ArrayList动态扩容机制

以ArrayList中add方法，讲解ArrayList动态扩容机制

1. 扩容判断
   每次增加元素时，用size+1计算出作为需要最小的容器大小minCapacity，然后和容器大小elementData.length作比较，如果比elementData.length小，就要进行扩容。
2. 扩容大小
   扩容时，将elementData.length作为oldCapacity，增大50%（oldCapacity >> 1），
   接着作两个判断
   如果增大后的值newCapacity还是小于minCapacity，就用minCapacity作为扩容值
   如果增大后的值newCapacity大于MAX_ARRAY_SIZE，就用Ar rayList最大可支持的容量hugeCapacity(minCapacity)

MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8

3. 源码展示

 public boolean add(E e) {
      //size表示The size of the ArrayList (the number of elements it contains)
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

 private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(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);
    }

private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }