HashSet源码分析（基于1.8）

其实对于HashSet，他的源码很简单，只是里面有些问题我感觉有必要拿出来跟大家共同探讨一下。

HashSet底层是HashMap实现

我们知道，HashMap中key值是不能“重复”的（这个是否重复是通过hashcode和equals比较出来的，这是一个值得探讨的问题），HashSet正是借鉴了HashMap的key的这样一个特性，以此产生了这样一个不能包含重复数据的集合。

源码分析

由于源码不是很多，所以我连同注释一同贴出来了：

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.
* 这个类由hash表支持，并实现了Set接口（实际上他是一个HashMap实例）。他不保证set中的遍历顺序，尤其
* 特别的是，他也不保证set中数据顺序的恒久不变。这个类允许null的存在。
* <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.
* 上面这段话跟HashMap中的是一样的
* <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;//底层是HashMap实现的

// Dummy value to associate with an Object in the backing Map
//PRESENT是一个假的value值，帮助用HashMap实现HashSet。
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).
*/
//注意HashMap的默认初始容量和负载因子
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 ? 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
*/
@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.
*/
//同样支持序列化传输
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);
}
}

HashSet中值得注意的方法

既然HashSet中不能包含重复的数据，那么我们就着重探讨一下add（）这个方法。

从上面的源码我们可以看出，add（）这个方法实际上是调用map中的put方法，而put（）又调用putVal这个方法，因此下面我贴出HashMap中putVal（）这个方法来着重分析一下。

//进行插入操作，分为三种情况，1.插入位置无数据，直接存入 2.插入位置有数据，但是较少且符合链表结构存储的条件，那么以链表操作存入
//3.插入位置有数据，但是以树结构进行存储，那么以树的相关操作进行存入

//下面这段代码我们应当注意的是，hash的值是用key计算出来的。
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0) //1.如果HashMap未被初始化，那么初始化它
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)//如果索引处的数据是null，那么单独处理这种情况
tab[i] = newNode(hash, key, value, null);
//下面处理不是null的情况
else {
Node<K,V> e; K k;
//下面的p在上面已经赋值过， p = tab[i = (n - 1)&hash]代表的是key索引位置处原来存在的数据
//另外，p的hash成员是通过key值计算出来的
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))

//我们这里要特别注意，上面条件判断的是：p.hash == hash &&p.key==key||(key != null && key.equals(k)),又“==”是比较地址的，因此只要key地址相同或者key的equals方法返回为true，就会得到e=p
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//下面处理key映射已经存在的情况
if (e != null) { // existing mapping for key  如果映射关系存在
V oldValue = e.value;//e的value给oldVaule
//在HashSet中，key已存在的话，e.value就是PRESENT，那么当已经存在key时，如果再次添加相同的key，下面if内的语句会执行，因此也就会执行添加动作。
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}

现在，HashMap的put动作（也就是HashSet的add动作）就分析完了，可能现在很多人还处于模糊状态。下面我再结合一些实际情况说一下。

1.对key对象的要求？

答：对于key对象，我们一般需要重写key对象对应类的hashcode（）和equals（）方法。因为如果2个对象通过equals（）方法返回为true，那么java要求此时2个对象的hashcode（）方法返回的hash值要相同。

2.如果添加了一个key和value，而在集合中之前已经有了该key，具体执行动作是怎样的？

答：如果key已经存在，那么就会执行到上面putVal（）源码中“e=p；”这一句，然后转向“下面处理映射已经存在的情况”，然后就会覆盖之前的value。

3.如果通过equals（）方法比较返回true，hashcode（）方法返回的结果却不相等，会产生什么后果？

答：通过上面的源码我们可以看到，对于一个key，程序会先通过该key计算出其hash值，下面给出源码中hash值的计算代码：

static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

可见，该值与hashcode（）方法有关，一般正常来说，通过equals（）方法比较为true的2个对象，我们是默认为他们是相同的对象，如果他们hashcode（）方法返回的值不同，他们通过hash（）方法返回的值也就不同，因此当A1已经存入HashMap时，与A1相等的A2再次执行put动作时，不会根据相同的hash值查找到之前与他相同的A1，而是会查找到另外一个位置，如果恰好该位置为null，那么就会成功执行插入动作，这时我们就会看到，2个相同的key均插入进同一个HashMap对象中了。