Java中Stack类的实现——上源码

/*
 * 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 <code>Stack</code> class represents a last-in-first-out
 * (LIFO) stack of objects. It extends class <tt>Vector</tt> with five
 * operations that allow a vector to be treated as a stack. The usual
 * <tt>push</tt> and <tt>pop</tt> operations are provided, as well as a
 * method to <tt>peek</tt> at the top item on the stack, a method to test
 * for whether the stack is <tt>empty</tt>, and a method to <tt>search</tt>
 * the stack for an item and discover how far it is from the top.
 * <p>
 * When a stack is first created, it contains no items.
 *
 * <p>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:
 * <pre>   {@code
 *   Deque<Integer> stack = new ArrayDeque<Integer>();}</pre>
 *
 * @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:
     * <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.
     */
      //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 <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;
}

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);
    }