ArrayList扩容1.5倍

小灰灰 2022-06-01 08:22 249阅读 0赞

### 先写一个小案例 ###

###### 代码 ######

package com.itheima.demo;
    
    import java.lang.reflect.Field;
    import java.util.ArrayList;
    
    public class ArrayListDemo {
    
        public static void main(String[] args) {
            ArrayList<Integer> list1 = new ArrayList<>();
            Integer capacity = getCapacity(list1);// 获取容量
            int size = list1.size();
            System.out.println("list1的容量:" + capacity);
            System.out.println("list1的大小:" + size);
            System.out.println("----------------------------");
    
            ArrayList<Integer> list2 = new ArrayList<>();
            list2.add(1);
            capacity = getCapacity(list2);// 获取容量,arraylist初始化容量是10
            size = list2.size();
            System.out.println("list2的容量:" + capacity);
            System.out.println("list2的大小:" + size);
            System.out.println("----------------------------");
    
            ArrayList<Integer> list3 = new ArrayList<>();
    
            capacity = getCapacity(list3);// 获取容量,arraylist初始化容量是10
            for (int i = 0; i < 10; i++) {
                list3.add(i);
            }
            capacity = getCapacity(list3);
            size = list3.size();
            System.out.println("list3的容量:" + capacity);
            System.out.println("list3的大小:" + size);
            System.out.println("----------------------------");
    
            ArrayList<Integer> list4 = new ArrayList<>();
            for (int i = 0; i < 11; i++) {
                list4.add(i);
            }
            capacity = getCapacity(list4);// 获取容量
            size = list4.size();
            System.out.println("list4的容量:" + capacity);
            System.out.println("list4的大小:" + size);
    
        }
    
        // 获取list容量
        public static Integer getCapacity(ArrayList list) {
            Integer length = null;
            Class clazz = list.getClass();
            Field field;
            try {
                field = clazz.getDeclaredField("elementData");
                field.setAccessible(true);
                Object[] object = (Object[]) field.get(list);
                length = object.length;
                return length;
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return length;
        }
    }

###### 运行结果 ######

![这里写图片描述][SouthEast]  
当ArrayList不为空时,并且它的大小不超过10时,它的容量都是10.但当大小从10增加到11时,容量变成了15,扩大了1.5倍.

### ArrayList的源代码 ###

/** * Appends the specified element to the end of this list. * * @param e element to be appended to this list * @return <tt>true</tt> (as specified by {@link Collection#add}) */
        public boolean add(E e) {
            ensureCapacityInternal(size + 1); 
             //调用了一ensureCapacityInternal方法，实现list的容量换算,继续往下点击源代码
            elementData[size++] = e;
            return true;
        }

private void ensureCapacityInternal(int minCapacity) {
            if (elementData == EMPTY_ELEMENTDATA) {
                minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
            }
    
            ensureExplicitCapacity(minCapacity);//不管怎么样,最后都要走这一步,继续往下点击

private void ensureExplicitCapacity(int minCapacity) {
            modCount++;
    
            // overflow-conscious code
            if (minCapacity - elementData.length > 0)
                grow(minCapacity);//最后时限扩容的是grow,所以继续往下看grow的源码
        }

private void grow(int minCapacity) {
            // overflow-conscious code
            int oldCapacity = elementData.length;
            int newCapacity = oldCapacity + (oldCapacity >> 1);//新容量扩大到原容量的1.5倍，右移一位相关于原数值除以2。
            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);
        }

从案例和源代码来说,ArrayList的扩容是原来的1.5倍,当然,实际操作中,ArrayList扩容可能大于1.5倍,毕竟ArrayList调用add时,所涉及到的代码不止上面这一小点.

[SouthEast]: /images/20220601/c22407a381344ee994d36829b53660fa.png