Vector和ArrayList的扩容

Vector介绍

构造器
public Vector()  // 实际上调用了Vector(10)

public Vector(int initialCapacity) // 实际上调用了Vector(initialCapacity,0)

public Vector(int initialCapacity, int capacityIncrement)

public Vector(Collection<? extends E> c)

扩容原则：
是否设置容量扩展值capacityIncrement，如果设置了，currentCapcity = oldCapacity + capacityIncrement
如果没设置容量扩展值，则currentCapcity =oldCapacity + oldCapacity（即扩容增加一倍）。
但是如果扩容了一次后，发现currentCapcity< minCapacity，最终使用currentCapcity = currentCapcity

源码：
java.util.Vector

    /**
     * Increases the capacity of this vector, if necessary, to ensure
     * that it can hold at least the number of components specified by
     * the minimum capacity argument.
     *
     * <p>If the current capacity of this vector is less than
     * {@code minCapacity}, then its capacity is increased by replacing its
     * internal data array, kept in the field {@code elementData}, with a
     * larger one.  The size of the new data array will be the old size plus
     * {@code capacityIncrement}, unless the value of
     * {@code capacityIncrement} is less than or equal to zero, in which case
     * the new capacity will be twice the old capacity; but if this new size
     * is still smaller than {@code minCapacity}, then the new capacity will
     * be {@code minCapacity}.
     *
     * @param minCapacity the desired minimum capacity
     */
    public synchronized void ensureCapacity(int minCapacity) {
        if (minCapacity > 0) {
            modCount++;
            ensureCapacityHelper(minCapacity);
        }
    }
    
    
    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

构造器
public ArrayList()

public ArrayList(Collection <? extends E> c)

public ArrayList(int initialCapacity)

扩容原则：
扩容增加0.5倍，currentCapcity = oldCapacity + 0.5 * oldCapacity
如果扩容后 currentCapcity < minCapacity,则currentCapcity = minCapacity

    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    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);
    }