ArrayList扩容机制实探

ArrayList初始化

问题：执行以下代码后，这个list的列表大小（size）和容量（capacity）分别是多大？

List<String> list = new ArrayList<>();

答案是：size = 0, capacity = 0，即我们使用无参构造方法创建ArrayList对象时，大小和容量都是为0。

那么如果我继续执行：

        for(int i=0;i<100;i++) {
            list.add(String.valueOf(i));
        }

每次add之后，size和capacity 分别是多少？

答案是：

size = 1, capacity = 10
size = 2, capacity = 10
size = 3, capacity = 10
size = 4, capacity = 10
size = 5, capacity = 10
size = 6, capacity = 10
size = 7, capacity = 10
size = 8, capacity = 10
size = 9, capacity = 10
size = 10, capacity = 10
size = 11, capacity = 15
size = 12, capacity = 15
size = 13, capacity = 15
size = 14, capacity = 15
size = 15, capacity = 15
size = 16, capacity = 22
size = 17, capacity = 22
size = 18, capacity = 22
size = 19, capacity = 22
size = 20, capacity = 22
size = 21, capacity = 22
size = 22, capacity = 22
size = 23, capacity = 33
size = 24, capacity = 33
size = 25, capacity = 33
size = 26, capacity = 33
size = 27, capacity = 33
size = 28, capacity = 33
size = 29, capacity = 33
size = 30, capacity = 33
size = 31, capacity = 33
size = 32, capacity = 33
size = 33, capacity = 33
size = 34, capacity = 49
size = 35, capacity = 49
size = 36, capacity = 49
size = 37, capacity = 49
size = 38, capacity = 49
size = 39, capacity = 49
size = 40, capacity = 49
size = 41, capacity = 49
size = 42, capacity = 49
size = 43, capacity = 49
size = 44, capacity = 49
size = 45, capacity = 49
size = 46, capacity = 49
size = 47, capacity = 49
size = 48, capacity = 49
size = 49, capacity = 49
size = 50, capacity = 73
size = 51, capacity = 73
size = 52, capacity = 73
size = 53, capacity = 73
size = 54, capacity = 73
size = 55, capacity = 73
size = 56, capacity = 73
size = 57, capacity = 73
size = 58, capacity = 73
size = 59, capacity = 73
size = 60, capacity = 73
size = 61, capacity = 73
size = 62, capacity = 73
size = 63, capacity = 73
size = 64, capacity = 73
size = 65, capacity = 73
size = 66, capacity = 73
size = 67, capacity = 73
size = 68, capacity = 73
size = 69, capacity = 73
size = 70, capacity = 73
size = 71, capacity = 73
size = 72, capacity = 73
size = 73, capacity = 73
size = 74, capacity = 109
size = 75, capacity = 109
size = 76, capacity = 109
size = 77, capacity = 109
size = 78, capacity = 109
size = 79, capacity = 109
size = 80, capacity = 109
size = 81, capacity = 109
size = 82, capacity = 109
size = 83, capacity = 109
size = 84, capacity = 109
size = 85, capacity = 109
size = 86, capacity = 109
size = 87, capacity = 109
size = 88, capacity = 109
size = 89, capacity = 109
size = 90, capacity = 109
size = 91, capacity = 109
size = 92, capacity = 109
size = 93, capacity = 109
size = 94, capacity = 109
size = 95, capacity = 109
size = 96, capacity = 109
size = 97, capacity = 109
size = 98, capacity = 109
size = 99, capacity = 109
size = 100, capacity = 109

从中看出了规律：

即：ArrayList无参初始化是，容量是0，往list里面添加一个元素，容量开始扩容，扩容大小是10，当我的size到达10之后，再继续添加时，容量是拿当前的容量乘以1.5倍，舍去小数取整，即为新容量大小。

那如果继续无限添加元素呢，答案是：

拿最后一次扩容的容量与 整型最大值 - 8比较，如果是大于的话，取整型最大值（即：2^31 -1），如果是小于，则取整型最大值 - 8

ArrayList扩容源码如下：

    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);
    }

capacity 是list的私有属性，也没有提供相关的方法获取这个值，不过我们可以通过反射来获取这个值：

    public int getArrayListCapacity(List arrayList) {
        Class<ArrayList> arrayListClass = ArrayList.class;
        int capacity = 0;
        try {

            Field field = arrayListClass.getDeclaredField("elementData"); //获取 elementData 字段
            field.setAccessible(true);// 设置为可访问
            Object[] objects = (Object[])field.get(arrayList);
            //返回当前ArrayList实例的容量值
            capacity = objects.length;
        } catch (Exception e) {
            e.printStackTrace();
            capacity = -1;
        }
        return capacity;
    }

测试代码：

        List<String> list = new ArrayList<>();
        System.out.println(getArrayListCapacity(list));
        for(int i=0;i<100;i++) {
            list.add(String.valueOf(i));
            System.out.println("size = " + list.size() + ", capacity = " + getArrayListCapacity(list));
        }