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ArrayList及HashMap的扩容规则讲解

2019年07月19日  | 移动技术网IT编程  | 我要评论

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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);
}

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