本文中涉及
HashMap的知识参考HashMap源码阅读
0. HashSet是什么
- 实现
Set接口，所以元素不重复，最多一个null元素 - 不保证元素的存储顺序
1. 实现的本质
HashSet是通过HashMap来存储元素的，也是通过它来保证Key的唯一性
在这个HashMap中要存储的元素作为Key，PRESENT作为Value来进行存储
private transient HashMap<E,Object> map;
// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();
2. 主要api解析
2.1 构造函数
在构造函数中初始化HashMap，其中的capacity和loadFactor也是指定HashMap的capacity和loadFactor。
public HashSet()
public HashSet(int initialCapacity)
public HashSet(int initialCapacity, float loadFactor)
public HashSet(Collection<? extends E> c)
2.2 add方法
往map中存入一个元素，key是需要存储的元素，value是PRESENT，由于HashMap中key是唯一的，所以Set中的元素就是唯一的，不能重复
/**
* 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 ? e2==null : 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;
}
2.3 remove方法
同样的，remove方法也是从map中删除元素
/**
* Removes the specified element from this set if it is present.
* More formally, removes an element <tt>e</tt> such that
* <tt>(o==null ? e==null : 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;
}
2.4 遍历
遍历是遍历map的keySet
/**
* Returns an iterator over the elements in this set. The elements
* are returned in no particular order.
*
* @return an Iterator over the elements in this set
* @see ConcurrentModificationException
*/
public Iterator<E> iterator() {
return map.keySet().iterator();
}
3. 参考
- HashSet源码build 1.8.0_121-b13版本
