Core Java 简单谈谈HashSet(推荐)

private transient HashMap<E,Object> map; // Dummy value to associate with an Object in the backing Map private static final Object PRESENT = new Object(); /**  * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has  * default initial capacity (16) and load factor (0.75).  */ public HashSet() {  map = new HashMap<>(); }

/**  * 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;