为什么Hashtable 11的initialCapacity为HashMap，而HashMap中的DEFAULT_INITIAL_CAPACITY为16并要求2的幂?

问题描述

比较JDK 1.6中的HashMap和Hashtable源代码，我在HashMap中看到了以下代码:

Comparing the HashMap and Hashtable source code in JDK 1.6, I saw the below code inside HashMap:

/**
 * The default initial capacity - MUST be a power of two.
 */
static final int DEFAULT_INITIAL_CAPACITY = 16;

    int capacity = 1;
    while (capacity < initialCapacity)
        capacity <<= 1;

但是，在Hashtable中，我看到了:

However, in Hashtable, I saw this:

table = new Entry[initialCapacity];

public Hashtable() {
    this(11, 0.75f);
}

所以我的问题是:为什么HashMap需要2的幂作为初始容量，而Hashtable选择11作为默认初始容量?我认为这与Hashtable是线程安全的并且不允许空键或值无关.

So my question is:Why does HashMap require a power of 2 as the initial capacity, while Hashtable chooses 11 as the default initial capacity?I assume this has nothing to do with the thing that Hashtable is thread-safe and does not allow null key or values.

推荐答案

下面的文章详细介绍了此问题: HashMap需要更好的hashCode()-JDK 1.4第II部分.

The following article addresses this question in some detail: HashMap requires a better hashCode() - JDK 1.4 Part II.

根据该文章，转换为2的幂次幂的主要原因是位掩码比整数除法快.这并非没有不良后果，其中一位原始作者对此进行了解释:

According to that article, the main reason to move to power-of-two sizes was that bit masking is faster than integer division. This is not without adverse consequences, which are explained by one of the original authors:

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