HashMap的容量与扩容

最新推荐文章于 2025-06-07 22:21:43 发布

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本文详细解析了HashMap的工作原理，包括不同构造方法的使用场景及其内部实现细节。探讨了HashMap的初始化过程、桶数组的建立与调整机制，以及负载因子的作用。

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有几个重要的常量：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> DEFAULT_INITIAL_CAPACITY = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//默认的桶数组大小</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> MAXIMUM_CAPACITY = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">30</span>;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//极限值（超过这个值就将threshold修改为Integer.MAX_VALUE（此时桶大小已经是2的31次方了），表明不进行扩容了）</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> DEFAULT_LOAD_FACTOR = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.75</span>f;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//负载因子（请阅读下面体会这个值的用处）</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

（一）调用HashMap()构造方法

这个构造方法长这样：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">HashMap</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.loadFactor = DEFAULT_LOAD_FACTOR; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//设置一个默认的负载因子，默认为0.75，下面说它有啥用</span>
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

这就完了，然后用于hash的桶数组为null，当我们第一次put的时候，会到达这段代码：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">Node<K,V>[] tab; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> n;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//table就是桶数组，初始为null</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((tab = table) == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span> || (n = tab.length) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>)
        n = (tab = resize()).length;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

所以会进入resize()方法，到达这段代码：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//取消多余的代码，我直接贴上进入的这个分支的代码</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
      newCap = DEFAULT_INITIAL_CAPACITY;
      newThr = (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

newCap的意思是：即将用于hash的桶数组的长度，这里的默认值是16
newThr的意思是：当桶中的键值对的个数超过这个值时就进行扩容，这时候上面提到的负载因子就起作用了，所以这里的newThr为0.75×16 = 12

所以经过这个默认构造方法，并且进行第一次put后，桶数组建立了，桶容量为16，极限值为12。

（二）调用HashMap(int initialCapacity)构造方法

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">HashMap</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> initialCapacity) {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//这个方法实际调用另一个构造方法，所以这个构造方法就不分析了，直接看第三个的分析</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>(initialCapacity, DEFAULT_LOAD_FACTOR);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

（三）调用HashMap(int initialCapacity, float loadFactor)构造方法

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">HashMap</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> initialCapacity, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> loadFactor) {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//桶数组的大小小于0时抛异常</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (initialCapacity < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>)
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> IllegalArgumentException(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Illegal initial capacity: "</span> +
                                               initialCapacity);
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如果桶数组的大小超过最大值，则简单的将桶容量修改为最大值2的30次方</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如果负载因子不符合规范，那么抛异常</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (loadFactor <= <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> || Float.isNaN(loadFactor))
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> IllegalArgumentException(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Illegal load factor: "</span> +
                                               loadFactor);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.loadFactor = loadFactor;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//这个意思是根据桶数组的大小求一个“极限值threshold”，当桶中的键值队个数超过这个大小就进行扩容</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//tableSizeFor方法求得的数字是刚超过initialCapacity的一个2的n次方的数，例如initialCapacity是1000，那么得到的threshold就是1024，这个方法很奇特，我没看懂，感兴趣的自己去研究。</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.threshold = tableSizeFor(initialCapacity);
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

所以经过这个方法之后，桶容量确定了，极限值也确定了，但是桶数组还是null。
当第一次put时，触发如下代码：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//发现桶数组是null，所以需要新建一个桶数组</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((tab = table) == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span> || (n = tab.length) == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>)
    n = (tab = resize()).length;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

接着看resize()中触发了哪些代码：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">Node<K,V>[] oldTab = table;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> oldCap = (oldTab == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) ? <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> : oldTab.length;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//看这里，oldCap会是0</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> oldThr = threshold;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//看这里，oldThr不会是0，因为有tableSizeFor()方法，确保oldThr至少是1</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> newCap, newThr = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (oldCap > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (oldCap >= MAXIMUM_CAPACITY) {
        threshold = Integer.MAX_VALUE;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> oldTab;
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((newCap = oldCap << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) < MAXIMUM_CAPACITY &&
            oldCap >= DEFAULT_INITIAL_CAPACITY)
        newThr = oldThr << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// double threshold</span>
}
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//所以肯定会进入这个分支，将桶数组的大小改为极限值大小。然后在下面创建一个newCap大小的桶数组</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (oldThr > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// initial capacity was placed in threshold</span>
    newCap = oldThr;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>

然后还会进入下面这个分支：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (newThr == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> ft = (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>)MAXIMUM_CAPACITY ?
                      (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>)ft : Integer.MAX_VALUE);
        }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

这个分支会按负载因子设置极限值

（四）超过极限值后的扩容

在每次put之后，会有下面这个判断：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (++size > threshold)
    resize();</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

就是说超过极限值时会进行扩容，扩容方式如下：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">Node<K,V>[] oldTab = table;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> oldCap = (oldTab == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) ? <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> : oldTab.length;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> oldThr = threshold;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> newCap, newThr = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//肯定会进入这个分支</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (oldCap > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (oldCap >= MAXIMUM_CAPACITY) {
        threshold = Integer.MAX_VALUE;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> oldTab;
    }
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如果我们是这样调用：HashMap(7，任意满足条件的值），那么经过第一次调用put()会初始化桶数组的大小和极限值一样，这里即为8。所以这里不会进入这个分支，但是newCap会变为16</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((newCap = oldCap << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) < MAXIMUM_CAPACITY &&
            oldCap >= DEFAULT_INITIAL_CAPACITY)
        newThr = oldThr << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// double threshold</span>
}
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//所以会进入这里</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (newThr == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> ft = (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>)newCap * loadFactor;
    newThr = (newCap < MAXIMUM_CAPACITY && ft < (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span>)MAXIMUM_CAPACITY ?
            (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>)ft : Integer.MAX_VALUE);
}
threshold = newThr;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//计算的出这里的threshold即极限值会由刚才的8变为12（和负载因子大小有关）</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li></ul>

接上面这个例子，下一次扩容会如何？

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (oldCap > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (oldCap >= MAXIMUM_CAPACITY) {
        threshold = Integer.MAX_VALUE;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> oldTab;
    }
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//因为桶容量变为了16，所以这次会进入这个分支，桶容量和极限值都会加倍。</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> ((newCap = oldCap << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) < MAXIMUM_CAPACITY &&
            oldCap >= DEFAULT_INITIAL_CAPACITY)
        newThr = oldThr << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// double threshold</span>
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

直到…………
当桶容量加倍到最大值会怎么样？

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//因为每次都是加倍，所以最终肯定会加倍到MAXIMUM_CAPACITY，会进入这个分支。</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (oldCap >= MAXIMUM_CAPACITY) {
    threshold = Integer.MAX_VALUE;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> oldTab;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

桶容量变为最大值时，紧接着的一次扩容只是简单的将极限值修改为Integer.MAX_VALUE。桶数组大小并不会继续加倍

（五）如果HashMap中的键值对数量超过Integer.MAX_VALUE了呢？

我们仔细看一下这段代码：

<code class="language-java hljs  has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background: transparent;">这个size类型是<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>，那么最大就是Integer.MAX_VALUE，所以不会有++size > threshold的情况，往后都不会进行扩容了。
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (++size > threshold)
    resize();</code>