深入源码分析HashSet
来源：互联网 发布：淘宝好货源 编辑：程序博客网 时间：2024/05/20 07:33
前面我们花了一定的篇幅学习了HashMap的一些底层原理，以及简单了解了HashSet和HashMap两种集合的渊源，现在我们从HashSet源码入手，来学习HashSet更细节的地方。
对于HashSet而言，它是基于HashMap实现的。HashSet底层采用HashMap来保存元素，因此HashSet底层其实比较简单。
<code class="hljs cs 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-radius: 0px; word-wrap: normal; background: transparent;">package java.util;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, java.io.Serializable{ <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> final <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> serialVersionUID = -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5024744406713321676</span>L; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// HashSet是通过map(HashMap对象)保存内容的</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> transient HashMap<E,Object> map; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 定义一个虚拟的Object PRESENT是向map中插入key-value对应的value</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 因为HashSet中只需要用到key，而HashMap是key-value键值对；</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 所以，向map中添加键值对时，键值对的值固定是PRESENT</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> final Object PRESENT = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Object(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 默认构造函数 底层创建一个HashMap</span> <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;">HashSet</span>() { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 调用HashMap的默认构造函数，创建map</span> map = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HashMap<E,Object>(); } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 带集合的构造函数</span> <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;">HashSet</span>(Collection<? extends E> c) { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 创建map。</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 为什么要调用Math.max((int) (c.size()/.75f) + 1, 16)，从 (c.size()/.75f) + 1 和 16 中选择一个比较大的树呢？ </span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 首先，说明(c.size()/.75f) + 1</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 因为从HashMap的效率(时间成本和空间成本)考虑，HashMap的加载因子是0.75。</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 当HashMap的“阈值”(阈值=HashMap总的大小*加载因子) < “HashMap实际大小”时，</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 就需要将HashMap的容量翻倍。</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 所以，(c.size()/.75f) + 1 计算出来的正好是总的空间大小。</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 接下来，说明为什么是 16 。</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// HashMap的总的大小，必须是2的指数倍。若创建HashMap时，指定的大小不是2的指数倍；</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// HashMap的构造函数中也会重新计算，找出比“指定大小”大的最小的2的指数倍的数。</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 所以，这里指定为16是从性能考虑。避免重复计算。</span> map = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HashMap<E,Object>(Math.max((<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>) (c.size()/<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">.75</span>f) + <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;">16</span>)); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 将集合(c)中的全部元素添加到HashSet中</span> addAll(c); } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 指定HashSet初始容量和加载因子的构造函数</span> <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;">HashSet</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) { map = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HashMap<E,Object>(initialCapacity, loadFactor); } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 指定HashSet初始容量的构造函数</span> <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;">HashSet</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> initialCapacity) { map = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HashMap<E,Object>(initialCapacity); } HashSet(<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, boolean dummy) { map = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> LinkedHashMap<E,Object>(initialCapacity, loadFactor); } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 返回HashSet的迭代器</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Iterator<E> <span class="hljs-title" style="box-sizing: border-box;">iterator</span>() { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 实际上返回的是HashMap的“key集合的迭代器”</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> map.keySet().iterator(); } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//调用HashMap的size()方法返回Entry的数量，得到该Set里元素的个数</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">size</span>() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> map.size(); } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//调用HashMap的isEmpty()来判断HaspSet是否为空</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//HashMap为null。对应的HashSet也为空</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> boolean <span class="hljs-title" style="box-sizing: border-box;">isEmpty</span>() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> map.isEmpty(); } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//调用HashMap的containsKey判断是否包含指定的key</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//HashSet的所有元素就是通过HashMap的key来保存的</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> boolean <span class="hljs-title" style="box-sizing: border-box;">contains</span>(Object o) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> map.containsKey(o); } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 将元素(e)添加到HashSet中，也就是将元素作为Key放入HashMap中</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> boolean <span class="hljs-title" style="box-sizing: border-box;">add</span>(E e) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> map.put(e, PRESENT)==<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>; } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 删除HashSet中的元素(o)，其实是在HashMap中删除了以o为key的Entry</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> boolean <span class="hljs-title" style="box-sizing: border-box;">remove</span>(Object o) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> map.remove(o)==PRESENT; } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//清空HashMap的clear方法清空所有Entry</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">clear</span>() { map.clear(); } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 克隆一个HashSet，并返回Object对象</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Object <span class="hljs-title" style="box-sizing: border-box;">clone</span>() { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> { HashSet<E> newSet = (HashSet<E>) super.clone(); newSet.map = (HashMap<E, Object>) map.clone(); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> newSet; } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (CloneNotSupportedException e) { <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> InternalError(); } } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// java.io.Serializable的写入函数</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 将HashSet的“总的容量，加载因子，实际容量，所有的元素”都写入到输出流中</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">writeObject</span>(java.io.ObjectOutputStream s) throws java.io.IOException { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Write out any hidden serialization magic</span> s.defaultWriteObject(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Write out HashMap capacity and load factor</span> s.writeInt(map.capacity()); s.writeFloat(map.loadFactor()); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Write out size</span> s.writeInt(map.size()); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Write out all elements in the proper order.</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (Iterator i=map.keySet().iterator(); i.hasNext(); ) s.writeObject(i.next()); } <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// java.io.Serializable的读取函数</span> <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 将HashSet的“总的容量，加载因子，实际容量，所有的元素”依次读出</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">readObject</span>(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Read in any hidden serialization magic</span> s.defaultReadObject(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Read in HashMap capacity and load factor and create backing HashMap</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> capacity = s.readInt(); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> loadFactor = s.readFloat(); map = (((HashSet)<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>) instanceof LinkedHashSet ? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> LinkedHashMap<E,Object>(capacity, loadFactor) : <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HashMap<E,Object>(capacity, loadFactor)); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Read in size</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> size = s.readInt(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Read in all elements in the proper order.</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i<size; i++) { E e = (E) s.readObject(); map.put(e, PRESENT); } }}</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); 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padding: 0px 5px;">101</li><li style="box-sizing: border-box; padding: 0px 5px;">102</li><li style="box-sizing: border-box; padding: 0px 5px;">103</li><li style="box-sizing: border-box; padding: 0px 5px;">104</li><li style="box-sizing: border-box; padding: 0px 5px;">105</li><li style="box-sizing: border-box; padding: 0px 5px;">106</li><li style="box-sizing: border-box; padding: 0px 5px;">107</li><li style="box-sizing: border-box; padding: 0px 5px;">108</li><li style="box-sizing: border-box; padding: 0px 5px;">109</li><li style="box-sizing: border-box; padding: 0px 5px;">110</li><li style="box-sizing: border-box; padding: 0px 5px;">111</li><li style="box-sizing: border-box; padding: 0px 5px;">112</li><li style="box-sizing: border-box; padding: 0px 5px;">113</li><li style="box-sizing: border-box; padding: 0px 5px;">114</li><li style="box-sizing: border-box; padding: 0px 5px;">115</li><li style="box-sizing: border-box; padding: 0px 5px;">116</li><li style="box-sizing: border-box; padding: 0px 5px;">117</li><li style="box-sizing: border-box; padding: 0px 5px;">118</li><li style="box-sizing: border-box; padding: 0px 5px;">119</li><li style="box-sizing: border-box; padding: 0px 5px;">120</li><li style="box-sizing: border-box; padding: 0px 5px;">121</li><li style="box-sizing: border-box; padding: 0px 5px;">122</li><li style="box-sizing: border-box; padding: 0px 5px;">123</li><li style="box-sizing: border-box; padding: 0px 5px;">124</li><li style="box-sizing: border-box; padding: 0px 5px;">125</li><li style="box-sizing: border-box; padding: 0px 5px;">126</li><li style="box-sizing: border-box; padding: 0px 5px;">127</li><li style="box-sizing: border-box; padding: 0px 5px;">128</li><li style="box-sizing: border-box; padding: 0px 5px;">129</li><li style="box-sizing: border-box; padding: 0px 5px;">130</li><li style="box-sizing: border-box; padding: 0px 5px;">131</li><li style="box-sizing: border-box; padding: 0px 5px;">132</li><li style="box-sizing: border-box; padding: 0px 5px;">133</li><li style="box-sizing: border-box; padding: 0px 5px;">134</li><li style="box-sizing: border-box; padding: 0px 5px;">135</li><li style="box-sizing: border-box; padding: 0px 5px;">136</li><li style="box-sizing: border-box; padding: 0px 5px;">137</li><li style="box-sizing: border-box; padding: 0px 5px;">138</li><li style="box-sizing: border-box; padding: 0px 5px;">139</li><li style="box-sizing: border-box; padding: 0px 5px;">140</li><li style="box-sizing: border-box; padding: 0px 5px;">141</li><li style="box-sizing: border-box; padding: 0px 5px;">142</li><li style="box-sizing: border-box; padding: 0px 5px;">143</li></ul><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><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li><li style="box-sizing: border-box; padding: 0px 5px;">65</li><li style="box-sizing: border-box; padding: 0px 5px;">66</li><li style="box-sizing: border-box; padding: 0px 5px;">67</li><li style="box-sizing: border-box; padding: 0px 5px;">68</li><li style="box-sizing: border-box; padding: 0px 5px;">69</li><li style="box-sizing: border-box; padding: 0px 5px;">70</li><li style="box-sizing: border-box; padding: 0px 5px;">71</li><li style="box-sizing: border-box; padding: 0px 5px;">72</li><li style="box-sizing: border-box; padding: 0px 5px;">73</li><li style="box-sizing: border-box; padding: 0px 5px;">74</li><li style="box-sizing: border-box; padding: 0px 5px;">75</li><li style="box-sizing: border-box; padding: 0px 5px;">76</li><li style="box-sizing: border-box; padding: 0px 5px;">77</li><li style="box-sizing: border-box; padding: 0px 5px;">78</li><li style="box-sizing: border-box; padding: 0px 5px;">79</li><li style="box-sizing: border-box; padding: 0px 5px;">80</li><li style="box-sizing: border-box; padding: 0px 5px;">81</li><li style="box-sizing: border-box; padding: 0px 5px;">82</li><li style="box-sizing: border-box; padding: 0px 5px;">83</li><li style="box-sizing: border-box; padding: 0px 5px;">84</li><li style="box-sizing: border-box; padding: 0px 5px;">85</li><li style="box-sizing: border-box; padding: 0px 5px;">86</li><li style="box-sizing: border-box; padding: 0px 5px;">87</li><li style="box-sizing: border-box; padding: 0px 5px;">88</li><li style="box-sizing: border-box; padding: 0px 5px;">89</li><li style="box-sizing: border-box; padding: 0px 5px;">90</li><li style="box-sizing: border-box; padding: 0px 5px;">91</li><li style="box-sizing: border-box; padding: 0px 5px;">92</li><li style="box-sizing: border-box; padding: 0px 5px;">93</li><li style="box-sizing: border-box; padding: 0px 5px;">94</li><li style="box-sizing: border-box; padding: 0px 5px;">95</li><li style="box-sizing: border-box; padding: 0px 5px;">96</li><li style="box-sizing: border-box; padding: 0px 5px;">97</li><li style="box-sizing: border-box; padding: 0px 5px;">98</li><li style="box-sizing: border-box; padding: 0px 5px;">99</li><li style="box-sizing: border-box; padding: 0px 5px;">100</li><li style="box-sizing: border-box; padding: 0px 5px;">101</li><li style="box-sizing: border-box; padding: 0px 5px;">102</li><li style="box-sizing: border-box; padding: 0px 5px;">103</li><li style="box-sizing: border-box; padding: 0px 5px;">104</li><li style="box-sizing: border-box; padding: 0px 5px;">105</li><li style="box-sizing: border-box; padding: 0px 5px;">106</li><li style="box-sizing: border-box; padding: 0px 5px;">107</li><li style="box-sizing: border-box; padding: 0px 5px;">108</li><li style="box-sizing: border-box; padding: 0px 5px;">109</li><li style="box-sizing: border-box; padding: 0px 5px;">110</li><li style="box-sizing: border-box; padding: 0px 5px;">111</li><li style="box-sizing: border-box; padding: 0px 5px;">112</li><li style="box-sizing: border-box; padding: 0px 5px;">113</li><li style="box-sizing: border-box; padding: 0px 5px;">114</li><li style="box-sizing: border-box; padding: 0px 5px;">115</li><li style="box-sizing: border-box; padding: 0px 5px;">116</li><li style="box-sizing: border-box; padding: 0px 5px;">117</li><li style="box-sizing: border-box; padding: 0px 5px;">118</li><li style="box-sizing: border-box; padding: 0px 5px;">119</li><li style="box-sizing: border-box; padding: 0px 5px;">120</li><li style="box-sizing: border-box; padding: 0px 5px;">121</li><li style="box-sizing: border-box; padding: 0px 5px;">122</li><li style="box-sizing: border-box; padding: 0px 5px;">123</li><li style="box-sizing: border-box; padding: 0px 5px;">124</li><li style="box-sizing: border-box; padding: 0px 5px;">125</li><li style="box-sizing: border-box; padding: 0px 5px;">126</li><li style="box-sizing: border-box; padding: 0px 5px;">127</li><li style="box-sizing: border-box; padding: 0px 5px;">128</li><li style="box-sizing: border-box; padding: 0px 5px;">129</li><li style="box-sizing: border-box; padding: 0px 5px;">130</li><li style="box-sizing: border-box; padding: 0px 5px;">131</li><li style="box-sizing: border-box; padding: 0px 5px;">132</li><li style="box-sizing: border-box; padding: 0px 5px;">133</li><li style="box-sizing: border-box; padding: 0px 5px;">134</li><li style="box-sizing: border-box; padding: 0px 5px;">135</li><li style="box-sizing: border-box; padding: 0px 5px;">136</li><li style="box-sizing: border-box; padding: 0px 5px;">137</li><li style="box-sizing: border-box; padding: 0px 5px;">138</li><li style="box-sizing: border-box; padding: 0px 5px;">139</li><li style="box-sizing: border-box; padding: 0px 5px;">140</li><li style="box-sizing: border-box; padding: 0px 5px;">141</li><li style="box-sizing: border-box; padding: 0px 5px;">142</li><li style="box-sizing: border-box; padding: 0px 5px;">143</li></ul>从上述HashSet源代码可以看出，它其实就是一个对HashMap的封装而已。所有放入HashSet中的集合元素实际上由HashMap的key来保存，而HashMap的value则存储了一个PRESENT，它是一个静态的Object对象。
HashSet的绝大部分方法都是通过调用HashMap的方法来实现的，因此HashSet和HashMap两个集合在实现本质上是相同的。
根据HashMap的一个特性: 将一个key-value对放入HashMap中时，首先根据key的hashCode()返回值决定该Entry的存储位置，如果两个key的hash值相同，那么它们的存储位置相同。如果这个两个key的equalus比较返回true。那么新添加的Entry的value会覆盖原来的Entry的value，key不会覆盖。因此,如果向HashSet中添加一个已经存在的元素，新添加的集合元素不会覆盖原来已有的集合元素。
现在我们通过一个实际的例子来看看是否真正理解了HashMap和HashSet存储元素的细节:
<code class="hljs cs 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-radius: 0px; word-wrap: normal; background: transparent;">class Name{ <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> String first; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> String last; <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;">Name</span>(String first, String last) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.first = first; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.last = last; } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> boolean <span class="hljs-title" style="box-sizing: border-box;">equals</span>(Object o) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span> == o) { <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>; } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (o.getClass() == Name.class) { Name n = (Name)o; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> n.first.equals(first) && n.last.equals(last); } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>; }}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> HashSetTest{ <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</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;">void</span> <span class="hljs-title" style="box-sizing: border-box;">main</span>(String[] args) { Set<Name> s = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HashSet<Name>(); s.add(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Name(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"abc"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"123"</span>)); System.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.println( s.contains(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Name(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"abc"</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"123"</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><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li></ul><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><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li></ul>上面程序中向HashSet里添加了一个new Name(“abc”,”123”)对象之后，立即通过程序判断该HashSet里是否包含一个new Name(“abc”,”123”)对象。粗看上去，很容易以为该程序会输出true。
实际上会输出false。因为HashSet判断两个对象相等的标准是想通过hashCode()方法计算出其hash值，当hash值相同的时候才继续判断equals()方法。而如上程序我们并没有重写hashCode()方法。所以两个Name类的hash值并不相同，因此HashSet会把其当成两个对象来处理。
所以，当我们要将一个类作为HashMap的key或者存储在HashSet的时候。通过重写hashCode()和equals(Object object)方法很重要，并且保证这两个方法的返回值一致。当两个类的hashCode()返回一致时，应该保证equasl()方法也返回true。当给上述Name类增加如下方法:
<code class="hljs cs 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-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-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">hashCode</span>(){<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> first.hashCode()+last.hashCode();}</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><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>此时我们测试的方法会返回true。
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