Vector源码分析

vector的底层存储结构是数组，和ArrayList一样，不同的是vector是线程安全的，而ArrayList不是线程安全的。进入源码分析

1.属性

 /**
     * The array buffer into which the components of the vector are
     * stored. The capacity of the vector is the length of this array buffer,
     * and is at least large enough to contain all the vector's elements.
     *
     * <p>Any array elements following the last element in the Vector are null.
     *
     * @serial
     */
    //vector的底层存储结构，vector中的元素就是存储在这个数组中
    protected Object[] elementData;

    /**
     * The number of valid components in this {@code Vector} object.
     * Components {@code elementData[0]} through
     * {@code elementData[elementCount-1]} are the actual items.
     *
     * @serial
     */
    //数组中元素的数量
    protected int elementCount;

    /**
     * The amount by which the capacity of the vector is automatically
     * incremented when its size becomes greater than its capacity.  If
     * the capacity increment is less than or equal to zero, the capacity
     * of the vector is doubled each time it needs to grow.
     *
     * @serial
     */
    //当扩容的时候的增量
    protected int capacityIncrement;

2.构造器

 public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        //创建一个数组对象，给予一定的大小
        this.elementData = new Object[initialCapacity];        
        //给capacityIncrement赋值

        this.capacityIncrement = capacityIncrement;

    }

3.方法

add(E e):将指定元素添加到此向量的末尾

  
    //注意此处添加了synchronized关键字，保证了线程安全
    public synchronized boolean add(E e) {
        //修改次数+1
        modCount++;
        //保证容量大小够用
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    }

  private void ensureCapacityHelper(int minCapacity) {
        // overflow-conscious code
        // 如果需要的最小容量 大于 数组的长度（现在的容量），那么就需要扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

   private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        //如果capacityIncrement>0，这个值就是扩容的增量；否则扩容的增量大小等于现在的容量
        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);
    }

remove(int index):移除指定位置的元素

    
  //有synchronized关键字，保证线程安全
  public synchronized E remove(int index) {
        modCount++;
        if (index >= elementCount)
            throw new ArrayIndexOutOfBoundsException(index);
        E oldValue = elementData(index);
        //后面数组的复制操作时，需要复制的长度
        int numMoved = elementCount - index - 1;
        if (numMoved > 0)
            //此处就是数组的复制操作，可以理解为把index+1开始的元素，长度为numMoved整体向前移动一个单位
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        //将最后一个元素值设为null
        elementData[--elementCount] = null; // Let gc do its work

        return oldValue;
    }

其它方法也是类似，这里就不再一一分析，主要是增加了synchronized关键字保证了线程安全，这是和ArrayList的最主要的区别。