通过阅读源码发现，HashSet底层的实现源码其实就是调用HashMap的方法实现的，所以如果你阅读过HashMap或对HashMap比较熟悉的话，那么阅读HashSet就很轻松，也很容易理解了。我之前也写了一篇关于hashMap源码阅读的文章，可以点击这里查看。

　　使用过HashSet的都清楚它保存的元素是不可以重复的，其实HashSet的元素都是保存在HashMap的key中的，而HashMap的key是没有重复的。

　　构造函数

/**
     * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
     * the specified initial capacity and the specified load factor.
     *
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
     */
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

　　HashSet的构造方法都是直接调用了HashMap的构造方法，HashSet有很多个构造方法全部都是直接调用了HashMap的构造方法。

　　add方法

/**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element <tt>e</tt> to this set if
     * this set contains no element <tt>e2</tt> such that
     * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns <tt>false</tt>.
     *
     * @param e element to be added to this set
     * @return <tt>true</tt> if this set did not already contain the specified
     * element
     */
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

　　contains方法

 /**
     * Returns <tt>true</tt> if this set contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this set
     * contains an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this set is to be tested
     * @return <tt>true</tt> if this set contains the specified element
     */
    public boolean contains(Object o) {
        return map.containsKey(o);
    }

　　remove方法

 /**
     * Removes the specified element from this set if it is present.
     * More formally, removes an element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,
     * if this set contains such an element.  Returns <tt>true</tt> if
     * this set contained the element (or equivalently, if this set
     * changed as a result of the call).  (This set will not contain the
     * element once the call returns.)
     *
     * @param o object to be removed from this set, if present
     * @return <tt>true</tt> if the set contained the specified element
     */
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

　其他的方法也都是直接调用HashMap的方法，所以在这里就不用贴出来了。

　　1、只要弄懂HashMap就很容易明白HashSet了，可以参考这篇文章：jdk源码浅读-HashMap https://www.cnblogs.com/rainple/p/9927263.html

　　2、我自己在看源码的时候也手写了HashMap、HashSet等数据结构的类，大家可以下载下来参考一下，有不懂或不理解的地方可以问我，如果有什么问题也随时欢迎骚扰。项目地址：https://github.com/rainple1860/MyCollection

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