HashSet源码解析

Set接口

在讲HashSet之前先说一下Set接口。

见源码：

package java.util;

//这里我截取部分源码中的注释
/**
* A collection that contains no duplicate elements.  More formally, sets
* contain no pair of elements <code>e1</code> and <code>e2</code> such that
* <code>e1.equals(e2)</code>, and at most one null element.  As implied by
* its name, this interface models the mathematical <i>set</i> abstraction.
*/
//set是一个不包含重复元素的集合。更正式一点来说，set不能包含这样的一对元素：通过equals方法比较返回true。并且至多有一个null元素。
/**
* @author  Josh Bloch(集合框架的作者，牛人)
* @author  Neal Gafter
* @see Collection
* @see List
* @see SortedSet
* @see HashSet
* @see TreeSet
* @see AbstractSet
* @see Collections#singleton(java.lang.Object)
* @see Collections#EMPTY_SET
* @since 1.2
*/

public interface Set<E> extends Collection<E> {
// Query Operations

int size();

boolean isEmpty();

boolean contains(Object o);

Iterator<E> iterator();

Object[] toArray();

<T> T[] toArray(T[] a);

// Modification Operations

boolean add(E e);

boolean remove(Object o);

// Bulk Operations

boolean containsAll(Collection<?> c);

boolean addAll(Collection<? extends E> c);

boolean retainAll(Collection<?> c);

boolean removeAll(Collection<?> c);

void clear();

// Comparison and hashing

boolean equals(Object o);//这个方法在Set中要特别注意！

int hashCode();//这个方法在Set中要特别注意！

//1.8之后的新特性
@Override
default Spliterator<E> spliterator() {
return Spliterators.spliterator(this, Spliterator.DISTINCT);
}
}

因为在AbstractSet中已经实现了大部分方法，因此，在HashSet中我们会感觉代码少很多。下面是HashSet的源码。

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;

private static final Object PRESENT = new Object();

public HashSet() {
map = new HashMap<>();
}

public HashSet(Collection<? extends E> c) {
map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
addAll(c);
}

public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}

public HashSet(int initialCapacity) {
map = new HashMap<>(initialCapacity);
}

HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}

public Iterator<E> iterator() {
return map.keySet().iterator();
}

public int size() {
return map.size();
}

public boolean isEmpty() {
return map.isEmpty();
}

public boolean contains(Object o) {
return map.containsKey(o);
}

public boolean add(E e) {
return map.put(e, PRESENT)==null;
}

public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}

public void clear() {
map.clear();
}

@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);
}
}

private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// Write out any hidden serialization magic
s.defaultWriteObject();

s.writeInt(map.capacity());
s.writeFloat(map.loadFactor());

s.writeInt(map.size());

for (E e : map.keySet())
s.writeObject(e);
}

private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {

s.defaultReadObject();

int capacity = s.readInt();
if (capacity < 0) {
throw new InvalidObjectException("Illegal capacity: " +
capacity);
}

float loadFactor = s.readFloat();
if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
throw new InvalidObjectException("Illegal load factor: " +
loadFactor);
}

int size = s.readInt();
if (size < 0) {
throw new InvalidObjectException("Illegal size: " +
size);
}

capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
HashMap.MAXIMUM_CAPACITY);

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

for (int i=0; i<size; i++) {
@SuppressWarnings("unchecked")
E e = (E) s.readObject();
map.put(e, PRESENT);
}
}

public Spliterator<E> spliterator() {
return new HashMap.KeySpliterator<E,Object>(map, 0, -1, 0, 0);
}
}

我们可以看到，HashSet是在HashMap的基础上，利用HashMap的key不能“相同”的特性实现的。其对应的HashMap的value值被“PRESENT”对象填充。

由于HashSet的特性是内部的元素不同，因此我们在这里再讲一下HashMap中key的特性：

这里我们先来看一下HashMap中的添加方法：

/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
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)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))//这里是保持Set特性关键，因此equals（）方法的实现是很重要的。
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;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}

从上面我们可以看到，Set保持“无重复”的特性，需要依赖equals（）方法比较的结果。

这里我来简要说一下HashMap中寻找value值的流程，首先根据传进来的key值，计算出其hash散列值，然后通过hash值找到位置，在存储位置上再进行equals（）的比较最后得到想寻找的值。从这个过程我们可以看到，hashcode（）方法和equals（）方法之间的相关联系，我们必须维持。

在JAVA编程中，有三个约定，这三个约定是使用集合框架的基础。