Java集合系列四HashSet和LinkedHashSet解析

Posted 2020-09-30 wlrhnh

2017-07-29 16:58:13

一、简介

1、Set概念

Set可以理解为集合，非常类似数据概念中的集合，集合三大特征：1、确定性；2、互异性；3、无序性，因此Set实现类也有类似的特征。

2、HashSet

HashSet继承自AbstractSet，实现了Set接口，但是其源码非常少，也非常简单。内部使用HashMap来存储数据，数据存储在HashMap的key中，value都是同一个默认值：

二、HashSet几个重要的方法

1、add(E e)

HashSet的确定性，也可以理解为唯一性，是通过HashMap的put方法来保证的，往HashMap中put数据时，如果key是一样的，只会替换key对应的value，不会新插入一条数据。所以往HashSet中add相同的元素没有什么用，这里的相同是通过equals方法保证的，具体的在HashMap中细说。

2、remove(Object o)

简单粗暴，从HashMap中移除一条数据。

3、contains(Object o)

4、iterator()

5、其他

其他的方法诸如：size()、isEmpty()、contains()、clear()等都完全委托给了HashMap。需要注意的是：HashSet没有提供set、get等方法。

 源码如下：

  1 package java.util;
  2 
  3 import java.io.InvalidObjectException;
  4 
  5 /**
  6  * This class implements the <tt>Set</tt> interface, backed by a hash table
  7  * (actually a <tt>HashMap</tt> instance).  It makes no guarantees as to the
  8  * iteration order of the set; in particular, it does not guarantee that the
  9  * order will remain constant over time.  This class permits the <tt>null</tt>
 10  * element.
 11  *
 12  * <p>This class offers constant time performance for the basic operations
 13  * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>),
 14  * assuming the hash function disperses the elements properly among the
 15  * buckets.  Iterating over this set requires time proportional to the sum of
 16  * the <tt>HashSet</tt> instance\'s size (the number of elements) plus the
 17  * "capacity" of the backing <tt>HashMap</tt> instance (the number of
 18  * buckets).  Thus, it\'s very important not to set the initial capacity too
 19  * high (or the load factor too low) if iteration performance is important.
 20  *
 21  * <p><strong>Note that this implementation is not synchronized.</strong>
 22  * If multiple threads access a hash set concurrently, and at least one of
 23  * the threads modifies the set, it <i>must</i> be synchronized externally.
 24  * This is typically accomplished by synchronizing on some object that
 25  * naturally encapsulates the set.
 26  *
 27  * If no such object exists, the set should be "wrapped" using the
 28  * {@link Collections#synchronizedSet Collections.synchronizedSet}
 29  * method.  This is best done at creation time, to prevent accidental
 30  * unsynchronized access to the set:<pre>
 31  *   Set s = Collections.synchronizedSet(new HashSet(...));</pre>
 32  *
 33  * <p>The iterators returned by this class\'s <tt>iterator</tt> method are
 34  * <i>fail-fast</i>: if the set is modified at any time after the iterator is
 35  * created, in any way except through the iterator\'s own <tt>remove</tt>
 36  * method, the Iterator throws a {@link ConcurrentModificationException}.
 37  * Thus, in the face of concurrent modification, the iterator fails quickly
 38  * and cleanly, rather than risking arbitrary, non-deterministic behavior at
 39  * an undetermined time in the future.
 40  *
 41  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 42  * as it is, generally speaking, impossible to make any hard guarantees in the
 43  * presence of unsynchronized concurrent modification.  Fail-fast iterators
 44  * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 45  * Therefore, it would be wrong to write a program that depended on this
 46  * exception for its correctness: <i>the fail-fast behavior of iterators
 47  * should be used only to detect bugs.</i>
 48  *
 49  * <p>This class is a member of the
 50  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 51  * Java Collections Framework</a>.
 52  *
 53  * @param <E> the type of elements maintained by this set
 54  *
 55  * @author  Josh Bloch
 56  * @author  Neal Gafter
 57  * @see     Collection
 58  * @see     Set
 59  * @see     TreeSet
 60  * @see     HashMap
 61  * @since   1.2
 62  */
 63 
 64 public class HashSet<E>
 65     extends AbstractSet<E>
 66     implements Set<E>, Cloneable, java.io.Serializable
 67 {
 68     static final long serialVersionUID = -5024744406713321676L;
 69 
 70     private transient HashMap<E,Object> map;
 71 
 72     // Dummy value to associate with an Object in the backing Map
 73     private static final Object PRESENT = new Object();
 74 
 75     /**
 76      * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
 77      * default initial capacity (16) and load factor (0.75).
 78      */
 79     public HashSet() {
 80         map = new HashMap<>();
 81     }
 82 
 83     /**
 84      * Constructs a new set containing the elements in the specified
 85      * collection.  The <tt>HashMap</tt> is created with default load factor
 86      * (0.75) and an initial capacity sufficient to contain the elements in
 87      * the specified collection.
 88      *
 89      * @param c the collection whose elements are to be placed into this set
 90      * @throws NullPointerException if the specified collection is null
 91      */
 92     public HashSet(Collection<? extends E> c) {
 93         map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
 94         addAll(c);
 95     }
 96 
 97     /**
 98      * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
 99      * the specified initial capacity and the specified load factor.
100      *
101      * @param      initialCapacity   the initial capacity of the hash map
102      * @param      loadFactor        the load factor of the hash map
103      * @throws     IllegalArgumentException if the initial capacity is less
104      *             than zero, or if the load factor is nonpositive
105      */
106     public HashSet(int initialCapacity, float loadFactor) {
107         map = new HashMap<>(initialCapacity, loadFactor);
108     }
109 
110     /**
111      * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
112      * the specified initial capacity and default load factor (0.75).
113      *
114      * @param      initialCapacity   the initial capacity of the hash table
115      * @throws     IllegalArgumentException if the initial capacity is less
116      *             than zero
117      */
118     public HashSet(int initialCapacity) {
119         map = new HashMap<>(initialCapacity);
120     }
121 
122     /**
123      * Constructs a new, empty linked hash set.  (This package private
124      * constructor is only used by LinkedHashSet.) The backing
125      * HashMap instance is a LinkedHashMap with the specified initial
126      * capacity and the specified load factor.
127      *
128      * @param      initialCapacity   the initial capacity of the hash map
129      * @param      loadFactor        the load factor of the hash map
130      * @param      dummy             ignored (distinguishes this
131      *             constructor from other int, float constructor.)
132      * @throws     IllegalArgumentException if the initial capacity is less
133      *             than zero, or if the load factor is nonpositive
134      */
135     HashSet(int initialCapacity, float loadFactor, boolean dummy) {
136         map = new LinkedHashMap<>(initialCapacity, loadFactor);
137     }
138 
139     /**
140      * Returns an iterator over the elements in this set.  The elements
141      * are returned in no particular order.
142      *
143      * @return an Iterator over the elements in this set
144      * @see ConcurrentModificationException
145      */
146     public Iterator<E> iterator() {
147         return map.keySet().iterator();
148     }
149 
150     /**
151      * Returns the number of elements in this set (its cardinality).
152      *
153      * @return the number of elements in this set (its cardinality)
154      */
155     public int size() {
156         return map.size();
157     }
158 
159     /**
160      * Returns <tt>true</tt> if this set contains no elements.
161      *
162      * @return <tt>true</tt> if this set contains no elements
163      */
164     public boolean isEmpty() {
165         return map.isEmpty();
166     }
167 
168     /**
169      * Returns <tt>true</tt> if this set contains the specified element.
170      * More formally, returns <tt>true</tt> if and only if this set
171      * contains an element <tt>e</tt> such that
172      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
173      *
174      * @param o element whose presence in this set is to be tested
175      * @return <tt>true</tt> if this set contains the specified element
176      */
177     public boolean contains(Object o) {
178         return map.containsKey(o);
179     }
180 
181     /**
182      * Adds the specified element to this set if it is not already present.
183      * More formally, adds the specified element <tt>e</tt> to this set if
184      * this set contains no element <tt>e2</tt> such that
185      * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
186      * If this set already contains the element, the call leaves the set
187      * unchanged and returns <tt>false</tt>.
188      *
189      * @param e element to be added to this set
190      * @return <tt>true</tt> if this set did not already contain the specified
191      * element
192      */
193     public boolean add(E e) {
194         return map.put(e, PRESENT)==null;
195     }
196 
197     /**
198      * Removes the specified element from this set if it is present.
199      * More formally, removes an element <tt>e</tt> such that
200      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,
201      * if this set contains such an element.  Returns <tt>true</tt> if
202      * this set contained the element (or equivalently, if this set
203      * changed as a result of the call).  (This set will not contain the
204      * element once the call returns.)
205      *
206      * @param o object to be removed from this set, if present
207      * @return <tt>true</tt> if the set contained the specified element
208      */
209     public boolean remove(Object o) {
210         return map.remove(o)==PRESENT;
211     }
212 
213     /**
214      * Removes all of the elements from this set.
215      * The set will be empty after this call returns.
216      */
217     public void clear() {
218         map.clear();
219     }
220 
221     /**
222      * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
223      * themselves are not cloned.
224      *
225      * @return a shallow copy of this set
226      */
227     @SuppressWarnings("unchecked")
228     public Object clone() {
229         try {
230             HashSet<E> newSet = (HashSet<E>) super.clone();
231             newSet.map = (HashMap<E, Object>) map.clone();
232             return newSet;
233         } catch (CloneNotSupportedException e) {
234             throw new InternalError(e);
235         }
236     }
237 
238     /**
239      * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
240      * serialize it).
241      *
242      * @serialData The capacity of the backing <tt>HashMap</tt> instance
243      *             (int), and its load factor (float) are emitted, followed by
244      *             the size of the set (the number of elements it contains)
245      *             (int), followed by all of its elements (each an Object) in
246      *             no particular order.
247      */
248     private void writeObject(java.io.ObjectOutputStream s)
249         throws java.io.IOException {
250         // Write out any hidden serialization magic
251         s.defaultWriteObject();
252 
253         // Write out HashMap capacity and load factor
254         s.writeInt(map.capacity());
255         s.writeFloat(map.loadFactor());
256 
257         // Write out size
258         s.writeInt(map.size());
259 
260         // Write out all elements in the proper order.
261         for (E e : map.keySet())
262             s.writeObject(e);
263     }
264 
265     /**
266      * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
267      * deserialize it).
268      */
269     private void readObject(java.io.ObjectInputStream s)
270         throws java.io.IOException, ClassNotFoundException {
271         // Read in any hidden serialization magic
272         s.defaultReadObject();
273 
274         // Read capacity and verify non-negative.
275         int capacity = s.readInt();
276         if (capacity < 0) {
277             throw new InvalidObjectException("Illegal capacity: " +
278                                              capacity);
279         }
280 
281         // Read load factor and verify positive and non NaN.
282         float loadFactor = s.readFloat();
283         if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
284             throw new InvalidObjectException("Illegal load factor: " +
285                                              loadFactor);
286         }
287 
288         // Read size and verify non-negative.
289         int size = s.readInt();
290         if (size < 0) {
291             throw new InvalidObjectException("Illegal size: " +
292                                              size);
293         }
294 
295         // Set the capacity according to the size and load factor ensuring that
296         // the HashMap is at least 25% full but clamping to maximum capacity.
297         capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
298                 HashMap.MAXIMUM_CAPACITY);
299 
300         // Create backing HashMap
301         map = (((HashSet<?>)this) instanceof LinkedHashSet ?
302                new LinkedHashMap<E,Object>(capacity, loadFactor) :
303                new HashMap<E,Object>(capacity, loadFactor));
304 
305         // Read in all elements in the proper order.
306         for (int i=0; i<size; i++) {
307             @SuppressWarnings("unchecked")
308                 E e = (E) s.readObject();
309             map.put(e, PRESENT);
310         }
311     }
312 
313     /**
314      * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
315      * and <em>fail-fast</em> {@link Spliterator} over the elements in this
316      * set.
317      *
318      * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
319      * {@link Spliterator#DISTINCT}.  Overriding implementations should document
320      * the reporting of additional characteristic values.
321      *
322      * @return a {@code Spliterator} over the elements in this set
323      * @since 1.8
324      */
325     public Spliterator<E> spliterator() {
326         return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
327     }
328 }

View Code

三、LinkedHashSet

1、LinekdHashSet简介

LinkedHashSet继承自HashSet，源码更少、更简单，唯一的区别是LinkedHashSet内部使用的是LinkHashMap。这样做的意义或者好处就是LinkedHashSet中的元素顺序是可以保证的，也就是说遍历序和插入序是一致的。

2、Demo使用

打印日志如上，HashSet和HashMap都不保证顺序，Link**能保证顺序。

源码如下：

 1 public class MainSet {
 2     public static void main(String[] args) {
 3         Object value = new Object();
 4         HashMap<String, Object> hashMap = new HashMap<>();
 5         HashSet<String> hashSet = new HashSet<>();
 6         LinkedHashMap<String, Object> linkedHashMap = new LinkedHashMap<>();
 7         LinkedHashSet<String> linkedHashSet = new LinkedHashSet<>();
 8         hashSet.add("java");
 9         hashMap.put("java", value);
10         linkedHashSet.add("java");
11         linkedHashMap.put("java", value);
12 
13         hashSet.add("golang");
14         hashMap.put("golang", value);
15         linkedHashSet.add("golang");
16         linkedHashMap.put("golang", value);
17 
18         hashSet.add("python");
19         hashMap.put("python", value);
20         linkedHashSet.add("python");
21         linkedHashMap.put("python", value);
22 
23         hashSet.add("ruby");
24         hashMap.put("ruby", value);
25         linkedHashSet.add("ruby");
26         linkedHashMap.put("ruby", value);
27 
28         hashSet.add("scala");
29         hashMap.put("scala", value);
30         linkedHashSet.add("scala");
31         linkedHashMap.put("scala", value);
32 
33         hashSet.add("c");
34         hashMap.put("c", value);
35         linkedHashSet.add("c");
36         linkedHashMap.put("c", value);
37 
38         System.out.println("默认插入序：\\njava\\tgolang\\tpython\\truby\\tscala\\tc");
39 
40         System.out.println(" \\nHashSet:-------------------");
41         for (String str : hashSet) {
42             System.out.print(str + "\\t");
43         }
44 
45         System.out.println(" \\n\\nHashMap:-------------------");
46         for (Map.Entry<String, Object> entry : hashMap.entrySet()) {
47             System.out.print(entry.getKey() + "\\t");
48         }
49 
50         System.out.println(" \\n\\nLinkedHashSet:-------------------");
51         for (String str : linkedHashSet) {
52             System.out.print(str + "\\t");
53         }
54 
55         System.out.println(" \\n\\nLinkedHashMap:-------------------");
56         for (Map.Entry<String, Object> entry : linkedHashMap.entrySet()) {
57             System.out.print(entry.getKey() + "\\t");
58         }
59     }
60 }

View Code

3、debug中奇异现象

如上，我们明明添加了6个元素，但是table中只有5个，怎么回事呢？初步猜测应该是“c”元素和其中某一个元素位于同一个bucket，验证如下：

我们发现，“java”竟然和“c”位于同一个bucket，他俩在同一个链表中。唯一的疑惑是：“c”是后加入的元素，按理说应该在链表的表头才对啊，这