概述
Hash table and linked list implementation of the <tt>Set</tt> interface, with predictable iteration order.
This implementation differs from <tt>HashSet</tt> in that it maintains a doubly-linked list running through all of its entries.
This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (<i>insertion-order</i>).
Hash表+LinkedList 的Set实现，支持可预测的iterator顺序；
LinkedHashSet是双向链表；
LinkedHashSet默认的iterator顺序是 元素被insert的顺序（插入排序）；
It can be used to produce a copy of a set that has the same order as the original, regardless of the original set's implementation:
void foo(Set s) { Set copy = new LinkedHashSet(s); }
LinkedHashSet通常被用来 当做与原集合相同顺序的copy；
This class provides all of the optional <tt>Set</tt> operations, and permits null elements.
Like <tt>HashSet</tt>, it provides constant-time performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and <tt>remove</tt>), assuming the hash function disperses elements properly among the buckets.
LinkedHashSet支持所有Set操作，允许null；
LinkedHashSet的add、contains、remove 花费 O(1)时间复杂度，因为Hash将元素分散在buckets；
A linked hash set has two parameters that affect its performance: <i>initial capacity</i> and <i>load factor</i>.
LinkedHashSet有2个参数影响性能：initial capacity、load factor；
Note that this implementation is not synchronized.
If multiple threads access a linked hash set concurrently, and at least one of the threads modifies the set, it <em>must</em> be synchronized externally.
If no such object exists, the set should be "wrapped" using the {@link Collections#synchronizedSet Collections.synchronizedSet} method.
LinkedHashSet是线程非同步的；
如果多个线程并发修改，需要在外部进行同步；
可以使用Collections.synchronizedSet；
The iterators returned by this class's <tt>iterator</tt> method are <em>fail-fast</em>: 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 will throw a {@link ConcurrentModificationException}.
LinkedHashSet的iterator是fail-fast：如果在iterator时，remove（除iterator的remove），将会抛出ConcurrentModificationException；
链路
new LinkedHashSet<>(10)
// java.util.LinkedHashSet.LinkedHashSet(int)
public LinkedHashSet(int initialCapacity) {
super(initialCapacity, .75f, true);
}
// java.util.HashSet.HashSet(int, float, boolean)
/**
* 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.
* 底层使用的是LinkedHashMap
* @param dummy 忽略（与其他构造器进行区分）
*/
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}
add
// java.util.HashSet.add
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}
// java.util.HashMap.put
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
// java.util.HashMap.putVal
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
HashMap.Node<K,V>[] tab; HashMap.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); // 创建新的LinkedHashMap的node && link到列表尾部
else {
HashMap.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 HashMap.TreeNode)
e = ((HashMap.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;
}
// java.util.LinkedHashMap.newNode
HashMap.Node<K,V> newNode(int hash, K key, V value, HashMap.Node<K,V> e) {
LinkedHashMap.Entry<K,V> p = new LinkedHashMap.Entry<K,V>(hash, key, value, e);
linkNodeLast(p);
return p;
}
// java.util.LinkedHashMap.linkNodeLast
private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {
LinkedHashMap.Entry<K,V> last = tail;
tail = p;
if (last == null)
head = p;
else {
p.before = last;
last.after = p;
}
}
linkedHashSet.iterator()
Set<String> linkedHashSet = new LinkedHashSet<>(10);
linkedHashSet.add("a");
linkedHashSet.add("d");
linkedHashSet.add("c");
linkedHashSet.add("b");
Iterator<String> iterator = linkedHashSet.iterator();
while (iterator.hasNext()) { // java.util.LinkedHashMap.LinkedHashIterator.hasNext
String next = iterator.next();// java.util.LinkedHashMap.LinkedKeyIterator.next -> java.util.LinkedHashMap.LinkedHashIterator.nextNode
iterator.remove(); // java.util.LinkedHashMap.LinkedHashIterator.remove
}
// java.util.HashSet.iterator
public Iterator<E> iterator() {
return map.keySet().iterator();
}
// java.util.LinkedHashMap.keySet
public Set<K> keySet() {
Set<K> ks = keySet;
if (ks == null) {
ks = new LinkedHashMap.LinkedKeySet();
keySet = ks;
}
return ks;
}
// java.util.LinkedHashMap.LinkedKeySet
final class LinkedKeySet extends AbstractSet<K> {
}
// java.util.LinkedHashMap.LinkedKeySet.iterator
final class LinkedKeySet extends AbstractSet<K> {
public final Iterator<K> iterator() {
return new LinkedHashMap.LinkedKeyIterator();
}
}
// java.util.LinkedHashMap.LinkedKeyIterator
final class LinkedKeyIterator extends LinkedHashIterator implements Iterator<K> {
public final K next() { return nextNode().getKey(); }
}
// java.util.LinkedHashMap.LinkedHashIterator
abstract class LinkedHashIterator {
public final boolean hasNext() {
return next != null;
}
final LinkedHashMap.Entry<K,V> nextNode() {
LinkedHashMap.Entry<K,V> e = next;
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (e == null)
throw new NoSuchElementException();
current = e;
next = e.after;
return e;
}
public final void remove() {
HashMap.Node<K,V> p = current;
if (p == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
current = null;
K key = p.key;
removeNode(hash(key), key, null, false, false);
expectedModCount = modCount;
}
}