Vector源码个人解读

Vector类的基本属性

 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
     */
    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
     */
    protected int capacityIncrement;

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    private static final long serialVersionUID = -2767605614048989439L;

其实跟ArrayList没有多少区别，底层也是一个elementData数组。

Vector构造器

  public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }

    /**
     * Constructs an empty vector so that its internal data array
     * has size {@code 10} and its standard capacity increment is
     * zero.
     */
    public Vector() {
        this(10);
    }

可以发现，如果是无参，那么默认长度就是10，有参的话就看参数的值。

与ArrayList的扩容机制不同，vector是按两倍扩容的

private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                         capacityIncrement : oldCapacity);//这个三元运算符，其实就是两倍扩容。capacityIncrement是写死的，一直位为0.
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

其余方法与ArrayList大同小异。

总结：1.ArrayList扩容机制是默认为0，无参则为10，有参则之后需要扩容按照1.5倍进行扩容。Vector扩容机制是默认就是10，之后按2倍扩容

2.ArrayList是线程不安全的，而Vector的绝大多数方法都是有synchronization(同步)的使用，所以是线程安全的，因此性能也就比ArrayList差。

3.当作数据操作主要为索引或者只是在集合末端增加、删除元素时，两者效率都比较高。