欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页  >  IT编程

ArrayList及HashMap的扩容规则讲解

程序员文章站 2024-02-20 21:37:10
1、arraylist 默认大小为10 /** * default initial capacity. */ private static final...

1、arraylist

默认大小为10

/**
 * default initial capacity.
*/
private static final int default_capacity = 10;

最大容量为2^30 - 8

 /**
 * the maximum size of array to allocate.
 * some vms reserve some header words in an array.
 * attempts to allocate larger arrays may result in
 * outofmemoryerror: requested array size exceeds vm limit
 */
private static final int max_array_size = integer.max_value - 8;
/**
 * a constant holding the maximum value an {@code int} can
 * have, 2<sup>31</sup>-1.
*/
public static final int  max_value = 0x7fffffff;

扩容规则为:oldcapacity*1.5

/**
 * increases the capacity to ensure that it can hold at least the
 * number of elements specified by the minimum capacity argument.
 * @param mincapacity the desired minimum capacity
 */
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);
}

2、hashmap

默认大小: 16

/**
 * the default initial capacity - must be a power of two.
*/
static final int default_initial_capacity = 1 << 4; // aka 16

最大容量为:2^30

/**
 * the maximum capacity, used if a higher value is implicitly specified
 * by either of the constructors with arguments.
 * must be a power of two <= 1<<30.
*/
static final int maximum_capacity = 1 << 30;

扩容规则为:大于oldcapacity的最小的2的n次方整数

/**
 * adds a new entry with the specified key, value and hash code to
 * the specified bucket. it is the responsibility of this
 * method to resize the table if appropriate.
 * subclass overrides this to alter the behavior of put method.
 */
void addentry(int hash, k key, v value, int bucketindex) {
  if ((size >= threshold) && (null != table[bucketindex])) {
    resize(2 * table.length);
    hash = (null != key) ? hash(key) : 0;
    bucketindex = indexfor(hash, table.length);
  }
  createentry(hash, key, value, bucketindex);
}

总结

以上就是这篇文章的全部内容了,希望本文的内容对大家的学习或者工作具有一定的参考学习价值,谢谢大家对的支持。如果你想了解更多相关内容请查看下面相关链接