HashSet源码解读
一：先看其实现了哪些接口和继承了哪些类
1.实现了Serializable接口，表明它支持序列化。
2.实现了Cloneable接口，表明它支持克隆，可以调用超类的clone（）方法进行浅拷贝。
3.继承了AbstractSet抽象类，和ArrayList和LinkedList一样，在他们的抽象父类中，都提供了equals（）方法和hashCode（）方法。它们自身并不实现这两个方法
4.从JDK源码可以看出，底层并没有使用我们常规认为的利用hashcode（）方法求的值进行比较，而是通过调用AbstractCollection的containsAll（）方法，如果他们中元素完全相同（与顺序无关），则他们的equals（）方法的比较结果就为true。
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/**
* Compares the specified object with this set for equality. Returns
* <tt>true</tt> if the given object is also a set, the two sets have
* the same size, and every member of the given set is contained in
* this set. This ensures that the <tt>equals</tt> method works
* properly across different implementations of the <tt>Set</tt>
* interface.<p>
*
* This implementation first checks if the specified object is this
* set; if so it returns <tt>true</tt>. Then, it checks if the
* specified object is a set whose size is identical to the size of
* this set; if not, it returns false. If so, it returns
* <tt>containsAll((Collection) o)</tt>.
*
* @param o object to be compared for equality with this set
* @return <tt>true</tt> if the specified object is equal to this set
*/
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof Set))
return false;
Collection<?> c = (Collection<?>) o;//保证个数相等
if (c.size() != size())
return false;
try {//调用了AbstractCollection的方法。
return containsAll(c);
} catch (ClassCastException unused) {
return false;
} catch (NullPointerException unused) {
return false;
}
}复制代码
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public boolean containsAll(Collection<?> c) {
//只需要逐个判断集合是否包含其中的元素。
for (Object e : c)
if (!contains(e))
return false;
return true;
}复制代码
4.实现了Set接口。
二：HashSet概述
1.HashSet是通过HashMap的键来存值，HashMap里面的所有值都为null;
2.学习这个之前先看HashMap;附上网址https://www.cnblogs.com/xhlwjy/p/11246618.html
三：HashSet的属性
private transient HashMap map;//用HashMap的Key来存值
// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();//用于填充HashMap的value
四：构造方法
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/**
* Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
* default initial capacity (16) and load factor (0.75).
*///这个是初始化一个HashMap
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
*///初始化HashMap,把c集合中的所有元素添加到map中
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
*///初始化HashMap的容量和加载因子
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
*/
//初始化HashMap的容量
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
*///这个初始化使用的是LinkedHashMap
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}复制代码
五：添加新元素
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//底层仍然利用了HashMap键进行了元素的添加。
//在HashMap的put()方法中，该方法的返回值是对应HashMap中键值对中的值，而值总是PRESENT,
//该PRESENT一直都是private static final Object PRESENT = new Object();
//PRESENT只是初始化了，并不能改变，所以PRESENT的值一直为null。
//所以只要插入成功了，put（）方法返回的值总是null。
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}复制代码
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}这段代码就不分析了，与HashMap的插入差不多，可以先去看HashMap
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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))))
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;
}复制代码
六：删除一个元素
//该方法底层实现了仍然使用了map的remove（）方法。
//map的remove（）方法的返回的是被删除键对应的值。（在HashSet的底层HashMap中的所有键值对的值都是PRESENT）
public boolean remove(Object o) {
return map.remove(o)==PRESENT;
}之后的代码就不粘贴出来了，自己可以去源码里面看
七：清空方法
public void clear() {
map.clear();
}八：克隆方法
底层仍然使用了Object的clone（）方法，得到的Object对象，并把它强制转化为HashSet,然后把它的底层的HashMap也克隆一份（调用的HashMap的clone（）方法），并把它赋值给newSet，最后返回newSet即可。
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@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);
}
}复制代码
九：是否包含某个元素
底层使用了HashMap的containsKey（）
public boolean contains(Object o) {
return map.containsKey(o);
}
十：判读是不是空
调用HashMap的方法
public boolean isEmpty() {
return map.isEmpty();
}
十一：统计HashSet中包含元素的个数
public int size() {
return map.size();
}
十二：生成迭代器
public Iterator<E> iterator() {
return map.keySet().iterator();
}
十三：序列化
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/**
* 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//容量写入流
s.writeInt(map.capacity());//加载因子写入流
s.writeFloat(map.loadFactor());
// Write out size//存放的数量写入流
s.writeInt(map.size());
// Write out all elements in the proper order.//把每个元素写入流
for (E e : map.keySet())
s.writeObject(e);
}复制代码
十四：反序列化
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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);
//HashMap中构建哈希桶数组是在第一个元素被添加的时候才构建，所以在构建之前检查它,
// 调用HashMap.tableSizeFor来计算实际分配的大小,
// 检查Map.Entry []类，因为它是最接近的公共类型实际创建的内容。
SharedSecrets.getJavaOISAccess()
.checkArray(s, Map.Entry[].class,HashMap.tableSizeFor(capacity));
//创建HashMap。
map = (((HashSet<?>)this) instanceof LinkedHashSet ?
new LinkedHashMap<E,Object>(capacity, loadFactor) :
new HashMap<E,Object>(capacity, loadFactor));
// 按写入流中的顺序再把元素依次读取出来放到map中。
for (int i=0; i<size; i++) {
@SuppressWarnings("unchecked")
E e = (E) s.readObject();
map.put(e, PRESENT);
}
}
