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
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.
今天的文章arraylist扩容1.5倍+1_arraylist默认长度和扩容分享到此就结束了,感谢您的阅读。
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