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
i
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nt 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;
}