HashSet的实现原理

1.HashSet概述

HashSet实现Set接口，由哈希表支持。它不保证set的迭代顺序：特别是它不保证该顺序恒久不变。此类允许用null元素。HashSet中不允许有重复元素，这是因为HashSet是根据HashMap实现的，HashSet中的元素都存放在HashMap的key上面，而value中的值都是统一的一个private static final Object PRESENT = new Object();HashSet跟HashMap一样，都是一个存放链表的数组。

HashSet中的add(E e)方法调用的是底层HashMap中的put(K key, V value)方法，而如果是在HashMap中调用put，首先会判断key是否存在，如果key存在则修改value值，如果key不存在则插入这个key-value。而在set中，因为value值没有用，也就不存在修改value值的说法，因此往HashSet中添加元素，首先判断元素（也就是key）是否存在，如果不存在则插入，如果存在则不插入，这样HashSet中就不存在重复值。

2.HashSet的实现

对于HashSet而言，它是基于HashMap实现的，HashSet底层使用HashMap来保存所有元素，更确切的来说，HashSet中的元素，只是存放在底层的HashMap的key上，而value使用一个static final的Object对象标识。因此HashSet的实现比较简单，相关HashSet的操作，基本上都是直接调用底层HashMap的相关方法来完成。注意对于HashSet中保存的对象，请注意正确重写其equals和hashCode方法，以保证放入的对象的唯一性。

3.HashSet源码

/*
* Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;

/**
* 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() {

4000
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 ? e==null : 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 ? 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;
}

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

/**
* 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 in HashMap capacity and load factor and create backing HashMap
int capacity = s.readInt();
float loadFactor = s.readFloat();
map = (((HashSet)this) instanceof LinkedHashSet ?
new LinkedHashMap<E,Object>(capacity, loadFactor) :
new HashMap<E,Object>(capacity, loadFactor));

// Read in size
int size = s.readInt();

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