【Java集合系列四】HashSet和LinkedHashSet解析

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：

 package java.util;

 import java.io.InvalidObjectException;

 /**
  * This class implements the <tt>Set</tt> interface, backed by a hash table
  * (actually a <tt>HashMap</tt> instance).  It makes no guarantees as to the
  * iteration order of the set; in particular, it does not guarantee that the
  * order will remain constant over time.  This class permits the <tt>null</tt>
  * element.
  *
  * <p>This class offers constant time performance for the basic operations
  * (<tt>add</tt>, <tt>remove</tt>, <tt>contains</tt> and <tt>size</tt>),
  * assuming the hash function disperses the elements properly among the
  * buckets.  Iterating over this set requires time proportional to the sum of
  * the <tt>HashSet</tt> instance's size (the number of elements) plus the
  * "capacity" of the backing <tt>HashMap</tt> instance (the number of
  * buckets).  Thus, it's very important not to set the initial capacity too
  * high (or the load factor too low) if iteration performance is important.
  *
  * <p><strong>Note that this implementation is not synchronized.</strong>
  * If multiple threads access a hash set concurrently, and at least one of
  * the threads modifies the set, it <i>must</i> be synchronized externally.
  * This is typically accomplished by synchronizing on some object that
  * naturally encapsulates the set.
  *
  * If no such object exists, the set should be "wrapped" using the
  * {@link Collections#synchronizedSet Collections.synchronizedSet}
  * method.  This is best done at creation time, to prevent accidental
  * unsynchronized access to the set:<pre>
  *   Set s = Collections.synchronizedSet(new HashSet(...));</pre>
  *
  * <p>The iterators returned by this class's <tt>iterator</tt> method are
  * <i>fail-fast</i>: if the set is modified at any time after the iterator is
  * created, in any way except through the iterator's own <tt>remove</tt>
  * method, the Iterator throws a {@link ConcurrentModificationException}.
  * Thus, in the face of concurrent modification, the iterator fails quickly
  * and cleanly, rather than risking arbitrary, non-deterministic behavior at
  * an undetermined time in the future.
  *
  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
  * as it is, generally speaking, impossible to make any hard guarantees in the
  * presence of unsynchronized concurrent modification.  Fail-fast iterators
  * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
  * Therefore, it would be wrong to write a program that depended on this
  * exception for its correctness: <i>the fail-fast behavior of iterators
  * should be used only to detect bugs.</i>
  *
  * <p>This class is a member of the
  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  * Java Collections Framework</a>.
  *
  * @param <E> the type of elements maintained by this set
  *
  * @author  Josh Bloch
  * @author  Neal Gafter
  * @see     Collection
  * @see     Set
  * @see     TreeSet
  * @see     HashMap
  * @since   1.2
  */

 public class HashSet<E>
     extends AbstractSet<E>
     implements Set<E>, Cloneable, java.io.Serializable
 {
     static final long serialVersionUID = -5024744406713321676L;

     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<>();
     }

     /**
      * Constructs a new set containing the elements in the specified
      * collection.  The <tt>HashMap</tt> is created with default load factor
      * (0.75) and an initial capacity sufficient to contain the elements in
      * the specified collection.
      *
      * @param c the collection whose elements are to be placed into this set
      * @throws NullPointerException if the specified collection is null
      */
     public HashSet(Collection<? extends E> c) {
         map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
         addAll(c);
     }

     /**
      * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
      * the specified initial capacity and the specified load factor.
      *
      * @param      initialCapacity   the initial capacity of the hash map
      * @param      loadFactor        the load factor of the hash map
      * @throws     IllegalArgumentException if the initial capacity is less
      *             than zero, or if the load factor is nonpositive
      */
     public HashSet(int initialCapacity, float loadFactor) {
         map = new HashMap<>(initialCapacity, loadFactor);
     }

     /**
      * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
      * the specified initial capacity and default load factor (0.75).
      *
      * @param      initialCapacity   the initial capacity of the hash table
      * @throws     IllegalArgumentException if the initial capacity is less
      *             than zero
      */
     public HashSet(int initialCapacity) {
         map = new HashMap<>(initialCapacity);
     }

     /**
      * Constructs a new, empty linked hash set.  (This package private
      * constructor is only used by LinkedHashSet.) The backing
      * HashMap instance is a LinkedHashMap with the specified initial
      * capacity and the specified load factor.
      *
      * @param      initialCapacity   the initial capacity of the hash map
      * @param      loadFactor        the load factor of the hash map
      * @param      dummy             ignored (distinguishes this
      *             constructor from other int, float constructor.)
      * @throws     IllegalArgumentException if the initial capacity is less
      *             than zero, or if the load factor is nonpositive
      */
     HashSet(int initialCapacity, float loadFactor, boolean dummy) {
         map = new LinkedHashMap<>(initialCapacity, loadFactor);
     }

     /**
      * 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();
     }

     /**
      * Returns the number of elements in this set (its cardinality).
      *
      * @return the number of elements in this set (its cardinality)
      */
     public int size() {
         return map.size();
     }

     /**
      * Returns <tt>true</tt> if this set contains no elements.
      *
      * @return <tt>true</tt> if this set contains no elements
      */
     public boolean isEmpty() {
         return map.isEmpty();
     }

     /**
      * Returns <tt>true</tt> if this set contains the specified element.
      * More formally, returns <tt>true</tt> if and only if this set
      * contains an element <tt>e</tt> such that
      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
      *
      * @param o element whose presence in this set is to be tested
      * @return <tt>true</tt> if this set contains the specified element
      */
     public boolean contains(Object o) {
         return map.containsKey(o);
     }

     /**
      * 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&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;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;
     }

     /**
      * Removes the specified element from this set if it is present.
      * More formally, removes an element <tt>e</tt> such that
      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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;
     }

     /**
      * Removes all of the elements from this set.
      * The set will be empty after this call returns.
      */
     public void clear() {
         map.clear();
     }

     /**
      * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
      * themselves are not cloned.
      *
      * @return a shallow copy of this set
      */
     @SuppressWarnings("unchecked")
     public Object clone() {
         try {
             HashSet<E> newSet = (HashSet<E>) super.clone();
             newSet.map = (HashMap<E, Object>) map.clone();
             return newSet;
         } catch (CloneNotSupportedException e) {
             throw new InternalError(e);
         }
     }

     /**
      * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
      * serialize it).
      *
      * @serialData The capacity of the backing <tt>HashMap</tt> instance
      *             (int), and its load factor (float) are emitted, followed by
      *             the size of the set (the number of elements it contains)
      *             (int), followed by all of its elements (each an Object) in
      *             no particular order.
      */
     private void writeObject(java.io.ObjectOutputStream s)
         throws java.io.IOException {
         // Write out any hidden serialization magic
         s.defaultWriteObject();

         // Write out HashMap capacity and load factor
         s.writeInt(map.capacity());
         s.writeFloat(map.loadFactor());

         // Write out size
         s.writeInt(map.size());

         // Write out all elements in the proper order.
         for (E e : map.keySet())
             s.writeObject(e);
     }

     /**
      * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
      * deserialize it).
      */
     private void readObject(java.io.ObjectInputStream s)
         throws java.io.IOException, ClassNotFoundException {
         // Read in any hidden serialization magic
         s.defaultReadObject();

         // Read capacity and verify non-negative.
         int capacity = s.readInt();
         if (capacity < 0) {
             throw new InvalidObjectException("Illegal capacity: " +
                                              capacity);
         }

         // Read load factor and verify positive and non NaN.
         float loadFactor = s.readFloat();
         if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
             throw new InvalidObjectException("Illegal load factor: " +
                                              loadFactor);
         }

         // Read size and verify non-negative.
         int size = s.readInt();
         if (size < 0) {
             throw new InvalidObjectException("Illegal size: " +
                                              size);
         }

         // Set the capacity according to the size and load factor ensuring that
         // the HashMap is at least 25% full but clamping to maximum capacity.
         capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
                 HashMap.MAXIMUM_CAPACITY);

         // Create backing HashMap
         map = (((HashSet<?>)this) instanceof LinkedHashSet ?
                new LinkedHashMap<E,Object>(capacity, loadFactor) :
                new HashMap<E,Object>(capacity, loadFactor));

         // Read in all elements in the proper order.
         for (int i=0; i<size; i++) {
             @SuppressWarnings("unchecked")
                 E e = (E) s.readObject();
             map.put(e, PRESENT);
         }
     }

     /**
      * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
      * and <em>fail-fast</em> {@link Spliterator} over the elements in this
      * set.
      *
      * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
      * {@link Spliterator#DISTINCT}.  Overriding implementations should document
      * the reporting of additional characteristic values.
      *
      * @return a {@code Spliterator} over the elements in this set
      * @since 1.8
      */
     public Spliterator<E> spliterator() {
         return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
     }
 }

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：

 public class MainSet {
     public static void main(String[] args) {
         Object value = new Object();
         HashMap<String, Object> hashMap = new HashMap<>();
         HashSet<String> hashSet = new HashSet<>();
         LinkedHashMap<String, Object> linkedHashMap = new LinkedHashMap<>();
         LinkedHashSet<String> linkedHashSet = new LinkedHashSet<>();
         hashSet.add("java");
         hashMap.put("java", value);
         linkedHashSet.add("java");
         linkedHashMap.put("java", value);

         hashSet.add("golang");
         hashMap.put("golang", value);
         linkedHashSet.add("golang");
         linkedHashMap.put("golang", value);

         hashSet.add("python");
         hashMap.put("python", value);
         linkedHashSet.add("python");
         linkedHashMap.put("python", value);

         hashSet.add("ruby");
         hashMap.put("ruby", value);
         linkedHashSet.add("ruby");
         linkedHashMap.put("ruby", value);

         hashSet.add("scala");
         hashMap.put("scala", value);
         linkedHashSet.add("scala");
         linkedHashMap.put("scala", value);

         hashSet.add("c");
         hashMap.put("c", value);
         linkedHashSet.add("c");
         linkedHashMap.put("c", value);

         System.out.println("默认插入序：\njava\tgolang\tpython\truby\tscala\tc");

         System.out.println(" \nHashSet:-------------------");
         for (String str : hashSet) {
             System.out.print(str + "\t");
         }

         System.out.println(" \n\nHashMap:-------------------");
         for (Map.Entry<String, Object> entry : hashMap.entrySet()) {
             System.out.print(entry.getKey() + "\t");
         }

         System.out.println(" \n\nLinkedHashSet:-------------------");
         for (String str : linkedHashSet) {
             System.out.print(str + "\t");
         }

         System.out.println(" \n\nLinkedHashMap:-------------------");
         for (Map.Entry<String, Object> entry : linkedHashMap.entrySet()) {
             System.out.print(entry.getKey() + "\t");
         }
     }
 }

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，这个问题还需要探究。

同时这里也介绍了一种能debug到HashMap内部数据结构的方法，但是需要注意2个问题：

1、需要在AS中设置一下，否则debug看到的信息不是这样的，如下图：

2、直接使用table[3].next是不行的， 需要像图上那样，些完整的包名才行。

4、继续debug

如上所示，设置HashSet之后，同样能看到如下信息：

看看LinkedHashMap，如下图，LinkedHashMap中多了head和tail，这是指向表头、表尾的指针，head指向“java”，tail指向“c”，这和我们的插入序保持一致，但是实际存储和之前是一样的。

LinkedHashSet如下图：