java 如何查看ArrayList的容量capacity

刚刚仔细研究了下ArrayList的源码，发现其自动扩容公式为capacity=capacity+(capacity>>1) (简化后)，想验证一下

capacity即element数组的大小，而element数组又是私有的，于是想到利用反射访问

import java.lang.reflect.Field;
import java.util.ArrayList;

public class Test{
    public static void main(String args[]) {
        ArrayList<Integer> arrayList = new ArrayList<>();

        System.out.println(getArrayListCapacity(arrayList));

        //增加元素，使其扩容
        arrayList.add(0);
        System.out.println(getArrayListCapacity(arrayList));

        for(int i = 0; i < 10; ++i)
            arrayList.add(0);
        System.out.println(getArrayListCapacity(arrayList));

        for(int i = 0; i < 5; ++i)
            arrayList.add(0);
        System.out.println(getArrayListCapacity(arrayList));
    }

    public static int getArrayListCapacity(ArrayList<?> arrayList) {
        Class<ArrayList> arrayListClass = ArrayList.class;
        try {
            Field field = arrayListClass.getDeclaredField("elementData");
            field.setAccessible(true);
            Object[] objects = (Object[])field.get(arrayList);
           return objects.length;
        } catch (NoSuchFieldException e) {
            e.printStackTrace();
            return -1;
        } catch (IllegalAccessException e) {
            e.printStackTrace();
            return -1;
        }
    }
}

输出：

0

10

15

22

初次扩容容量置为10，以后每次扩容容量增大一半。