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 HashMap 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 e to this set if
* this set contains no element e2 such that
* (e==null ? e2==null : e.equals(e2)).
* If this set already contains the element, the call leaves the set
* unchanged and returns false.
*
* @param e element to be added to this set
* @return true 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 true if this set contains the specified element.
* More formally, returns true if and only if this set
* contains an element e such that
* (o==null ? e==null : o.equals(e)).
*
* @param o element whose presence in this set is to be tested
* @return true 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 e such that
* (o==null ? e==null : o.equals(e)),
* if this set contains such an element. Returns true 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 true if the set contained the specified element
*/
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}
作者：Java邵先生
链接：https://www.jianshu.com/p/45e01d99cdbe
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