Java基礎(chǔ)之容器Vector詳解

更新時(shí)間：2021年04月20日 09:51:29 作者：吃透Java

這篇文章主要介紹了Java基礎(chǔ)之容器Vector詳解,文中有非常詳細(xì)的代碼示例,對(duì)正在學(xué)習(xí)java的小伙伴們有很好的幫助,需要的朋友可以參考下

一、前言

知識(shí)補(bǔ)充：Arrays.copyOf函數(shù)：

public static int[] copyOf(int[] original, int newLength) {  
        int[] copy = new int[newLength];  
        System.arraycopy(original, 0, copy, 0,  
                         Math.min(original.length, newLength));  
        return copy;  
    }

可見copyOf()在內(nèi)部新建一個(gè)數(shù)組，調(diào)用arrayCopy()將original內(nèi)容復(fù)制到copy中去，并且長度為newLength。返回copy;

繼續(xù)看一下System.arraycopy函數(shù)：

public static native void arraycopy(Object src,  int  srcPos,  
                                        Object dest, int destPos,  
                                        int length);

src - 源數(shù)組。

srcPos - 源數(shù)組中的起始位置。

dest - 目標(biāo)數(shù)組。

destPos - 目標(biāo)數(shù)據(jù)中的起始位置。

length - 要復(fù)制的數(shù)組元素的數(shù)量。

該方法是用了native關(guān)鍵字，調(diào)用的為C++編寫的底層函數(shù)，可見其為JDK中的底層函數(shù)。

二、Vector簡介

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

Vector類實(shí)現(xiàn)了一個(gè)可增長的對(duì)象數(shù)組，內(nèi)部是以動(dòng)態(tài)數(shù)組的形式來存儲(chǔ)數(shù)據(jù)的。
Vector具有數(shù)組所具有的特性、通過索引支持隨機(jī)訪問、所以通過隨機(jī)訪問Vector中的元素效率非常高、但是執(zhí)行插入、刪除時(shí)效率比較低下。
繼承了AbstractList,此類提供 List 接口的骨干實(shí)現(xiàn)，以最大限度地減少實(shí)現(xiàn)”隨機(jī)訪問”數(shù)據(jù)存儲(chǔ)（如數(shù)組）支持的該接口所需的工作.對(duì)于連續(xù)的訪問數(shù)據(jù)（如鏈表），應(yīng)優(yōu)先使用 AbstractSequentialList，而不是此類.
實(shí)現(xiàn)了List接口,意味著Vector元素是有序的,可以重復(fù)的,可以有null元素的集合.
實(shí)現(xiàn)了RandomAccess接口標(biāo)識(shí)著其支持隨機(jī)快速訪問,實(shí)際上,我們查看RandomAccess源碼可以看到,其實(shí)里面什么都沒有定義.因?yàn)锳rrayList底層是數(shù)組,那么隨機(jī)快速訪問是理所當(dāng)然的,訪問速度O(1).
實(shí)現(xiàn)了Cloneable接口,標(biāo)識(shí)著可以它可以被復(fù)制.注意,ArrayList里面的clone()復(fù)制其實(shí)是淺復(fù)制
實(shí)現(xiàn)了Serializable 標(biāo)識(shí)著集合可被序列化。

三、Vector源碼

public class Vector<E>
    extends AbstractList<E>
    implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    //保存Vector數(shù)據(jù)的數(shù)組
    protected Object[] elementData;

    //實(shí)際數(shù)據(jù)的數(shù)量
    protected int elementCount;

    //容量增長的系數(shù)
    protected int capacityIncrement;

    // Vector的序列版本號(hào)
    private static final long serialVersionUID = -2767605614048989439L;

    //指定Vector初始大小和增長系數(shù)的構(gòu)造函數(shù)
    public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    }

    //指定初始容量的構(gòu)造函數(shù)
    public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }

    //Vector構(gòu)造函數(shù)，默認(rèn)容量為10
    public Vector() {
        this(10);
    }

    //初始化一個(gè)指定集合數(shù)據(jù)的構(gòu)造函數(shù)
    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);
    }

    //將Vector全部元素拷貝到anArray數(shù)組中
    public synchronized void copyInto(Object[] anArray) {
        System.arraycopy(elementData, 0, anArray, 0, elementCount);
    }

    //當(dāng)前的數(shù)組中元素個(gè)數(shù)大于記錄的元素個(gè)數(shù)時(shí)，重新賦值給當(dāng)前數(shù)組所記錄的元素
    public synchronized void trimToSize() {
        modCount++;
        int oldCapacity = elementData.length;
        if (elementCount < oldCapacity) {
            elementData = Arrays.copyOf(elementData, elementCount);
        }
    }

   //確定Vector的容量
    public synchronized void ensureCapacity(int minCapacity) {
        if (minCapacity > 0) {
            // 將Vector的改變統(tǒng)計(jì)數(shù)+1
            modCount++;
            ensureCapacityHelper(minCapacity);
        }
    }

    //確定容量的幫助函數(shù)，如果所需容量大于當(dāng)前的容量時(shí)則執(zhí)行擴(kuò)容
    private void ensureCapacityHelper(int minCapacity) {
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    //數(shù)組所允許的最大容量
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    //執(zhí)行擴(kuò)容函數(shù)
    private void grow(int minCapacity) {
        // overflow-conscious code
        //記錄當(dāng)前容量
        int oldCapacity = elementData.length;
        //如果擴(kuò)容系數(shù)大于0則新容量等于當(dāng)前容量+擴(kuò)容系數(shù)，如果擴(kuò)容系數(shù)小于等于0則新容量等于當(dāng)前容量的2倍
        int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                         capacityIncrement : oldCapacity);
        //如果新容量小于當(dāng)前需要的容量，則把需要的容量賦值給需要擴(kuò)容的新容量
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
         //如果新擴(kuò)容容量大于最大數(shù)組容量，則執(zhí)行巨大擴(kuò)容
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    //巨大擴(kuò)容函數(shù)，如果所需容量大于最大數(shù)組容量，則返回int形最大值(2^31 -1),否則返回最大數(shù)組容量
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    //設(shè)置容量值為newSize，如果newSize大于當(dāng)前容量，則擴(kuò)容，否則newSize以后的所有元素置null
    public synchronized void setSize(int newSize) {
        modCount++;
        if (newSize > elementCount) {
            ensureCapacityHelper(newSize);
        } else {
            for (int i = newSize ; i < elementCount ; i++) {
                elementData[i] = null;
            }
        }
        elementCount = newSize;
    }

    //返回當(dāng)前Vector的容量
    public synchronized int capacity() {
        return elementData.length;
    }

    //返回Vector元素的個(gè)數(shù)
    public synchronized int size() {
        return elementCount;
    }

    //Vector元素個(gè)數(shù)是否為0
    public synchronized boolean isEmpty() {
        return elementCount == 0;
    }

    //返回Vector元素的Enumeration，Enumeration 接口是Iterator迭代器的“古老版本”
    //Enumeration接口中的方法名稱難以記憶，而且沒有Iterator的remove()方法。如果現(xiàn)在編寫Java程序，應(yīng)該盡量采用
    //Iterator迭代器，而不是用Enumeration迭代器。
    //之所以保留Enumeration接口的原因，主要為了照顧以前那些“古老”的程序，那些程序里大量使用Enumeration接口，如果新版
    //本的Java里直接刪除Enumeration接口，將會(huì)導(dǎo)致那些程序全部出錯(cuò)。
    public Enumeration<E> elements() {
        return new Enumeration<E>() {
            int count = 0;

            public boolean hasMoreElements() {
                return count < elementCount;
            }

            public E nextElement() {
                synchronized (Vector.this) {
                    if (count < elementCount) {
                        return elementData(count++);
                    }
                }
                throw new NoSuchElementException("Vector Enumeration");
            }
        };
    }

    //返回Vector中是否包含對(duì)象o
    public boolean contains(Object o) {
        return indexOf(o, 0) >= 0;
    }

    // 查找并返回元素(o)在Vector中的索引值
    public int indexOf(Object o) {
        return indexOf(o, 0);
    }

    // 從index位置開始向后查找元素(o)。
    // 若找到，則返回元素的索引值；否則，返回-1
    public synchronized int indexOf(Object o, int index) {
        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;
    }

    // 從后向前查找元素(o)。并返回元素的索引
    public synchronized int lastIndexOf(Object o) {
        return lastIndexOf(o, elementCount-1);
    }

    // 從index位置開始向前查找元素(o)。
    // 若找到，則返回元素的索引值；否則，返回-1
    public synchronized int lastIndexOf(Object o, int index) {
        if (index >= elementCount)
            throw new IndexOutOfBoundsException(index + " >= "+ elementCount);

        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Vector中index位置的元素。
    // 若index越界，則拋出異常
    public synchronized E elementAt(int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        }

        return elementData(index);
    }

    // 返回Vector中第0位置的元素。
    public synchronized E firstElement() {
        if (elementCount == 0) {
            throw new NoSuchElementException();
        }
        return elementData(0);
    }

    // 返回Vector中最后一個(gè)元素。
    public synchronized E lastElement() {
        if (elementCount == 0) {
            throw new NoSuchElementException();
        }
        return elementData(elementCount - 1);
    }

    // 設(shè)置index位置的元素值為obj
    public synchronized void setElementAt(E obj, int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                     elementCount);
        }
        elementData[index] = obj;
    }

    //刪除index位置處的元素
    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);
        }
        elementCount--;
        elementData[elementCount] = null; /* to let gc do its work */
    }

    //在index位置插入元素obj
    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后面添加對(duì)象obj
    public synchronized void addElement(E obj) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    }

    // 在Vector中查找并刪除元素obj。
    // 成功的話，返回true；否則，返回false。
    public synchronized boolean removeElement(Object obj) {
        modCount++;
        int i = indexOf(obj);
        if (i >= 0) {
            removeElementAt(i);
            return true;
        }
        return false;
    }

    //刪除Vector中所有元素
    public synchronized void removeAllElements() {
        modCount++;
        // Let gc do its work
        for (int i = 0; i < elementCount; i++)
            elementData[i] = null;

        elementCount = 0;
    }

    //返回Vector的克隆。 該副本將包含對(duì)內(nèi)部數(shù)據(jù)數(shù)組的克隆的引用，而不是對(duì)此對(duì)象的原始內(nèi)部數(shù)據(jù)數(shù)組的引用。
    public synchronized Object clone() {
        try {
            @SuppressWarnings("unchecked")
                Vector<E> v = (Vector<E>) super.clone();
            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);
        }
    }

    //返回包含Vector所有元素的數(shù)組
    public synchronized Object[] toArray() {
        return Arrays.copyOf(elementData, elementCount);
    }

    // 返回Vector的模板數(shù)組。所謂模板數(shù)組，即可以將T設(shè)為任意的數(shù)據(jù)類型
    @SuppressWarnings("unchecked")
    public synchronized <T> T[] toArray(T[] a) {
        // 若數(shù)組a的大小 < Vector的元素個(gè)數(shù)；
        // 則新建一個(gè)T[]數(shù)組，數(shù)組大小是“Vector的元素個(gè)數(shù)”，并將“Vector”全部拷貝到新數(shù)組中
        if (a.length < elementCount)
            return (T[]) Arrays.copyOf(elementData, elementCount, a.getClass());
        // 若數(shù)組a的大小 >= Vector的元素個(gè)數(shù)；
        // 則將Vector的全部元素都拷貝到數(shù)組a中。
        System.arraycopy(elementData, 0, a, 0, elementCount);

        if (a.length > elementCount)
            a[elementCount] = null;

        return a;
    }

    // Positional Access Operations

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    //獲取index處的元素
    public synchronized E get(int index) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);

        return elementData(index);
    }

    //設(shè)置index處的元素為element，并返回被替換掉的元素
    public synchronized E set(int index, E element) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);

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

    //Vector末尾添加元素
    public synchronized boolean add(E e) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    }

    //移除Vector中第一個(gè)出現(xiàn)對(duì)象o的元素
    public boolean remove(Object o) {
        return removeElement(o);
    }

    //在index位置添加對(duì)象element
    public void add(int index, E element) {
        insertElementAt(element, index);
    }

    //移除index位置的元素
    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Vector
    public void clear() {
        removeAllElements();
    }

    // Bulk Operations

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

    //在Vector末尾添加集合c
    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;
    }

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

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

   //在index位置添加集合c中的元素
    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;
    }

    // 返回兩個(gè)對(duì)象是否相等
    public synchronized boolean equals(Object o) {
        return super.equals(o);
    }

   // 計(jì)算哈希值
    public synchronized int hashCode() {
        return super.hashCode();
    }

    // 調(diào)用父類的toString()
    public synchronized String toString() {
        return super.toString();
    }

    // 獲取Vector中fromIndex(包括)到toIndex(不包括)的子集
    public synchronized List<E> subList(int fromIndex, int toIndex) {
        return Collections.synchronizedList(super.subList(fromIndex, toIndex),
                                            this);
    }

    // 刪除Vector中fromIndex到toIndex的元素
    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的寫入函數(shù)
    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();
    }

    public synchronized ListIterator<E> listIterator(int index) {
        if (index < 0 || index > elementCount)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    public synchronized ListIterator<E> listIterator() {
        return new ListItr(0);
    }
    public synchronized Iterator<E> iterator() {
        return new Itr();
    }

    private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        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();
        }
    }

    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++;
    }

    @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;
        }
    }
}

四、總結(jié)

Vector實(shí)際上是通過一個(gè)數(shù)組去保存數(shù)據(jù)的。當(dāng)我們構(gòu)造Vecotr時(shí)；若使用默認(rèn)構(gòu)造函數(shù)，則Vector的默認(rèn)容量大小是10。
當(dāng)Vector容量不足以容納全部元素時(shí)，Vector的容量會(huì)增加。若容量增加系數(shù) >0，則將容量的值增加“容量增加系數(shù)”；否則，將容量大小增加一倍。
Vector的克隆函數(shù)，即是將全部元素克隆到一個(gè)數(shù)組中。

五、Vector遍歷方式

1. 隨機(jī)訪問遍歷，通過索引值去遍歷

由于Vector實(shí)現(xiàn)了RandomAccess接口，它支持通過索引值去隨機(jī)訪問元素。

Integer value = null;
int size = vec.size();
for (int i=0; i<size; i++) {
    value = (Integer)vec.get(i);        
}

2. 通過迭代器遍歷。即通過Iterator去遍歷

Integer value = null;
Iterator<Integer> iterator = vec.iterator();
   while (iterator.hasNext()) {
       value = iterator.next();
   }

3. 通過增強(qiáng)for循環(huán)去遍歷

Integer value = null;
for (Integer integ:vec) {
    value = integ;
}

4. 通過Enumeration遍歷

Integer value = null;
Enumeration enu = vec.elements();
while (enu.hasMoreElements()) {
    value = (Integer)enu.nextElement();
}

測(cè)試這些遍歷方式效率的代碼如下：

public class Test {

    public static void main(String[] args) {
        Vector<Integer> vector = new Vector<>();
        for (int i = 0; i < 100000; i++)
            vector.add(i);

        iteratorThroughRandomAccess(vector);
        iteratorThroughIterator(vector);
        iteratorThroughFor2(vector);
        iteratorThroughEnumeration(vector);
    }

    public static void iteratorThroughRandomAccess(List list) {
        long startTime, endTime;
        startTime = System.currentTimeMillis();
        for (int i = 0; i < list.size(); i++) {

        }
        endTime = System.currentTimeMillis();
        long time = endTime - startTime;
        System.out.println("iteratorThroughRandomAccess：" + time + " ms");
    }

    public static void iteratorThroughIterator(List list) {
        long startTime, endTime;
        startTime = System.currentTimeMillis();
        Iterator<Integer> iterator = list.iterator();
        while (iterator.hasNext()) {
            iterator.next();
        }
        endTime = System.currentTimeMillis();
        long time = endTime - startTime;
        System.out.println("iteratorThroughIterator：" + time + " ms");
    }

    public static void iteratorThroughFor2(List list) {
        long startTime, endTime;
        startTime = System.currentTimeMillis();
        for (Object o : list) {

        }
        endTime = System.currentTimeMillis();
        long time = endTime - startTime;
        System.out.println("iteratorThroughFor2：" + time + " ms");
    }

    public static void iteratorThroughEnumeration(Vector vec) {
        long startTime, endTime;
        startTime = System.currentTimeMillis();
        for (Enumeration enu = vec.elements(); enu.hasMoreElements(); ) {
            enu.nextElement();
        }
        endTime = System.currentTimeMillis();
        long time = endTime - startTime;
        System.out.println("iteratorThroughEnumeration：" + time + " ms");
    }


}

輸出如下：