ArrayList 扩容

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

private void ensureCapacityInternal(int minCapacity) {
        // 如果是个空数组
        if (elementData == EMPTY_ELEMENTDATA) {
            // 取minCapacity和10的较大者
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        // 如果数组已经有数据了
        ensureExplicitCapacity(minCapacity);
    }

    // 确保数组容量大于ArrayList中元素个数
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++; // 将“修改统计数”+1

        // 如果实际数据容量大于数组容量，就给数组扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

// 增大数组空间
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1); // 在原来容量的基础上加上
                                                            // oldCapacity/2
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity; // 最少保证容量和minCapacity一样
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity); // 最多不能超过最大容量
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }