1 package java.util;
2
3 public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable,
4 java.io.Serializable {
5 static final long serialVersionUID = -5024744406713321676L;
6
7 // 内部的HashMap对象，不可被串行化。存储的元素作为key，保证不会重复
8 private transient HashMap<E, Object> map;
9
10 // Dummy value to associate with an Object in the backing Map
11 private static final Object PRESENT = new Object();
12
13 /**
14 * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
15 * default initial capacity (16) and load factor (0.75).
16 */
17 public HashSet() {
18 map = new HashMap<E, Object>();
19 }
20
21 /**
22 * Constructs a new set containing the elements in the specified collection.
23 * The <tt>HashMap</tt> is created with default load factor (0.75) and an
24 * initial capacity sufficient to contain the elements in the specified
25 * collection.
26 *
27 * @param c
28 * the collection whose elements are to be placed into this set
29 * @throws NullPointerException
30 * if the specified collection is null
31 */
32 public HashSet(Collection<? extends E> c) {
33 map = new HashMap<E, Object>(Math.max((int) (c.size() / .75f) + 1, 16));
34 // 对集合c中的每个元素，调用add(e)方法
35 addAll(c);
36 }
37
38 /**
39 * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
40 * the specified initial capacity and the specified load factor.
41 *
42 * @param initialCapacity
43 * the initial capacity of the hash map
44 * @param loadFactor
45 * the load factor of the hash map
46 * @throws IllegalArgumentException
47 * if the initial capacity is less than zero, or if the load
48 * factor is nonpositive
49 */
50 public HashSet(int initialCapacity, float loadFactor) {
51 map = new HashMap<E, Object>(initialCapacity, loadFactor);
52 }
53
54 /**
55 * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
56 * the specified initial capacity and default load factor (0.75).
57 *
58 * @param initialCapacity
59 * the initial capacity of the hash table
60 * @throws IllegalArgumentException
61 * if the initial capacity is less than zero
62 */
63 public HashSet(int initialCapacity) {
64 map = new HashMap<E, Object>(initialCapacity);
65 }
66
67 /**
68 * Constructs a new, empty linked hash set. (This package private
69 * constructor is only used by LinkedHashSet.) The backing HashMap instance
70 * is a LinkedHashMap with the specified initial capacity and the specified
71 * load factor.
72 *
73 * @param initialCapacity
74 * the initial capacity of the hash map
75 * @param loadFactor
76 * the load factor of the hash map
77 * @param dummy
78 * ignored (distinguishes this constructor from other int, float
79 * constructor.)
80 * @throws IllegalArgumentException
81 * if the initial capacity is less than zero, or if the load
82 * factor is nonpositive
83 */
84 HashSet(int initialCapacity, float loadFactor, boolean dummy) {
85 map = new LinkedHashMap<E, Object>(initialCapacity, loadFactor);
86 }
87
88 /**
89 * Returns an iterator over the elements in this set. The elements are
90 * returned in no particular order.
91 *
92 * @return an Iterator over the elements in this set
93 * @see ConcurrentModificationException
94 */
95 public Iterator<E> iterator() {
96 // 依赖HashMap中对iterator()的实现
97 return map.keySet().iterator();
98 }
99
100 /**
101 * Returns the number of elements in this set (its cardinality).
102 *
103 * @return the number of elements in this set (its cardinality)
104 */
105 public int size() {
106 return map.size();
107 }
108
109 /**
110 * Returns <tt>true</tt> if this set contains no elements.
111 *
112 * @return <tt>true</tt> if this set contains no elements
113 */
114 public boolean isEmpty() {
115 return map.isEmpty();
116 }
117
118 /**
119 * Returns <tt>true</tt> if this set contains the specified element. More
120 * formally, returns <tt>true</tt> if and only if this set contains an
121 * element <tt>e</tt> such that
122 * <tt>(o==null ? e==null : o.equals(e))</tt>.
123 *
124 * @param o
125 * element whose presence in this set is to be tested
126 * @return <tt>true</tt> if this set contains the specified element
127 */
128 public boolean contains(Object o) {
129 return map.containsKey(o);
130 }
131
132 /**
133 * Adds the specified element to this set if it is not already present. More
134 * formally, adds the specified element <tt>e</tt> to this set if this set
135 * contains no element <tt>e2</tt> such that
136 * <tt>(e==null ? e2==null : e.equals(e2))</tt>. If this
137 * set already contains the element, the call leaves the set unchanged and
138 * returns <tt>false</tt>.
139 *
140 * @param e
141 * element to be added to this set
142 * @return <tt>true</tt> if this set did not already contain the specified
143 * element
144 */
145 public boolean add(E e) {
146 // map.put(key,value)方法在key之前不存在的时候，返回null
147 return map.put(e, PRESENT) == null;
148 }
149
150 /**
151 * Removes the specified element from this set if it is present. More
152 * formally, removes an element <tt>e</tt> such that
153 * <tt>(o==null ? e==null : o.equals(e))</tt>, if this
154 * set contains such an element. Returns <tt>true</tt> if this set contained
155 * the element (or equivalently, if this set changed as a result of the
156 * call). (This set will not contain the element once the call returns.)
157 *
158 * @param o
159 * object to be removed from this set, if present
160 * @return <tt>true</tt> if the set contained the specified element
161 */
162 public boolean remove(Object o) {
163 // map.remove(key)方法返回相应的value，或者null
164 return map.remove(o) == PRESENT;
165 }
166
167 /**
168 * Removes all of the elements from this set. The set will be empty after
169 * this call returns.
170 */
171 public void clear() {
172 map.clear();
173 }
174
175 /**
176 * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
177 * themselves are not cloned.
178 *
179 * @return a shallow copy of this set
180 */
181 public Object clone() {
182 try {
183 HashSet<E> newSet = (HashSet<E>) super.clone();
184 newSet.map = (HashMap<E, Object>) map.clone();
185 return newSet;
186 } catch (CloneNotSupportedException e) {
187 throw new InternalError();
188 }
189 }
190
191 /**
192 * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
193 * serialize it).
194 *
195 * @serialData The capacity of the backing <tt>HashMap</tt> instance (int),
196 * and its load factor (float) are emitted, followed by the size
197 * of the set (the number of elements it contains) (int),
198 * followed by all of its elements (each an Object) in no
199 * particular order.
200 */
201 private void writeObject(java.io.ObjectOutputStream s)
202 throws java.io.IOException {
203 // Write out any hidden serialization magic
204 s.defaultWriteObject();
205
206 // Write out HashMap capacity and load factor
207 s.writeInt(map.capacity());
208 s.writeFloat(map.loadFactor());
209
210 // Write out size
211 s.writeInt(map.size());
212
213 // Write out all elements in the proper order.
214 for (Iterator i = map.keySet().iterator(); i.hasNext();)
215 s.writeObject(i.next());
216 }
217
218 /**
219 * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
220 * deserialize it).
221 */
222 private void readObject(java.io.ObjectInputStream s)
223 throws java.io.IOException, ClassNotFoundException {
224 // Read in any hidden serialization magic
225 s.defaultReadObject();
226
227 // Read in HashMap capacity and load factor and create backing HashMap
228 int capacity = s.readInt();
229 float loadFactor = s.readFloat();
230 // 根据对象类型，创建LinkedHashMap或者HashMap
231 map = (((HashSet) this) instanceof LinkedHashSet ? new LinkedHashMap<E, Object>(
232 capacity, loadFactor)
233 : new HashMap<E, Object>(capacity, loadFactor));
234
235 // Read in size
236 int size = s.readInt();
237
238 // Read in all elements in the proper order.
239 for (int i = 0; i < size; i++) {
240 E e = (E) s.readObject();
241 map.put(e, PRESENT);
242 }
243 }
244 }
245
小结：
- HashSet内部用一个HashMap的keyset保存数据元素。由HashMap的特性——所有的key不会重复，保证了HashSet中的所有元素不会重复。
- HashSet的方法很简单，一般都是直接调用HashMap的实现方法。HashSet甚至没有实现iterator()方法，也是依赖HashMap的实现。
- 串行化的readObject()和writeObject()方法设计思想值得学习。