1. LinkedHashMap概览
- LinkedHashMap类定义
public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>
LinkedHashMap直接继承自HashMap。所以说LinkedHashMap内部的实现也是数组+链表/红黑树实现
- LinkedHashMap的Entry
static class Entry<K,V> extends HashMap.Node<K,V> { Entry<K,V> before, after; Entry(int hash, K key, V value, Node<K,V> next) { super(hash, key, value, next); } }
LinkedHashMap.Entry继承自HashMap.Node。同时新增了before和after两个字段，用来维护LinkedHashMap中Entry的顺序。LinkedHashMap的entrySet()遍历就是通过该双链表来实现的。
1.LinkedHashMap的成员变量
transient LinkedHashMap.Entry<K,V> head;//指向双链表的头 transient LinkedHashMap.Entry<K,V> tail;//指向双链表的尾 final boolean accessOrder;//访问是否排序，访问包括get和put
LinkedHashMap的构造函数
public LinkedHashMap(int initialCapacity, float loadFactor) { super(initialCapacity, loadFactor); accessOrder = false; } public LinkedHashMap(int initialCapacity) { super(initialCapacity); accessOrder = false; } public LinkedHashMap() { super(); accessOrder = false; } public LinkedHashMap(Map<? extends K, ? extends V> m) { super(); accessOrder = false; putMapEntries(m, false); } public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) { super(initialCapacity, loadFactor); this.accessOrder = accessOrder; }
从定义的构造函数可以看出，相比HashMap只是多了accessOrder的定义。后面我们会详细讲解accessOrder的作用
2. LinkedHashMap接口方法详解
1. V get(Object key)
public V get(Object key) { Node<K,V> e; //getNode方法是HashMap提供的实现方法，通过Hash值定位下标，然后比对key和hash值是否相等 if ((e = getNode(hash(key), key)) == null) return null; //如果设置了accessOrder if (accessOrder) //这个方法后面重点讲解下 afterNodeAccess(e); return e.value; }
2. V put(K key, V value)
put方法完全复用了HashMap的put方法,具体解释可以参考HashMap
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; //如果是覆盖key的value调用afterNodeAccess(e) afterNodeAccess(e); return oldValue; } } ++modCount; if (++size > threshold) resize(); //如果是新插入的值 会调用afterNodeInsertion afterNodeInsertion(evict); return null; }
newNode,在LinkedHashMap中新插入一个Node,会链接到双链表的尾端
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); linkNodeLast(p); return p; } 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; } }
注意 在HashMap中afterNodeAccess(e)、afterNodeInsertion(evict)、afterNodeRemoval()都是空实现。但是在LinkedHashMap中。他们是有具体实现的。接下来我们分别来看下他们的实现
1.afterNodeAccess()
访问包括 get(key) put(key,value) put只限于原来key有value才算访问，新插入的不会调用afterNodeAccess()
void afterNodeAccess(Node<K,V> e) { // 把Entry移动到双链表的尾端 LinkedHashMap.Entry<K,V> last; if (accessOrder && (last = tail) != e) { LinkedHashMap.Entry<K,V> p = (LinkedHashMap.Entry<K,V>)e, b = p.before, a = >p.after; p.after = null; if (b == null) head = a; else b.after = a; if (a != null) a.before = b; else last = b; if (last == null) head = p; else { p.before = last; last.after = p; } tail = p; ++modCount; } }
2.afterNodeInsertion()
当插入了新的节点后，会根据需要去删除双链表头指向的节点
void afterNodeInsertion(boolean evict) { // 如果允许删除在HashMap中最老的Entry。双链表head指向的就是最老的Entry LinkedHashMap.Entry<K,V> first; if (evict && (first = head) != null && removeEldestEntry(first)) { K key = first.key; removeNode(hash(key), key, null, false, true); } }
3.afterNodeRemoval()
删除了数据需要 从双链表中删除
void afterNodeRemoval(Node<K,V> e) { // unlink LinkedHashMap.Entry<K,V> p = (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after; p.before = p.after = null; if (b == null) head = a; else b.after = a; if (a == null) tail = b; else a.before = b; }
3. remove(Object key)
remove(Object key)复用了HashMap的remove(Object key)方法。唯一的区别就是LinkedHashMap删除数据后，会从双链表删除数据
4. entrySet()
entrySet()的功能和HashMap一样也是为了遍历map中的元素。但是LinkedHashMap和HashMap遍历的实现确不相同。HashMap实现遍历的方式是从table数组的0下标开始查找，直到查到一个非空的元素，如果该元素还有链表或者红黑树，遍历之，遍历完接着遍历数组。LinkedHashMap遍历的方式是从双链表的head一直往尾端遍历。他们的区别就是HashMap遍历是无序的。LinkedHashMap遍历是有序的
1. 生成entrySet public Set<Map.Entry<K,V>> entrySet() { Set<Map.Entry<K,V>> es; return (es = entrySet) == null ? (entrySet = new LinkedEntrySet()) : es; } 2.iterator()方法 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; } final class LinkedEntryIterator extends LinkedHashIterator implements Iterator<Map.Entry<K,V>> { public final Map.Entry<K,V> next() { return nextNode(); } } abstract class LinkedHashIterator { LinkedHashMap.Entry<K,V> next; LinkedHashMap.Entry<K,V> current; int expectedModCount; LinkedHashIterator() { //从双链表的head开始 next = head; expectedModCount = modCount; current = null; } public final boolean hasNext() { return next != null; } //遍历双链表 从head开始 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; } }
