package java.util;
import java.util.function.Consumer;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.io.IOException;
import HashMap.Node;
/**
* 哈希表和链表实现Map接口，并具有固定迭代顺序。
* LinkedHashMap实现和HashMap的不同之处在于LinkedHashMap维护了一个双向链表来存储所有的Entry。
* 这个链表定义了迭代顺序，即关键字插入Map的顺序。重复插入关键字不会影响顺序。
* Hashtable和HashMap提供无序的键值对。
* 有一个特殊的构造函数LinkedHashMap(int,float,boolean) constructor用于创建一个链式Hash Map，它的迭代顺序是Entry上次访问顺序，
* 从最近访问最少到最近访问最多。非常适合建立LRU caches。
* 集合视图上的操作不会影响Map的遍历。
* removeEldestEntry(Map.Entry)允许重写自定义规则，在新映射添加的时候自动移除一些映射。
* 该类实现了Map的所有可选操作，允许空null值。
* HashMap提供常数时间的基本操作（add, contains, remove），假设哈希函数将元素均匀的散列在各槽中。
* 由于需要维护链表，LinkedHashMap的性能略低于HashMap。
* 但有一个例外：在LinkedHashMap的集合视图中遍历的时间和Size成正比，和capacity无关，
* 而HashMap的集合视图中遍历的时间和capacity成正比。
*
* LinkedHashMap有两个参数影响其性能：初始容量initial capacity，装载因子load factor，HashMap也是如此。
*
* 该实现也是非线程安全的。多线程访问时，需要外部同步。例如，需要Collections#synchronizedMap Collections.synchronizedMap方法包装：
* Map m = Collections.synchronizedMap(new LinkedHashMap(...));
* 在以访问决定顺序的链式HashMap中只有get是结构上的操作。
* LinkedHashMap的集合视图也都是快速失败的。
*
* 底层使用哈希表与双向链表来保存所有元素，其基本操作与父类HashMap相似，它通过重写父类相关的方法来实现自己的链表特性。
* 即LinkedHashMap的结点有：final int hash; //不可变的哈希值————由关键字key得来
final K key; //关键字不可变
V value;
Node<K,V> next;
Entry<K,V> before;
Entry<K,V> after;
*/
public class LinkedHashMap<K,V>
extends HashMap<K,V>
implements Map<K,V>
{
/*
* LinkedHashMap的结点Entry也可以转换成树的形式。
* 结点类中使用（head，tail）来维护一个双向链表（before，after）。
* 回调函数的风格也不太一样。
*/
/**
* LinkedHashMap的Entry类型继承自HashMap.Node
* 该Entry除了保存当前对象的引用外，还保存了其上一个元素before和下一个元素after的引用
* 从而在哈希表的基础上又构成了双向链表
*/
static class Entry<K,V> extends HashMap.Node<K,V> {
Entry<K,V> before, after; //前一个结点和后一个结点
Entry(int hash, K key, V value, Node<K,V> next) {
super(hash, key, value, next);
}
}
private static final long serialVersionUID = 3801124242820219131L;
/**
* 双向链表的头结点（最早的结点）.
*/
transient LinkedHashMap.Entry<K,V> head;
/**
* 双向链表的尾结点（最晚的结点）.
*/
transient LinkedHashMap.Entry<K,V> tail;
/**
* LinkedHashMap的迭代顺序：true是访问顺序（即最近最少使用次序），false是插入顺序
*/
final boolean accessOrder;
// 内部工具
// 链到链表末端
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;
}
}
// apply src's links to dst
/**
* 将src的链接链到dst上————用dst替换src
*/
private void transferLinks(LinkedHashMap.Entry<K,V> src,
LinkedHashMap.Entry<K,V> dst) {
LinkedHashMap.Entry<K,V> b = dst.before = src.before;
LinkedHashMap.Entry<K,V> a = dst.after = src.after;
//分别考虑首尾情况
//src是头结点
if (b == null)
head = dst;
else
b.after = dst;
//src是尾结点
if (a == null)
tail = dst;
else
a.before = dst;
}
// 重写HashMap的方法
void reinitialize() {
super.reinitialize();
//重新初始化head，tail，均为null
head = tail = null;
}
//创建新节点并链接到尾部
Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
LinkedHashMap.Entry<K,V> p =
new LinkedHashMap.Entry<K,V>(hash, key, value, e);
//将p链到尾部
linkNodeLast(p);
return p;
}
//替换结点，用next替换p
Node<K,V> replacementNode(Node<K,V> p, Node<K,V> next) {
LinkedHashMap.Entry<K,V> q = (LinkedHashMap.Entry<K,V>)p;
LinkedHashMap.Entry<K,V> t =
new LinkedHashMap.Entry<K,V>(q.hash, q.key, q.value, next);
//用t替换q
transferLinks(q, t);
return t;
}
TreeNode<K,V> newTreeNode(int hash, K key, V value, Node<K,V> next) {
TreeNode<K,V> p = new TreeNode<K,V>(hash, key, value, next);
linkNodeLast(p);
return p;
}
TreeNode<K,V> replacementTreeNode(Node<K,V> p, Node<K,V> next) {
LinkedHashMap.Entry<K,V> q = (LinkedHashMap.Entry<K,V>)p;
TreeNode<K,V> t = new TreeNode<K,V>(q.hash, q.key, q.value, next);
transferLinks(q, t);
return t;
}
//删除结点e后调整
void afterNodeRemoval(Node<K,V> e) { // 解链
LinkedHashMap.Entry<K,V> p =
(LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
p.before = p.after = null;
//分别考虑首尾情况
if (b == null)
head = a;
else
b.after = a;
if (a == null)
tail = b;
else
a.before = b;
}
void afterNodeInsertion(boolean evict) { // 可能需要移除最老结点
LinkedHashMap.Entry<K,V> first;
//removeEldestEntry(first)始终返回false，所以这里移除结点后不会有操作，不会移除最老结点
//这样做的目的是方便开发者可以把Map当cache来用，并且可以限制大小，只需继承LinkedHashMap并重写removeEldestEntry()
if (evict && (first = head) != null && removeEldestEntry(first)) {
K key = first.key;
removeNode(hash(key), key, null, false, true);
}
}
/**
* 当accessOrder为true（按最近最少使用）时，结点访问（get，put操作）后需要调整结点顺序，
* 将当前被操作节点移动到head结点之前，即链表的尾部
*/
void afterNodeAccess(Node<K,V> e) { // move node to last
LinkedHashMap.Entry<K,V> last;
//先将结点e从原链表中解除出来，再链到head之前
if (accessOrder && (last = tail) != e) {
LinkedHashMap.Entry<K,V> p =
(LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
//需要分别考虑p结点是head和tail的情况
//断开p=e与其后继结点的关联
p.after = null;
//若p=e的前一个结点为空，则p为原头结点head，所以删除后，p的原后继结点a成为新的头结点head
if (b == null)
head = a;
//p不是头结点，则b的后继指向a结点
else
b.after = a;
//后继结点a不为空，a指向p的前驱结点b
if (a != null)
a.before = b;
//后继结点为空，则p结点为尾结点tail，所以删除后，p的前驱成为新的尾结点
else
last = b;
//若last=b为空，则p为头结点，现在
if (last == null)
head = p;
//last不为空
else {
p.before = last;
last.after = p;
}
//将p结点链到尾部
tail = p;
++modCount;
}
}
void internalWriteEntries(java.io.ObjectOutputStream s) throws IOException {
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after) {
s.writeObject(e.key);
s.writeObject(e.value);
}
}
/**
* 构造方法1：购造一个指定初始容量和负载因子的、按照插入顺序的LinkedHashMap
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public LinkedHashMap(int initialCapacity, float loadFactor) {
//调用父类HashMap的相关构造方法来构造一个底层存放的table数组，
//但额外可以增加accessOrder这个参数，若不设置，默认false，代表按插入顺序迭代
//true为按访问顺序进行迭代
//HashMap的构造器中会调用init()方法，进行相关的初始化，这个方法在HashMap的实现中并无意义，只是提供给子类实现相关的初始化调用
super(initialCapacity, loadFactor);
accessOrder = false;
}
/**
* 构造方法2：构造一个指定初始容量的LinkedHashMap，取得键值对的顺序是插入顺序，默认装载因子0.75
* @param initialCapacity the initial capacity
* @throws IllegalArgumentException if the initial capacity is negative
*/
public LinkedHashMap(int initialCapacity) {
super(initialCapacity);
accessOrder = false;
}
/**
* 构造方法3：用默认的初始化容量16和负载因子0.75创建一个LinkedHashMap，取得键值对的顺序是插入顺序
*/
public LinkedHashMap() {
super();
accessOrder = false;
}
/**
* 构造方法4：通过传入的Map创建一个LinkedHashMap，容量为默认容量（16）和(map.size()/DEFAULT_LOAD_FACTORY)+1的较大者，、
* 装载因子为默认值0.75
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public LinkedHashMap(Map<? extends K, ? extends V> m) {
super();
accessOrder = false;
putMapEntries(m, false);
}
/**
* 构造方法5：根据指定容量、装载因子和键值对保持顺序创建一个LinkedHashMap
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @param accessOrder the ordering mode - <tt>true</tt> for
* access-order, <tt>false</tt> for insertion-order
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder) {
super(initialCapacity, loadFactor);
this.accessOrder = accessOrder;
}
/**
* 若有一个以上关键字映射到该值，返回true
* 重写父类的containsValue（Object value）方法，直接通个关header遍历链表判断是否有值和value相等
* 不用查询table数组
* @param value value whose presence in this map is to be tested
* @return <tt>true</tt> if this map maps one or more keys to the
* specified value
*/
public boolean containsValue(Object value) {
//通过header遍历链表判断是否有值和value相等
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after) {
V v = e.value;
if (v == value || (value != null && value.equals(v)))
return true;
}
return false;
}
/**
* 返回指定关键字映射到的值，不存在则返回null
*/
public V get(Object key) {
Node<K,V> e;
if ((e = getNode(hash(key), key)) == null)
return null;
//如果按照访问顺序（accessOrder为true），需要调整结点顺序
if (accessOrder)
afterNodeAccess(e);
return e.value;
}
/**
* {@inheritDoc}
*/
public V getOrDefault(Object key, V defaultValue) {
Node<K,V> e;
if ((e = getNode(hash(key), key)) == null)
return defaultValue;
if (accessOrder)
afterNodeAccess(e);
return e.value;
}
/**
* 清空LinkedHashMap
*/
public void clear() {
//调用父类的方法clear()
super.clear();
//将链表的header结点的before和after引用都指向null
//这样就无法访问到原链表中剩余的其他结点，他们都将被GC回收
head = tail = null;
}
/**
* 如果Map需要移除最老结点，返回true
* 这样做的目的是方便开发者可以把Map当cache来用，并且可以限制大小，只需继承LinkedHashMap并重写removeEldestEntry()
* put和putAll方法之后需要调用该函数
* <p>Sample use: this override will allow the map to grow up to 100
* entries and then delete the eldest entry each time a new entry is
* added, maintaining a steady state of 100 entries.
* <pre>
* private static final int MAX_ENTRIES = 100;
*
* protected boolean removeEldestEntry(Map.Entry eldest) {
* return size() > MAX_ENTRIES;
* }
* </pre>
*
* <p>This method typically does not modify the map in any way,
* instead allowing the map to modify itself as directed by its
* return value. It <i>is</i> permitted for this method to modify
* the map directly, but if it does so, it <i>must</i> return
* <tt>false</tt> (indicating that the map should not attempt any
* further modification). The effects of returning <tt>true</tt>
* after modifying the map from within this method are unspecified.
*
* <p>This implementation merely returns <tt>false</tt> (so that this
* map acts like a normal map - the eldest element is never removed).
*
* @param eldest The least recently inserted entry in the map, or if
* this is an access-ordered map, the least recently accessed
* entry. This is the entry that will be removed it this
* method returns <tt>true</tt>. If the map was empty prior
* to the <tt>put</tt> or <tt>putAll</tt> invocation resulting
* in this invocation, this will be the entry that was just
* inserted; in other words, if the map contains a single
* entry, the eldest entry is also the newest.
* @return <tt>true</tt> if the eldest entry should be removed
* from the map; <tt>false</tt> if it should be retained.
*/
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
return false;
}
/**
* Returns a {@link Set} view of the keys contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation), the results of
* the iteration are undefined. The set supports element removal,
* which removes the corresponding mapping from the map, via the
* <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
* operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
* operations.
* Its {@link Spliterator} typically provides faster sequential
* performance but much poorer parallel performance than that of
* {@code HashMap}.
*
* @return a set view of the keys contained in this map
*/
public Set<K> keySet() {
Set<K> ks;
return (ks = keySet) == null ? (keySet = new LinkedKeySet()) : ks;
}
final class LinkedKeySet extends AbstractSet<K> {
public final int size() { return size; }
public final void clear() { LinkedHashMap.this.clear(); }
public final Iterator<K> iterator() {
return new LinkedKeyIterator();
}
public final boolean contains(Object o) { return containsKey(o); }
public final boolean remove(Object key) {
return removeNode(hash(key), key, null, false, true) != null;
}
public final Spliterator<K> spliterator() {
return Spliterators.spliterator(this, Spliterator.SIZED |
Spliterator.ORDERED |
Spliterator.DISTINCT);
}
public final void forEach(Consumer<? super K> action) {
if (action == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
action.accept(e.key);
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa. If the map is
* modified while an iteration over the collection is in progress
* (except through the iterator's own <tt>remove</tt> operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Collection.remove</tt>, <tt>removeAll</tt>,
* <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
* support the <tt>add</tt> or <tt>addAll</tt> operations.
* Its {@link Spliterator} typically provides faster sequential
* performance but much poorer parallel performance than that of
* {@code HashMap}.
*
* @return a view of the values contained in this map
*/
public Collection<V> values() {
Collection<V> vs;
return (vs = values) == null ? (values = new LinkedValues()) : vs;
}
final class LinkedValues extends AbstractCollection<V> {
public final int size() { return size; }
public final void clear() { LinkedHashMap.this.clear(); }
public final Iterator<V> iterator() {
return new LinkedValueIterator();
}
public final boolean contains(Object o) { return containsValue(o); }
public final Spliterator<V> spliterator() {
return Spliterators.spliterator(this, Spliterator.SIZED |
Spliterator.ORDERED);
}
public final void forEach(Consumer<? super V> action) {
if (action == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
action.accept(e.value);
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
/**
* Returns a {@link Set} view of the mappings contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation, or through the
* <tt>setValue</tt> operation on a map entry returned by the
* iterator) the results of the iteration are undefined. The set
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
* <tt>clear</tt> operations. It does not support the
* <tt>add</tt> or <tt>addAll</tt> operations.
* Its {@link Spliterator} typically provides faster sequential
* performance but much poorer parallel performance than that of
* {@code HashMap}.
*
* @return a set view of the mappings contained in this map
*/
public Set<Map.Entry<K,V>> entrySet() {
Set<Map.Entry<K,V>> es;
return (es = entrySet) == null ? (entrySet = new LinkedEntrySet()) : es;
}
final class LinkedEntrySet extends AbstractSet<Map.Entry<K,V>> {
public final int size() { return size; }
public final void clear() { LinkedHashMap.this.clear(); }
public final Iterator<Map.Entry<K,V>> iterator() {
return new LinkedEntryIterator();
}
public final boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<?,?> e = (Map.Entry<?,?>) o;
Object key = e.getKey();
Node<K,V> candidate = getNode(hash(key), key);
return candidate != null && candidate.equals(e);
}
public final boolean remove(Object o) {
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>) o;
Object key = e.getKey();
Object value = e.getValue();
return removeNode(hash(key), key, value, true, true) != null;
}
return false;
}
public final Spliterator<Map.Entry<K,V>> spliterator() {
return Spliterators.spliterator(this, Spliterator.SIZED |
Spliterator.ORDERED |
Spliterator.DISTINCT);
}
public final void forEach(Consumer<? super Map.Entry<K,V>> action) {
if (action == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
action.accept(e);
if (modCount != mc)
throw new ConcurrentModificationException();
}
}
// Map overrides
public void forEach(BiConsumer<? super K, ? super V> action) {
if (action == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
action.accept(e.key, e.value);
if (modCount != mc)
throw new ConcurrentModificationException();
}
public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
if (function == null)
throw new NullPointerException();
int mc = modCount;
for (LinkedHashMap.Entry<K,V> e = head; e != null; e = e.after)
e.value = function.apply(e.key, e.value);
if (modCount != mc)
throw new ConcurrentModificationException();
}
// Iterators
abstract class LinkedHashIterator {
LinkedHashMap.Entry<K,V> next;
LinkedHashMap.Entry<K,V> current;
int expectedModCount;
LinkedHashIterator() {
next = head;
expectedModCount = modCount;
current = null;
}
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() {
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;
}
}
final class LinkedKeyIterator extends LinkedHashIterator
implements Iterator<K> {
public final K next() { return nextNode().getKey(); }
}
final class LinkedValueIterator extends LinkedHashIterator
implements Iterator<V> {
public final V next() { return nextNode().value; }
}
final class LinkedEntryIterator extends LinkedHashIterator
implements Iterator<Map.Entry<K,V>> {
public final Map.Entry<K,V> next() { return nextNode(); }
}
}
时间： 2024-07-31 08:12:47