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29 * This class implements the <tt>Set</tt> interface, backed by a hash table
30 * (actually a <tt>HashMap</tt> instance). It makes no guarantees as to the
31 * iteration order of the set; in particular, it does not guarantee that the
32 * order will remain constant over time. This class permits the <tt>null</tt>
35 * <p>This class offers constant time performance for the basic operations
36 * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>),
37 * assuming the hash function disperses the elements properly among the
38 * buckets. Iterating over this set requires time proportional to the sum of
39 * the <tt>HashSet</tt> instance's size (the number of elements) plus the
40 * "capacity" of the backing <tt>HashMap</tt> instance (the number of
41 * buckets). Thus, it's very important not to set the initial capacity too
42 * high (or the load factor too low) if iteration performance is important.
44 * <p><strong>Note that this implementation is not synchronized.</strong>
45 * If multiple threads access a hash set concurrently, and at least one of
46 * the threads modifies the set, it <i>must</i> be synchronized externally.
47 * This is typically accomplished by synchronizing on some object that
48 * naturally encapsulates the set.
50 * If no such object exists, the set should be "wrapped" using the
51 * {@link Collections#synchronizedSet Collections.synchronizedSet}
52 * method. This is best done at creation time, to prevent accidental
53 * unsynchronized access to the set:<pre>
54 * Set s = Collections.synchronizedSet(new HashSet(...));</pre>
56 * <p>The iterators returned by this class's <tt>iterator</tt> method are
57 * <i>fail-fast</i>: if the set is modified at any time after the iterator is
58 * created, in any way except through the iterator's own <tt>remove</tt>
59 * method, the Iterator throws a {@link ConcurrentModificationException}.
60 * Thus, in the face of concurrent modification, the iterator fails quickly
61 * and cleanly, rather than risking arbitrary, non-deterministic behavior at
62 * an undetermined time in the future.
64 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
65 * as it is, generally speaking, impossible to make any hard guarantees in the
66 * presence of unsynchronized concurrent modification. Fail-fast iterators
67 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
68 * Therefore, it would be wrong to write a program that depended on this
69 * exception for its correctness: <i>the fail-fast behavior of iterators
70 * should be used only to detect bugs.</i>
72 * <p>This class is a member of the
73 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
74 * Java Collections Framework</a>.
76 * @param <E> the type of elements maintained by this set
87 public class HashSet<E>
88 extends AbstractSet<E>
89 implements Set<E>, Cloneable, java.io.Serializable
91 static final long serialVersionUID = -5024744406713321676L;
93 private transient HashMap<E,Object> map;
95 // Dummy value to associate with an Object in the backing Map
96 private static final Object PRESENT = new Object();
99 * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
100 * default initial capacity (16) and load factor (0.75).
103 map = new HashMap<>();
107 * Constructs a new set containing the elements in the specified
108 * collection. The <tt>HashMap</tt> is created with default load factor
109 * (0.75) and an initial capacity sufficient to contain the elements in
110 * the specified collection.
112 * @param c the collection whose elements are to be placed into this set
113 * @throws NullPointerException if the specified collection is null
115 public HashSet(Collection<? extends E> c) {
116 map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
121 * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
122 * the specified initial capacity and the specified load factor.
124 * @param initialCapacity the initial capacity of the hash map
125 * @param loadFactor the load factor of the hash map
126 * @throws IllegalArgumentException if the initial capacity is less
127 * than zero, or if the load factor is nonpositive
129 public HashSet(int initialCapacity, float loadFactor) {
130 map = new HashMap<>(initialCapacity, loadFactor);
134 * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
135 * the specified initial capacity and default load factor (0.75).
137 * @param initialCapacity the initial capacity of the hash table
138 * @throws IllegalArgumentException if the initial capacity is less
141 public HashSet(int initialCapacity) {
142 map = new HashMap<>(initialCapacity);
146 * Constructs a new, empty linked hash set. (This package private
147 * constructor is only used by LinkedHashSet.) The backing
148 * HashMap instance is a LinkedHashMap with the specified initial
149 * capacity and the specified load factor.
151 * @param initialCapacity the initial capacity of the hash map
152 * @param loadFactor the load factor of the hash map
153 * @param dummy ignored (distinguishes this
154 * constructor from other int, float constructor.)
155 * @throws IllegalArgumentException if the initial capacity is less
156 * than zero, or if the load factor is nonpositive
158 HashSet(int initialCapacity, float loadFactor, boolean dummy) {
159 map = new LinkedHashMap<>(initialCapacity, loadFactor);
163 * Returns an iterator over the elements in this set. The elements
164 * are returned in no particular order.
166 * @return an Iterator over the elements in this set
167 * @see ConcurrentModificationException
169 public Iterator<E> iterator() {
170 return map.keySet().iterator();
174 * Returns the number of elements in this set (its cardinality).
176 * @return the number of elements in this set (its cardinality)
183 * Returns <tt>true</tt> if this set contains no elements.
185 * @return <tt>true</tt> if this set contains no elements
187 public boolean isEmpty() {
188 return map.isEmpty();
192 * Returns <tt>true</tt> if this set contains the specified element.
193 * More formally, returns <tt>true</tt> if and only if this set
194 * contains an element <tt>e</tt> such that
195 * <tt>(o==null ? e==null : o.equals(e))</tt>.
197 * @param o element whose presence in this set is to be tested
198 * @return <tt>true</tt> if this set contains the specified element
200 public boolean contains(Object o) {
201 return map.containsKey(o);
205 * Adds the specified element to this set if it is not already present.
206 * More formally, adds the specified element <tt>e</tt> to this set if
207 * this set contains no element <tt>e2</tt> such that
208 * <tt>(e==null ? e2==null : e.equals(e2))</tt>.
209 * If this set already contains the element, the call leaves the set
210 * unchanged and returns <tt>false</tt>.
212 * @param e element to be added to this set
213 * @return <tt>true</tt> if this set did not already contain the specified
216 public boolean add(E e) {
217 return map.put(e, PRESENT)==null;
221 * Removes the specified element from this set if it is present.
222 * More formally, removes an element <tt>e</tt> such that
223 * <tt>(o==null ? e==null : o.equals(e))</tt>,
224 * if this set contains such an element. Returns <tt>true</tt> if
225 * this set contained the element (or equivalently, if this set
226 * changed as a result of the call). (This set will not contain the
227 * element once the call returns.)
229 * @param o object to be removed from this set, if present
230 * @return <tt>true</tt> if the set contained the specified element
232 public boolean remove(Object o) {
233 return map.remove(o)==PRESENT;
237 * Removes all of the elements from this set.
238 * The set will be empty after this call returns.
240 public void clear() {
245 * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
246 * themselves are not cloned.
248 * @return a shallow copy of this set
250 public Object clone() {
252 HashSet<E> newSet = (HashSet<E>) super.clone();
253 newSet.map = (HashMap<E, Object>) map.clone();
255 } catch (CloneNotSupportedException e) {
256 throw new InternalError();