Java集合 --- Vector底层实现和原理（源码分析）

概述

文章的内容基于JDK1.7进行分析，之所以选用这个版本，是因为1.8的有些类做了改动，增加了阅读的难度，虽然是1.7，但是对于1.8做了重大改动的内容，文章也会进行说明。

Vector作为List的另外一个典型实现类，完全支持List的全部功能，Vector类也封装了一个动态的，允许在分配的Object[]数组，Vector是一个比较古老的集合，JDK1.0就已经存在，建议尽量不要使用这个集合，Vector与ArrayList的主要是区别是，Vector是线程安全的，但是性能比ArrayList要低。

数据结构

继承关系

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

实现接口

Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess 

子类

Stack

基本属性

 protected Object[] elementData;  //存放元素的数组
 protected int elementCount;    //已经放入数组的元素个数
 protected int capacityIncrement; //数组的增长系数

Vector源码解析

package java.util;

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

    //已经放入数组的元素数量
    protected int elementCount;

    //增长的系数
    protected int capacityIncrement;

    //可序列化版本号
    private static final long serialVersionUID = -2767605614048989439L;

    //构造方法，提供初始大小，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;
    }

    //构造方法，提供初始大小，增长系数为零
    public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }

    //无参构造方法
    public Vector() {
        this(10);
    }

    //构造方法，将指定的集合元素转化为Vector
    public Vector(Collection<? extends E> c) {
        elementData = c.toArray();
        elementCount = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        //判断c.toArray是否是Object[]类型
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, elementCount, Object[].class);
    }

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

    //将数组长度设置为等于vector的个数
    public synchronized void trimToSize() {
        modCount++;
        int oldCapacity = elementData.length;
        if (elementCount < oldCapacity) {
            elementData = Arrays.copyOf(elementData, elementCount);
        }
    }

    //扩充容量
    public synchronized void ensureCapacity(int minCapacity) {
        if (minCapacity > 0) {
            modCount++;
            ensureCapacityHelper(minCapacity);
        }
    }

    //扩充容量帮助函数
    private void ensureCapacityHelper(int minCapacity) {
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    //最大容量
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    //扩充容量执行方法
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        //根据capacityIncrement进行判断，capacityIncrement> 0 增加capacityIncrement个容量，否则容量扩充当前容量的一倍
        int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                         capacityIncrement : oldCapacity);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        //扩容操作，生成已给新的数组，容量为newCapacity,并将elementData中的元素全部拷贝到新数组中，并将新生成的数组在赋值给elementData 
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    //设置size
    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;
    }

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

    //返回vector的元素个数
    public synchronized int size() {
        return elementCount;
    }

    //是否为空
    public synchronized boolean isEmpty() {
        return elementCount == 0;
    }

    //返回vector中全部元素对应的Enumeration
    public Enumeration<E> elements() {
        //匿名内部类实现
        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");
            }
        };
    }

    //是否包含Object对线o 
    public boolean contains(Object o) {
        return indexOf(o, 0) >= 0;
    }

    //返回 o 对象的位置
    public int indexOf(Object o) {
        return indexOf(o, 0);
    }

    //从index位置开始，向后查找Object对象 （o）
    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;
    }

    //倒序查找对象 o 
    public synchronized int lastIndexOf(Object o) {
        return lastIndexOf(o, elementCount-1);
    }

    //从最后一个元素开始，向前查找对象o ,找到返回元素的索引，否则返回 -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;
    }

    //返回索引为index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);
    }

    //将index位置的元素设置为obj 
    public synchronized void setElementAt(E obj, int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                     elementCount);
        }
        elementData[index] = obj;
    }

    //删除指定位置的元素，Object[]对象数组从index+1开始向前依次移动一个位置
    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 */
    }

    //将obj元素插入index位置
    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++;
    }

    //添加元素
    public synchronized void addElement(E obj) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    }

    //删除元素 ，删除成功返回true, 否则返回false
    public synchronized boolean removeElement(Object obj) {
        modCount++;
        int i = indexOf(obj);
        if (i >= 0) {
            removeElementAt(i);
            return true;
        }
        return false;
    }

    //清空所有的元素
    public synchronized void removeAllElements() {
        modCount++;
        // Let gc do its work
        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();
            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();
        }
    }

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

    //转化为指定类型的数组
    @SuppressWarnings("unchecked")
    public synchronized <T> T[] toArray(T[] a) {
        if (a.length < elementCount)
            return (T[]) Arrays.copyOf(elementData, elementCount, a.getClass());

        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];
    }

    //得到索引为index的元素
    public synchronized E get(int index) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);

        return elementData(index);
    }

    //设置index位置的元素为element 
    public synchronized E set(int index, E element) {
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    //添加方法
    public synchronized boolean add(E e) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    }

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

    //将element添加到index位置上
    public void add(int index, E element) {
        insertElementAt(element, index);
    }

    //删除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;
    }

    //清除
    public void clear() {
        removeAllElements();
    }

    // Bulk Operations

   //是否包含集合c中所有的元素
    public synchronized boolean containsAll(Collection<?> c) {
        return super.containsAll(c);
    }

    //将集合c中所有的元素添加到列表中，借助System.copyOf()方法实现
    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中所有的元素
    public synchronized boolean removeAll(Collection<?> c) {
        return super.removeAll(c);
    }

   
    public synchronized boolean retainAll(Collection<?> c) {
        return super.retainAll(c);
    }

    //将集合c 添加到index之后的位置上
    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;
    }

    //判断方法
    public synchronized boolean equals(Object o) {
        return super.equals(o);
    }

    //计算hashCode值
    public synchronized int hashCode() {
        return super.hashCode();
    }

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

    //返回从fromIndex到toIndex之间的子集合
    public synchronized List<E> subList(int fromIndex, int toIndex) {
        return Collections.synchronizedList(super.subList(fromIndex, toIndex),
                                            this);
    }

    //范围删除元素
    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;
    }

    //将对象写入到输出流中
    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();
    }

    //省略了内部类的实现
}

总结

Vector和ArrayList的实现方式可以看出非常的想象，既然Vector类建议尽量少的使用，还是最好不要用了，通过上面的源码发现，每个方法中都添加了synchronized的关键字来保证同步，所以它是线程安全的，但正是这些方法的同步，让它的效率大大的降低了，比ArrayList的效率要慢。

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少年听雨歌楼上，红烛昏罗帐。  
壮年听雨客舟中，江阔云低，断雁叫西风。
感谢支持！
                                        ---起个名忒难

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