java集合vector与stack

Vector与ArrayList的最大区别就是Vector是线程安全的，而ArrayList不是线程安全的。另外区别还有：
- ArrayList不可以设置扩展的容量，默认1.5倍；Vector可以设置扩展的容量，如果没有设置，默认2倍
- ArrayList的无参构造方法中初始容量为0，而Vector的无参构造方法中初始容量为10。
- Vector线程安全，ArrayList线程不安全。

Vector源码介绍
Vector有四个构造方法，其内部有两个重要的参数，一个是elementCount代表当前元素个数，一个是capacityIncrement，代表当列表元素满了之后增加的容量。如果不设置capacityIncrement，那么Vector容量扩展时默认将扩展两倍，在ArrayList源码分析中，我们知道ArrayList在扩容时默认将扩展1.5倍，所以这又是ArrayList与Vector的一个区别。

public class Vector<E>
extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable

public Vector(int initialCapacity, int capacityIncrement) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
this.elementData = new Object[initialCapacity];
this.capacityIncrement = capacityIncrement;
    }


public Vector(int initialCapacity) {
this(initialCapacity, 0);
    }


public Vector() {
this(10);
    }  


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);
    }   

添加操作

public synchronized boolean add(E e) {
        modCount++;
//确保容量足够
        ensureCapacityHelper(elementCount + 1);
//添加元素
        elementData[elementCount++] = e;
return true;
    }
从上面代码可以看到，add()方法用synchronized关键字修饰，所以是线程安全的，ensureCapacityHelper()方法用于确保容量足够，不够时扩展容量，其实现如下：
private void ensureCapacityHelper(int minCapacity) {
// overflow-conscious code
if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

可以看到，当需要扩容时，将调用grow()方法
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);
    }

Stack源码分析

public
class Stack<E> extends Vector<E> {
    /**
     * Creates an empty Stack.
     */
    public Stack() {
    }

    /**
     * Pushes an item onto the top of this stack. This has exactly
     * the same effect as:
     * <blockquote><pre>
     * addElement(item)</pre></blockquote>
     *
     * @param   item   the item to be pushed onto this stack.
     * @return  the <code>item</code> argument.
     * @see     java.util.Vector#addElement
     */
    public E push(E item) {
        addElement(item);

 return item;
    }

    /**
     * Removes the object at the top of this stack and returns that
     * object as the value of this function.
     *
     * @return  The object at the top of this stack (the last item
     *          of the <tt>Vector</tt> object).
     * @throws  EmptyStackException  if this stack is empty.
     */
    public synchronized E pop() {
        E       obj;
        int     len = size();

        obj = peek();
        removeElementAt(len - 1);

 return obj;
    }

    /**
     * Looks at the object at the top of this stack without removing it
     * from the stack.
     *
     * @return  the object at the top of this stack (the last item
     *          of the <tt>Vector</tt> object).
     * @throws  EmptyStackException  if this stack is empty.
     */
    public synchronized E peek() {
        int     len = size();

if (len == 0)
            throw new EmptyStackException();
 return elementAt(len - 1);
    }

    /**
     * Tests if this stack is empty.
     *
     * @return  <code>true</code> if and only if this stack contains
     *          no items; <code>false</code> otherwise.
     */
    public boolean empty() {
 return size() == 0;
    }

    /**
     * Returns the 1-based position where an object is on this stack.
     * If the object <tt>o</tt> occurs as an item in this stack, this
     * method returns the distance from the top of the stack of the
     * occurrence nearest the top of the stack; the topmost item on the
     * stack is considered to be at distance <tt>1</tt>. The <tt>equals</tt>
     * method is used to compare <tt>o</tt> to the
     * items in this stack.
     *
     * @param   o   the desired object.
     * @return  the 1-based position from the top of the stack where
     *          the object is located; the return value <code>-1</code>
     *          indicates that the object is not on the stack.
     */
    public synchronized int search(Object o) {
        int i = lastIndexOf(o);

if (i >= 0) {
 return size() - i;
        }
 return -1;
    }

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = 1224463164541339165L;
}