jdk源码浅读-HashSet

　　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

　　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