/*
* Copyright (c) 1994, 2010, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util;
/**
* The Stack
class represents a last-in-first-out
* (LIFO) stack of objects. It extends class Vector with five
* operations that allow a vector to be treated as a stack. The usual
* push and pop operations are provided, as well as a
* method to peek at the top item on the stack, a method to test
* for whether the stack is empty, and a method to search
* the stack for an item and discover how far it is from the top.
*
* When a stack is first created, it contains no items.
*
*
A more complete and consistent set of LIFO stack operations is
* provided by the {@link Deque} interface and its implementations, which
* should be used in preference to this class. For example:
*
{@code
* Deque stack = new ArrayDeque();}
*
* @author Jonathan Payne
* @since JDK1.0
*/
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:
*
* addElement(item)
*
* @param item the item to be pushed onto this stack.
* @return the item
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 Vector object).
* @throws EmptyStackException if this stack is empty.
*/
//从此可以看出Stack类是线程安全的
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 Vector 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 true
if and only if this stack contains
* no items; false
otherwise.
*/
public boolean empty() {
return size() == 0;
}
/**
* Returns the 1-based position where an object is on this stack.
* If the object o 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 1. The equals
* method is used to compare o 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 -1
* 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;
}
从此可以看出Stack类是继承于Vertor向量,Vector也是基于数组实现的,下面简要分析一下Vector的源码:
public class Vector<E>
extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
//用来保存Vector向量的数组
protected Object[] elementData;
//用来保存当前数组长度
protected int elementCount;
/*当向量大小大于其容量时,容量自动增长的量。capacityIncrement<=0时候,向量容量增长一倍*/
protected int capacityIncrement;
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = -2767605614048989439L;
/*下面是构造函数*/
//使用指定的初始容量和容量增量构造一个空的向量
public Vector(int initialCapacity, int capacityIncrement) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);
this.elementData = new Object[initialCapacity];
this.capacityIncrement = capacityIncrement;
}
//使用指定的初始容量和等于零的容量增量构造一个空向量
public Vector(int initialCapacity) {
this(initialCapacity, 0);
}
//构造一个空向量,使其内部数据数组的大小为 10,其标准容量增量为零
public Vector() {
this(10);
}
//构造一个包含指定 collection 中的元素的向量,这些元素按其 collection 的迭代器返回元素的顺序排列
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);
}
//下面会列举在Stack类中用到的Vector中的函数
//添加新组件(其实我也不知道为什么要叫组件,暂且就认为是一种元素吧),时候要确保向量容量够大,否则就要增加其容量
public synchronized void addElement(E obj) {
modCount++;//这个变量的定义没找到。。。
ensureCapacityHelper(elementCount + 1);
elementData[elementCount++] = obj;
}
//如果向量大小超过其容量,曾要grow其容量
private void ensureCapacityHelper(int minCapacity) {
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
/*这里调用了Arrays类的静态方法copyOf(int[] original, int newLength) 复制指定的数组,截取或用 0 填充(如有必要),以使副本具有指定的长度*/
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);
}
//返回当前向量的大小,注意函数加了synchronized
public synchronized int size() {
return elementCount;
}
//移除向量中指定位置处的元素
public synchronized void removeElementAt(int index) {
modCount++;
if (index >= elementCount) {
throw new ArrayIndexOutOfBoundsException(index + " >= " +
elementCount);
}
else if (index < 0) {
throw new ArrayIndexOutOfBoundsException(index);
}
int j = elementCount - index - 1;
if (j > 0) {
System.arraycopy(elementData, index + 1, elementData, index, j);//我想这个应该就是数组中后面元素前移的动作吧
}
elementCount--;
elementData[elementCount] = null; /* to let gc do its work */ /*哈哈,很清楚的告诉我们,如果删除数组中对象,将其设置为null,这样垃圾回收器好尽快回收*/
}
//返回指定位置处的元素值
public synchronized E elementAt(int index) {
if (index >= elementCount) {
throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
}
return elementData(index);
}
//数组的访问很简单,只要给出下标即可
E elementData(int index) {
return (E) elementData[index];
}
}
下面简单比较一下ArrayList和Vector,两者均实现了List接口。
Vector和ArrayList在使用上非常相似,都可用来表示一组数量可变的对象应用的集合,并且可以随机地访问其中的元素。
1 Vector的方法都是同步的(Synchronized),是线程安全的(thread-safe),而ArrayList的方法不是,由于线程的同步必然要影响性能,因此,ArrayList的性能比Vector好。
2 当Vector或ArrayList中的元素超过它的初始大小时,Vector会将它的容量翻倍,而ArrayList只增加50%的大小,这样,ArrayList就有利于节约内存空间
下面代码是ArrayList类中容量增长方式的实现:
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;
int newCapacity = oldCapacity + (oldCapacity >> 1);//右移一位,表示除以2
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);
}