JDK源码看ArrayList和Vector的一些区别

最近在看JDK源码，从源码的角度记录一下ArrayList和Vector的一些区别

1、new

a、不指定长度

　　Vector默认创建10个元素的数组

    public Vector() {
        this(10);
    }

ArrayList默认指向一个0个元素的数组

    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }

b、指定长度

Vector创建指定元素个数的数组，默认增长量为0

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

ArrayList创建指定元素个数的数组

    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

c、指定长度，指定增量

Vector创建指定元素个数的数组，指定增长量

    public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    }

ArrayList无此种构造方法

d、elementData

Vector

    /**
     * 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
     */
    protected Object[] elementData;

ArrayList

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == EMPTY_ELEMENTDATA will be expanded to
     * DEFAULT_CAPACITY when the first element is added.
     */
    private transient Object[] elementData;

可以看到ArrayList的elementData有transient修饰，也就是说ArrayList的elementData不能被序列化；

同时ArrayList的elementData是private修饰，而Vector的elementData是protected修饰，

也就是说ArrayList的elementData只有ArrayList才能访问，而Vector的elementData同一个package和子类可以访问

2、增长

Vector默认增长当前数组元素个数的一倍，如果new对象时指定了initialIncrement，则增长量为initialIncrement

    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        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);
    }

ArrayList默认增长当前数组元素个数的一半

    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

Vector和ArrayList都是newCapacity<minCapacity则newCapacity=minCapacity，确保最小容量

3、线程安全

Vector是线程安全的，很多方法有synchronized

    public synchronized boolean add(E e) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = e;
        return true;
    }

ArrayList是线程不安全的

    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }