collection#add})
* @since 1.2
* 插入元素到数组的末尾
*/
public synchronized boolean add(E e) {
modCount++;
ensureCapacityHelper(elementCount + 1);
elementData[elementCount++] = e;
return true;
}
/**
* Adds the specified component to the end of this vector,
* increasing its size by one. The capacity of this vector is
* increased if its size becomes greater than its capacity.
*
* <p>This method is identical in functionality to the
* {@link #add(Object) add(E)}
* method (which is part of the {@link List} interface).
*
* @param obj the component to be added
* 与add(E e)方法相同，但是没有返回值
*/
public synchronized void addElement(E obj) {
modCount++;
ensureCapacityHelper(elementCount + 1);
elementData[elementCount++] = obj;
}
/**
* This implements the unsynchronized semantics of ensureCapacity.
* Synchronized methods in this class can internally call this
* method for ensuring capacity without incurring the cost of an
* extra synchronization.
*
* @see #ensureCapacity(int)
* 超出容量即进行扩增
*/
private void ensureCapacityHelper(int minCapacity) {
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
* 允许数组最大容量值
*/
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
// 容量扩增处理
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);
}
/**
* Inserts the specified element at the specified position in this Vector.
* Shifts the element currently at that position (if any) and any
* subsequent elements to the right (adds one to their indices).
*
* @param index index at which the specified element is to be inserted
* @param element element to be inserted
* @throws ArrayIndexOutOfBoundsException if the index is out of range
* ({@code index < 0 || index > size()})
* @since 1.2
* 在某个位置插入元素
*/
public void add(int index, E element) {
insertElementAt(element, index);
}
/**
* Inserts the specified object as a component in this vector at the
* specified {@code index}. Each component in this vector with
* an index greater or equal to the specified {@code index} is
* shifted upward to have an index one greater than the value it had
* previously.
*
* <p>The index must be a value greater than or equal to {@code 0}
* and less than or equal to the current size of the vector. (If the
* index is equal to the current size of the vector, the new element
* is appended to the Vector.)
*
* <p>This method is identical in functionality to the
* {@link #add(int, Object) add(int, E)}
* method (which is part of the {@link List} interface). Note that the
* {@code add} method reverses the order of the parameters, to more closely
* match array usage.
*
* @param obj the component to insert
* @param index where to insert the new component
* @throws ArrayIndexOutOfBoundsException if the index is out of range
* ({@code index < 0 || index > size()})
* 在某个位置插入元素
*/
public synchronized void insertElementAt(E obj, int index) {
modCount++;
if (index > elementCount) {
throw new ArrayIndexOutOfBoundsException(index
+ " > " + elementCount);
}
ensureCapacityHelper(elementCount + 1);
System.arraycopy(elementData, index, elementData, index + 1, elementCount - index);
elementData[index] = obj;
elementCount++;
}
