HashMap最大容量为什么是2的32次方

1
2
3
4
5
6
观察jdk中HashMap的源码，我们知道极限值为2的31次方。

void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;//将threshold置为Integer.MAX_VALUE
return;//当HashMap的容量已经是2的31次方的时候，直接返回。
}

Entry[] newTable = new Entry[newCapacity];
transfer(newTable);
table = newTable;
threshold = (int)(newCapacity * loadFactor);
}
1
2
3
4
5
6
7
8
9
10
11
12
13
观察jdk中源码可发现当HashMap的容量已经是2的31次方的时候，就不会在进行扩容了。

/**
* The next size value at which to resize (capacity * load factor).
* @serial
*/
int threshold;
1
2
3
4
5
如上为对threshold的定义。

/**
* 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) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
if (size++ >= threshold)
resize(2 * table.length);
}
1
2
3
4
5
6
7
8
9
10
11
12
13
观察如上源码可知，当添加完元素后的容量大于threshold，就调用resize方法。
---------------------
作者：liudezhicsdn
来源：CSDN
原文：https://blog.csdn.net/liudezhicsdn/article/details/51234292?utm_source=copy
版权声明：本文为博主原创文章，转载请附上博文链接！