Java集合系列[5]-Vector分析与源码解析

概述

之前已经学习过了ArrayList和LinkedList，也学习过了fail-fast机制。今天我们接着学习Collection框架中的另一个重要成员Vector。大概还是分为下面几个部分：

1. Vector简介
2. Vector的数据结构
3. Vector源码解析(基于JDK1.8)
4. Vector的遍历方式

Vector简介

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

Vector是矢量队列，继承于AbstractList，实现了List、RandomAccess、Cloneable和java.io.Serializable。

• Vector继承了AbstractList，实现了List，所以它是一个队列，支持相关CRUD和遍历等的功能；
• Vector实现了RandomAccess，说明它提供了随机访问的功能。RandomAccess是java中用来被List发现，为List提供快速访问功能的。在vector中，我们即可以通过元素的索引值来快速获取元素，这就是快速随机访问；
• Vector实现了Cloneable，说明它实现了clone()方法，能被克隆；
• Vector实现了java.io.Serializable，说明它能够被序列化；

和ArrayList不同的是，Vector是同步的，所以它的操作是线程安全的。

Vector的构造方法

public Vector(); //构造一个initialCapacity为10，capacityIncrement为0（容量溢出时double）的Vector
public Vector(int initialCapacity);//构造一个容量为指定initialCapacity，capactiryIncrement为0（容量溢出时double）的Vector
public Vector(int initialCapacity, int capacityIncrement);//构造一个容量为initialCapacity，容量增量为capacityIncrement的Vector
public Vector(Collection<? extends E> c);//创建一个包含Collection的Vector，容量为集合的数量，容量增量为0（容量溢出时double）的Vector

Vector的API

public synchronized void copyInto(Object[] anArray); //把集合内容拷贝到指定数组中
public synchronized void trimToSize();//调整容量为当前的元素数量
public synchronized void ensureCapacity(int minCapacity);//提升vector的容量
public synchronized void setSize(int newSize);//设置vector的大小
public synchronized int capacity();//获取当前vector的容量
public synchronized int size();//获取当前vector的元素数量
public synchronized boolean isEmpty();//判断当前vector是否有元素
public Enumeration<E> elements();//返回一个列举器
public boolean contains(Object o);//判断是否包含该元素
public int indexOf(Object o);//返回指定元素的索引值
public synchronized int indexOf(Object o, int index);//从指定位置开始查找,返回指定元素的索引值
public synchronized int lastIndexOf(Object o);//从尾部往前查找,查找指定元素的索引值
public synchronized E elementAt(int index);//返回指定索引值上的元素
public synchronized E firstElement();//获取vector的第一个元素
public synchronized E lastElement();//获取vector的最后一个元素
public synchronized void setElementAt(E obj, int index);//更新指定索引值上的元素为指定元素
public synchronized void removeElementAt(int index);//删除指定索引值上的元素
public synchronized void insertElementAt(E obj, int index);//在指定索引值上插入元素
public synchronized void addElement(E obj);//添加元素到vector的尾部
public synchronized boolean removeElement(Object obj);//删除在vector低位置的指定元素
public synchronized void removeAllElements();//删除所有元素(置null)
public synchronized Object clone();//返回当前vector的一个复制
public synchronized Object[] toArray();//返回包含当前vector所有元素的一个数组
public synchronized <T> T[] toArray(T[] a);//返回指定类型的一个包含当前vector全部元素的数组
public synchronized E get(int index);//获取指定索引值上的元素
public synchronized E set(int index, E element);//用指定元素代替索引值上的元素
public synchronized boolean add(E e);//添加一个元素到尾部
public boolean remove(Object o);//删除元素
public void add(int index, E element);//在指定位置上添加元素
public synchronized E remove(int index);//删除指定位置上的元素
public void clear();//清空所有元素
public synchronized boolean containsAll(Collection<?> c);//判断当前vector是否包含指定集合的所有元素
public synchronized boolean addAll(Collection<? extends E> c);//把指定集合的所有元素添加到vector的尾部
public synchronized boolean removeAll(Collection<?> c);//删除当前vector与c中元素相等的元素
public synchronized boolean retainAll(Collection<?> c);//删除当前vector与c中元素不相等的元素
public synchronized List<E> subList(int fromIndex, int toIndex);//返回[fromIndex, toIndex)的元素的数组
public synchronized ListIterator<E> listIterator();//返回一个列表迭代器
public synchronized ListIterator<E> listIterator(int index);//返回一个在指定位置上的列表迭代器
public synchronized Iterator<E> iterator();//返回一个迭代器
public synchronized void forEach(Consumer<? super E> action);//遍历每一个元素并在每个元素上执行指定的Consumer
public synchronized boolean removeIf(Predicate<? super E> filter);//删除所有满足指定Predicate条件的元素
public synchronized void replaceAll(UnaryOperator<E> operator);//用指定UnaryOperator处理的结果替换每一个元素
public synchronized void sort(Comparator<? super E> c);//根据指定的比较器对当前vector进行排序
public Spliterator<E> spliterator();//返回一个分割迭代器

数据结构分析

Vector的继承关系

java.lang.Object
    java.util.AbstractCollection<E>
        java.util.AbstractList<E>
            java.util.Vector<E>

Vector与Collection的关系

Vector的数据结构和ArrayList差不多,它包含了3个成员变量:elementData(对应ArrayList的elementDate), elementCount(对应ArrayList的size), capacityIncrement(ArrayList没有).

1. elementDate 是Object[]类型的数组,它保存了保存到Vector的元素.elementData是一个动态数组,如果初始化Vector的时候没有指定动态数组的容量,则使用默认容量10.随着Vector中元素的增加,Vector的容量也会随着增长, capacityIncrement是与增长相关的增长系数,具体的增长方式,可查看源码中的ensureCapacity方法.
2. elementCount是动态数组的实际大小.
3. capacityIncrement是动态数组的增长系数.如果在创建Vector指定了capacityIncrement,那么每次vector增长的时候,增加的大小都是capacityIncrement;如果创建的时候没有指定,则为容量double.

   int newCapacity = oldCapacity + ((capacityIncrement > 0) ? capacityIncrement : oldCapacity);

Vector的源码分析(基于JDK 1.8.0_73)

源码分析

package java.util;

import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;

public class Vector<E>
    extends AbstractList<E>
    implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    //存放元素的动态数组
    protected Object[] elementData;

    //元素数量
    protected int elementCount;

    //增长系数
    protected int capacityIncrement;

    private static final long serialVersionUID = -2767605614048989439L;

    //指定初始容量和增长系数
    public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    }

    //指定初始容量
    public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }

    //默认容量为10
    public Vector() {
        this(10);
    }

    public Vector(Collection<? extends E> c) {
        //获取集合c的数组,并将其赋值给elementData
        elementData = c.toArray();
        //设置动态数组的长度
        elementCount = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
    }

    //将vector的元素全部拷贝到anArray数组中
    public synchronized void copyInto(Object[] anArray) {
        System.arraycopy(elementData, 0, anArray, 0, elementCount);
    }

    //将当前容量设置=实际元素长度
    public synchronized void trimToSize() {
        modCount++;
        int oldCapacity = elementData.length;
        if (elementCount < oldCapacity) {
            elementData = Arrays.copyOf(elementData, elementCount);
        }
    }

    //确保容量不低于minCapacity
    public synchronized void ensureCapacity(int minCapacity) {
        //如果minCapacity=<0,那么确保肯定成立,所以不用处理
        if (minCapacity > 0) {
            modCount++;
            ensureCapacityHelper(minCapacity);
        }
    }

    //容量确认的帮助器
    private void ensureCapacityHelper(int minCapacity) {
        //如果minCapacity大于当前长度,那么肯定是要扩容了.
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    //由于1-部分VM对数组长度有限制(无法改变)/2-Java堆空间(-Xmx修改),
    //对数组的最大长度限制一下,确保不会抛出OutOfMemoryError
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    //容量增长方法
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        //如果增长系数大于0,容量就增加增长系数的数量;
        //否则,动态数组的容量就double.
        int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                         capacityIncrement : oldCapacity);
        //每次增长都确保容量不低于minCapacity,上面虽然动态数组的容量自己已经增加了,
        //但是不能保证不低于minCapacity,所以这里还要判断一下
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //当然啦,也不能超过数组的最大长度
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        //动态数组的容量确认了,是时候把所有元素拷贝到一个新容量的数组了
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    //获取最大容量
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // 溢出了
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    //设置vector的大小为newSize
    public synchronized void setSize(int newSize) {
        //修改计数器+1
        modCount++;
        //如果newSize大于当前的元素长度,那么就扩容;
        //否则,把超过newSize位置开始的元素都清空掉;
        if (newSize > elementCount) {
            ensureCapacityHelper(newSize);
        } else {
            for (int i = newSize ; i < elementCount ; i++) {
                elementData[i] = null;
            }
        }
        elementCount = newSize;
    }

    //获取当前容量
    public synchronized int capacity() {
        return elementData.length;
    }

    //获取当前元素长度elementCount
    public synchronized int size() {
        return elementCount;
    }

    //判断vector是不是空
    public synchronized boolean isEmpty() {
        return elementCount == 0;
    }

    //获取当前vector的一个列举器
    public Enumeration<E> elements() {
        //通过匿名类实现Enumeration
        return new Enumeration<E>() {
            int count = 0;

            //下一个要访问的元素是否存在
            public boolean hasMoreElements() {
                return count < elementCount;
            }

            //获取下一个访问的元素
            public E nextElement() {
                // 获取当前Vector对象的同步锁(原来匿名类是这么访问外部类的对象锁的)
                synchronized (Vector.this) {
                    if (count < elementCount) {
                        return elementData(count++);
                    }
                }
                throw new NoSuchElementException("Vector Enumeration");
            }
        };
    }

    //判断vector中是否包含对象o
    public boolean contains(Object o) {
        return indexOf(o, 0) >= 0;
    }

    //从前往后寻找对象o,返回其索引值;
    public int indexOf(Object o) {
        return indexOf(o, 0);
    }

    //从index位置开始从前往后寻找对象o,
    //存在多个,则返回最低位置的索引值;
    //不存在,则返回-1;
    public synchronized int indexOf(Object o, int index) {
        //如果是对象o是null,那么就用"=="来判断
        //否则,用"equals()"来判断
        if (o == null) {
            for (int i = index ; i < elementCount ; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = index ; i < elementCount ; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    //从后往前寻找,获取对象o的索引值
    public synchronized int lastIndexOf(Object o) {
        return lastIndexOf(o, elementCount-1);
    }

    //从index位置开始,从后往前寻找,返回对象o的索引值
    public synchronized int lastIndexOf(Object o, int index) {
        if (index >= elementCount)
            throw new IndexOutOfBoundsException(index + " >= "+ elementCount);

        //如果对象o是null,那么用"=="来比对
        //否则,用"equals()"来比对
        if (o == null) {
            for (int i = index; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = index; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    //返回index位置上的元素
    public synchronized E elementAt(int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        }

        return elementData(index);
    }

    //返回第一个元素
    public synchronized E firstElement() {
        if (elementCount == 0) {
            throw new NoSuchElementException();
        }
        return elementData(0);
    }

    //返回最后一个元素
    public synchronized E lastElement() {
        if (elementCount == 0) {
            throw new NoSuchElementException();
        }
        return elementData(elementCount - 1);
    }

    //替换index位置上的元素为对象obj
    public synchronized void setElementAt(E obj, int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                     elementCount);
        }
        elementData[index] = obj;
    }

    //删除index位置上对象,后面的元素往前移动
    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) {
            //把index位置后的元素往前移动1位
            System.arraycopy(elementData, index + 1, elementData, index, j);
        }
        elementCount--;
        elementData[elementCount] = null; /* to let gc do its work */
    }

    //在index位置上插入对象obj,该位置的元素及后面的元素一次往后tui
    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++;
    }

    //在vector最后添加对象obj
    public synchronized void addElement(E obj) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    }

    //删除对象obj
    public synchronized boolean removeElement(Object obj) {
        modCount++;
        int i = indexOf(obj);
        if (i >= 0) {
            removeElementAt(i);
            return true;
        }
        return false;
    }

    //清空所有元素
    public synchronized void removeAllElements() {
        modCount++;
        // 让GC去处理它们吧
        for (int i = 0; i < elementCount; i++)
            elementData[i] = null;

        elementCount = 0;
    }

    //克隆方法
    public synchronized Object clone() {
        try {
            @SuppressWarnings("unchecked")
                Vector<E> v = (Vector<E>) super.clone();
            //将当前vector的全部元素拷贝到新的vector中
            v.elementData = Arrays.copyOf(elementData, elementCount);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

    //返回Object数组
    public synchronized Object[] toArray() {
        return Arrays.copyOf(elementData, elementCount);
    }

    //返回vector的模板数组,所谓模板数组,就是可以把T设为任意的数据类型
    @SuppressWarnings("unchecked")
    public synchronized <T> T[] toArray(T[] a) {
        //如果给定数组的大小小于vector元素个数,那么就新建一个T类型数组吧
        //并且把所有的元素都拷贝到这个新的数组中
        if (a.length < elementCount)
            return (T[]) Arrays.copyOf(elementData, elementCount, a.getClass());

        //如果给定数组的大小不小于vector元素个数,那么就直接把vector所有元素都拷贝进去
        System.arraycopy(elementData, 0, a, 0, elementCount);

        //并且把多余的位置全部置null
        if (a.length > elementCount)
            a[elementCount] = null;

        return a;
    }

    // 基于位置的访问

    //获取index位置上的元素
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    //获取index位置上的元素
    public synchronized E get(int index) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);

        return elementData(index);
    }

    //替换index位置上的元素为element,并返回该位置上的老元素
    public synchronized E set(int index, E element) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    //添加对象e到vector尾部
    public synchronized boolean add(E e) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    }

    //删除o元素
    public boolean remove(Object o) {
        return removeElement(o);
    }

    //在index上添加元素
    public void add(int index, E element) {
        insertElementAt(element, index);
    }

    //删除index位置上的元素
    public synchronized E remove(int index) {
        modCount++;
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        E oldValue = elementData(index);

        int numMoved = elementCount - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--elementCount] = null; // Let gc do its work

        return oldValue;
    }

    //清空所有元素
    public void clear() {
        removeAllElements();
    }

    // 容器操作

    //判断vector是否包含集合c
    public synchronized boolean containsAll(Collection<?> c) {
        return super.containsAll(c);
    }

    //把集合c的元素全部添加到vector中
    public synchronized boolean addAll(Collection<? extends E> c) {
        modCount++;
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityHelper(elementCount + numNew);
        System.arraycopy(a, 0, elementData, elementCount, numNew);
        elementCount += numNew;
        return numNew != 0;
    }

    //删除集合c中的全部元素
    public synchronized boolean removeAll(Collection<?> c) {
        return super.removeAll(c);
    }

    //删除非集合c中的元素
    public synchronized boolean retainAll(Collection<?> c) {
        return super.retainAll(c);
    }

    //从index位置开始,添加集合c
    public synchronized boolean addAll(int index, Collection<? extends E> c) {
        modCount++;
        if (index < 0 || index > elementCount)
            throw new ArrayIndexOutOfBoundsException(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityHelper(elementCount + numNew);

        int numMoved = elementCount - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);

        System.arraycopy(a, 0, elementData, index, numNew);
        elementCount += numNew;
        return numNew != 0;
    }

    //返回两个对象是否相等
    public synchronized boolean equals(Object o) {
        return super.equals(o);
    }

    //计算当前vector的哈希值
    public synchronized int hashCode() {
        return super.hashCode();
    }

    public synchronized String toString() {
        return super.toString();
    }

    //获取vector上[fromIndex, toIndex)位置上的元素子集
    public synchronized List<E> subList(int fromIndex, int toIndex) {
        return Collections.synchronizedList(super.subList(fromIndex, toIndex),
                                            this);
    }

    //删除[fromIndex, toIndex)位置上的元素子集
    protected synchronized void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = elementCount - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // Let gc do its work
        int newElementCount = elementCount - (toIndex-fromIndex);
        while (elementCount != newElementCount)
            elementData[--elementCount] = null;
    }

    //java.io.serializable的写入方法
    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        final java.io.ObjectOutputStream.PutField fields = s.putFields();
        final Object[] data;
        synchronized (this) {
            fields.put("capacityIncrement", capacityIncrement);
            fields.put("elementCount", elementCount);
            data = elementData.clone();
        }
        fields.put("elementData", data);
        s.writeFields();
    }

    //返回一个fail-fast机制的列表迭代器
    public synchronized ListIterator<E> listIterator(int index) {
        if (index < 0 || index > elementCount)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    //fail-fast机制的列表迭代器
    public synchronized ListIterator<E> listIterator() {
        return new ListItr(0);
    }

   //返回一个fail-fast机制的迭代器
    public synchronized Iterator<E> iterator() {
        return new Itr();
    }

    /**
     * AbstractList.Itr的优化版本
     */
    private class Itr implements Iterator<E> {
        int cursor;       // 下一个返回元素的位置
        int lastRet = -1; // 最后一个返回元素的位置
        int expectedModCount = modCount; //期望修改值

        public boolean hasNext() {
            // Racy but within spec, since modifications are checked
            // within or after synchronization in next/previous
            return cursor != elementCount;
        }

        public E next() {
            synchronized (Vector.this) {
                checkForComodification();
                int i = cursor;
                if (i >= elementCount)
                    throw new NoSuchElementException();
                cursor = i + 1;
                return elementData(lastRet = i);
            }
        }

        public void remove() {
            if (lastRet == -1)
                throw new IllegalStateException();
            synchronized (Vector.this) {
                checkForComodification();
                Vector.this.remove(lastRet);
                expectedModCount = modCount;
            }
            cursor = lastRet;
            lastRet = -1;
        }

        @Override
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            synchronized (Vector.this) {
                final int size = elementCount;
                int i = cursor;
                if (i >= size) {
                    return;
                }
        @SuppressWarnings("unchecked")
                final E[] elementData = (E[]) Vector.this.elementData;
                if (i >= elementData.length) {
                    throw new ConcurrentModificationException();
                }
                while (i != size && modCount == expectedModCount) {
                    action.accept(elementData[i++]);
                }
                // update once at end of iteration to reduce heap write traffic
                cursor = i;
                lastRet = i - 1;
                checkForComodification();
            }
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    /**
     * AbstractList.ListItr的一个优化版本
     */
    final class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }

        public boolean hasPrevious() {
            return cursor != 0;
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor - 1;
        }

        public E previous() {
            synchronized (Vector.this) {
                checkForComodification();
                int i = cursor - 1;
                if (i < 0)
                    throw new NoSuchElementException();
                cursor = i;
                return elementData(lastRet = i);
            }
        }

        public void set(E e) {
            if (lastRet == -1)
                throw new IllegalStateException();
            synchronized (Vector.this) {
                checkForComodification();
                Vector.this.set(lastRet, e);
            }
        }

        public void add(E e) {
            int i = cursor;
            synchronized (Vector.this) {
                checkForComodification();
                Vector.this.add(i, e);
                expectedModCount = modCount;
            }
            cursor = i + 1;
            lastRet = -1;
        }
    }

    @Override
    public synchronized void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        final int expectedModCount = modCount;
        @SuppressWarnings("unchecked")
        final E[] elementData = (E[]) this.elementData;
        final int elementCount = this.elementCount;
        for (int i=0; modCount == expectedModCount && i < elementCount; i++) {
            action.accept(elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

    @Override
    @SuppressWarnings("unchecked")
    public synchronized boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        // figure out which elements are to be removed
        // any exception thrown from the filter predicate at this stage
        // will leave the collection unmodified
        int removeCount = 0;
        final int size = elementCount;
        final BitSet removeSet = new BitSet(size);
        final int expectedModCount = modCount;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            @SuppressWarnings("unchecked")
            final E element = (E) elementData[i];
            if (filter.test(element)) {
                removeSet.set(i);
                removeCount++;
            }
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }

        // shift surviving elements left over the spaces left by removed elements
        final boolean anyToRemove = removeCount > 0;
        if (anyToRemove) {
            final int newSize = size - removeCount;
            for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
                i = removeSet.nextClearBit(i);
                elementData[j] = elementData[i];
            }
            for (int k=newSize; k < size; k++) {
                elementData[k] = null;  // Let gc do its work
            }
            elementCount = newSize;
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            modCount++;
        }

        return anyToRemove;
    }

    @Override
    @SuppressWarnings("unchecked")
    public synchronized void replaceAll(UnaryOperator<E> operator) {
        Objects.requireNonNull(operator);
        final int expectedModCount = modCount;
        final int size = elementCount;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            elementData[i] = operator.apply((E) elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }

    @SuppressWarnings("unchecked")
    @Override
    public synchronized void sort(Comparator<? super E> c) {
        final int expectedModCount = modCount;
        Arrays.sort((E[]) elementData, 0, elementCount, c);
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }

    //返回一个late-binding机制的分割迭代器
    @Override
    public Spliterator<E> spliterator() {
        return new VectorSpliterator<>(this, null, 0, -1, 0);
    }

    /** Similar to ArrayList Spliterator */
    static final class VectorSpliterator<E> implements Spliterator<E> {
        private final Vector<E> list;
        private Object[] array;
        private int index; // current index, modified on advance/split
        private int fence; // -1 until used; then one past last index
        private int expectedModCount; // initialized when fence set

        /** Create new spliterator covering the given  range */
        VectorSpliterator(Vector<E> list, Object[] array, int origin, int fence,
                          int expectedModCount) {
            this.list = list;
            this.array = array;
            this.index = origin;
            this.fence = fence;
            this.expectedModCount = expectedModCount;
        }

        private int getFence() { // initialize on first use
            int hi;
            if ((hi = fence) < 0) {
                synchronized(list) {
                    array = list.elementData;
                    expectedModCount = list.modCount;
                    hi = fence = list.elementCount;
                }
            }
            return hi;
        }

        public Spliterator<E> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null :
                new VectorSpliterator<E>(list, array, lo, index = mid,
                                         expectedModCount);
        }

        @SuppressWarnings("unchecked")
        public boolean tryAdvance(Consumer<? super E> action) {
            int i;
            if (action == null)
                throw new NullPointerException();
            if (getFence() > (i = index)) {
                index = i + 1;
                action.accept((E)array[i]);
                if (list.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                return true;
            }
            return false;
        }

        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> action) {
            int i, hi; // hoist accesses and checks from loop
            Vector<E> lst; Object[] a;
            if (action == null)
                throw new NullPointerException();
            if ((lst = list) != null) {
                if ((hi = fence) < 0) {
                    synchronized(lst) {
                        expectedModCount = lst.modCount;
                        a = array = lst.elementData;
                        hi = fence = lst.elementCount;
                    }
                }
                else
                    a = array;
                if (a != null && (i = index) >= 0 && (index = hi) <= a.length) {
                    while (i < hi)
                        action.accept((E) a[i++]);
                    if (lst.modCount == expectedModCount)
                        return;
                }
            }
            throw new ConcurrentModificationException();
        }

        public long estimateSize() {
            return (long) (getFence() - index);
        }

        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }
}

总结

1. Vector实际上是通过一个数组去保存数据的.当我们使用默认构造器去构造Vector的时候,Vector的默认容量是10;
2. 当Vector容量不足以容纳全部元素时,Vector会自动扩容.若增长系数>0, 则将容量增加增长系数的数值, 否则容量double.
3. Vector的克隆方法,是把所有的元素克隆到一个数组中.
4. Vector之所以是线程安全,是因为它的方法都用了synchronized修饰,是同步的.

Vector的遍历

第一种:随机遍历

int j;
int size = vector.size();
//for (int i = 0; i < vector.size(); ++i) {
for (int i = 0; i < size; ++i) {
    j = vector.get(i);
}

第二种:迭代器遍历

int j;
Iterator<Integer> iterator = vector.iterator();
while (iterator.hasNext()) {
    j = iterator.next();
}

第三种:forEach遍历

int j;
for(Integer i : vector) {
    j = i;
}

第四种:Enumeration遍历

int j;
Enumeration<Integer> enumeration = vector.elements();
while (enumeration.hasMoreElements()) {
    j = enumeration.nextElement();
}

测试效率的代码如下:

package com.github.archerda.vector;

import java.util.Enumeration;
import java.util.Iterator;
import java.util.Vector;

/**
 * Create by archerda on 2016/09/03
 */
public class Reverse {

    public static void main(String[] args) {
        Vector<Integer> list = new Vector<>();
        for (int i = 0; i < 1000000; ++i) {
            list.add(i);
        }

        randomAccess(list);
        foreachAccess(list);
        iteratorAccess(list);
        enumerationAccess(list);

        System.out.println("---");

        randomAccess(list);
        foreachAccess(list);
        iteratorAccess(list);
        enumerationAccess(list);

        System.out.println("---");

        randomAccess(list);
        foreachAccess(list);
        iteratorAccess(list);
        enumerationAccess(list);
    }

    private static void randomAccess(Vector<Integer> vector) {
        long start = System.currentTimeMillis();
        int j;
        int size = vector.size();
        for (int i = 0; i < size; ++i) {
        //for (int i = 0; i < vector.size(); ++i) {
            j = vector.get(i);
        }
        System.out.println("randomAccess:" + (System.currentTimeMillis() - start) + "ms");
    }

    private static void iteratorAccess(Vector<Integer> vector) {
        long start = System.currentTimeMillis();
        int j;
        Iterator<Integer> iterator = vector.iterator();
        while (iterator.hasNext()) {
            j = iterator.next();
        }
        System.out.println("iteratorAccess:" + (System.currentTimeMillis() - start) + "ms");
    }

    private static void foreachAccess(Vector<Integer> vector) {
        long start = System.currentTimeMillis();
        int j;
        for(Integer i : vector) {
            j = i;
        }
        System.out.println("foreachAccess:" + (System.currentTimeMillis() - start) + "ms");
    }

    private static void enumerationAccess(Vector<Integer> vector) {
        long start = System.currentTimeMillis();
        int j;
        Enumeration<Integer> enumeration = vector.elements();
        while (enumeration.hasMoreElements()) {
            j = enumeration.nextElement();
        }
        System.out.println("enumerationAccess:" + (System.currentTimeMillis() - start) + "ms");
    }

}

结果:

randomAccess:30ms
foreachAccess:33ms
iteratorAccess:38ms
enumerationAccess:34ms
---
randomAccess:32ms
foreachAccess:27ms
iteratorAccess:29ms
enumerationAccess:28ms
---
randomAccess:29ms
foreachAccess:31ms
iteratorAccess:31ms
enumerationAccess:34ms

结论:

1. 遍历Vector的时候,使用随机访问的效率最高,使用迭代器的效率最低;
2. 使用随机访问的时候,如果在for语句里面用了i < vector.size()每次都获取size,那么随机访问的效率会变到最低;

参考文档