(原)HashMap之java8新特性

首先说一下HashMap存储结构，数组、链表、树这三种数据结构形成了hashMap。
存储结构下图所示，根据key的hash与table长度确定table位置，同一个位置的key以链表形式存储，超过一定限制链表转为树。
数组的具体存取规则是tab[(n-1) & hash],其中tab为node数组，n为数组的长度，hash为key的hash值。

//链表中数据的临界值，如果达到8，就进行resize扩展,如果数组大于64则转换为树.

static final int TREEIFY_THRESHOLD = 8; 

//如果链表的数据小于6，则从树转换为链表.

static final int UNTREEIFY_THRESHOLD = 6; 

//如果数组的size大于64，则把链表进行转化为树

static final int MIN_TREEIFY_CAPACITY = 64 

//根据key匹配Node,如果匹配不到key，则返回defaultValue

@Override public V getOrDefault(Object key, V defaultValue) { Node<K,V> e; return (e = getNode(hash(key), key)) == null ? defaultValue : e.value; } 

//根据key匹配Node,如果匹配不到则增加key-value,返回null,如果匹配到Node,如果oldValue不等于null则不进行value覆盖，返回oldValue

@Override public V putIfAbsent(K key, V value) { return putVal(hash(key), key, value, true, true); } 

//根据key匹配node，如果value也相同则删除

@Override public boolean remove(Object key, Object value) { return removeNode(hash(key), key, value, true, true) != null; } 

//根据key匹配node,如果value也相同则使用newValue覆盖返回true，否则返回false

@Override public boolean replace(K key, V oldValue, V newValue) { Node<K,V> e; V v; if ((e = getNode(hash(key), key)) != null && ((v = e.value) == oldValue || (v != null && v.equals(oldValue)))) { e.value = newValue; afterNodeAccess(e); return true; } return false; } 

//根据key做匹配Node，（匹配不到则新建然后重排）如果Node有value，则直接返回oldValue，如果没有value则根据Function接口的apply方法获取value，返回value。
//Function接口的apply的入参为key，调用computeIfAbsent时重写Function接口可以根据key进行逻辑处理，apply的返回值即为要存储的value。

@Override public V computeIfAbsent(K key, Functionsuper K, ? extends V> mappingFunction) { if (mappingFunction == null) throw new NullPointerException(); int hash = hash(key); Node<K,V>[] tab; Node<K,V> first; int n, i; int binCount = 0; TreeNode<K,V> t = null; Node<K,V> old = null; if (size > threshold || (tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; if ((first = tab[i = (n - 1) & hash]) != null) { //如果已经转为树,按照树的规则进行处理 if (first instanceof TreeNode) old = (t = (TreeNode<K,V>)first).getTreeNode(hash, key); else { Node<K,V> e = first; K k; //查找整个链表，找到对应的key do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { old = e; break; } ++binCount; } while ((e = e.next) != null); } V oldValue; if (old != null && (oldValue = old.value) != null) { afterNodeAccess(old); return oldValue; } } //根据重写逻辑计算返回value V v = mappingFunction.apply(key); if (v == null) { return null; } else if (old != null) { old.value = v; afterNodeAccess(old); return v; } else if (t != null) t.putTreeVal(this, tab, hash, key, v); else { //如果匹配不到则table加入数据 tab[i] = newNode(hash, key, v, first); if (binCount >= TREEIFY_THRESHOLD - 1) treeifyBin(tab, hash); } ++modCount; ++size; afterNodeInsertion(true); return v; } 

//V computeIfPresent(K key,BiFunction remappingFunction)：根据key做匹配，如果匹配不上则返回null,匹配上根据BiFunction的apply方法获取value，返回value。BiFunction接口的apply的入参为key、oldValue，调用computeIfPresent时重写Function接口可以根据key和oldValue进行逻辑处理，apply的返回值如果为null则删除该节点，否则即为要存储的value。

public V computeIfPresent(K key, BiFunctionsuper K, ? super V, ? extends V> remappingFunction) { if (remappingFunction == null) throw new NullPointerException(); Node<K,V> e; V oldValue; int hash = hash(key); if ((e = getNode(hash, key)) != null && (oldValue = e.value) != null) { //使用key和原value作为入参 V v = remappingFunction.apply(key, oldValue); if (v != null) { e.value = v; afterNodeAccess(e); return v; } else removeNode(hash, key, null, false, true); } return null; } 

//V compute(K key,BiFunction remappingFunction)：根据key做匹配，根据BiFunction的apply返回做存储的value。匹配到Node做value替换，匹配不到新增node。apply的返回值如果为null则删除该节点，否则即为要存储的value。

@Override public V compute(K key, BiFunctionsuper K, ? super V, ? extends V> remappingFunction) { if (remappingFunction == null) throw new NullPointerException(); int hash = hash(key); Node<K,V>[] tab; Node<K,V> first; int n, i; int binCount = 0; TreeNode<K,V> t = null; Node<K,V> old = null; if (size > threshold || (tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; if ((first = tab[i = (n - 1) & hash]) != null) { if (first instanceof TreeNode) old = (t = (TreeNode<K,V>)first).getTreeNode(hash, key); else { Node<K,V> e = first; K k; do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { old = e; break; } ++binCount; } while ((e = e.next) != null); } } V oldValue = (old == null) ? null : old.value; //使用key和原value作为入参 V v = remappingFunction.apply(key, oldValue); if (old != null) { if (v != null) { old.value = v; afterNodeAccess(old); } else removeNode(hash, key, null, false, true); } else if (v != null) { if (t != null) t.putTreeVal(this, tab, hash, key, v); else { tab[i] = newNode(hash, key, v, first); if (binCount >= TREEIFY_THRESHOLD - 1) treeifyBin(tab, hash); } ++modCount; ++size; afterNodeInsertion(true); } return v; } 

// V merge(K key, V value,BiFunction remappingFunction)：功能大部分与compute相同，不同之处在于BiFunction中apply的参数，入参为oldValue、value，调用merge时根据两个value进行逻辑处理并返回value。

@Override public V merge(K key, V value, BiFunctionsuper V, ? super V, ? extends V> remappingFunction) { if (value == null) throw new NullPointerException(); if (remappingFunction == null) throw new NullPointerException(); int hash = hash(key); Node<K,V>[] tab; Node<K,V> first; int n, i; int binCount = 0; TreeNode<K,V> t = null; Node<K,V> old = null; if (size > threshold || (tab = table) == null || (n = tab.length) == 0) n = (tab = resize()).length; if ((first = tab[i = (n - 1) & hash]) != null) { if (first instanceof TreeNode) old = (t = (TreeNode<K,V>)first).getTreeNode(hash, key); else { Node<K,V> e = first; K k; do { if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) { old = e; break; } ++binCount; } while ((e = e.next) != null); } } if (old != null) { V v; if (old.value != null) //使用新老value作为入参 v = remappingFunction.apply(old.value, value); else v = value; if (v != null) { old.value = v; afterNodeAccess(old); } else removeNode(hash, key, null, false, true); return v; } if (value != null) { if (t != null) t.putTreeVal(this, tab, hash, key, value); else { tab[i] = newNode(hash, key, value, first); if (binCount >= TREEIFY_THRESHOLD - 1) treeifyBin(tab, hash); } ++modCount; ++size; afterNodeInsertion(true); } return value; } 

// void forEach(BiConsumer action)：调用此方法时实现BiConsumer接口重写void accept(Object o, Object o2)方法，其中o为key，o2为value，可根据自己的实现对map中所有数据进行处理。

@Override public void forEach(BiConsumersuper K, ? super V> action) { Node<K,V>[] tab; if (action == null) throw new NullPointerException(); if (size > 0 && (tab = table) != null) { int mc = modCount; for (int i = 0; i < tab.length; ++i) { for (Node<K,V> e = tab[i]; e != null; e = e.next) action.accept(e.key, e.value); } if (modCount != mc) throw new ConcurrentModificationException(); } } 

// void replaceAll(BiFunction function)：调用此方法时重写BiFunction的Object apply(Object o, Object o2)方法，其中o为key，o2为value，根据重写方法逻辑进行重新赋值。

@Override public void replaceAll(BiFunctionsuper K, ? super V, ? extends V> function) { Node<K,V>[] tab; if (function == null) throw new NullPointerException(); if (size > 0 && (tab = table) != null) { int mc = modCount; for (int i = 0; i < tab.length; ++i) { for (Node<K,V> e = tab[i]; e != null; e = e.next) { e.value = function.apply(e.key, e.value); } } if (modCount != mc) throw new ConcurrentModificationException(); } } 

computeIfAbsent、computeIfPresent、compute对比

computeIfAbsent：如果key已存在，返回oldVlaue；不存在创建，返回新创建value

computeIfPresent：如果key不存在，返回null；如果已存在，value为null则删除此节点，不为null替换节点value并返回此value。

compute：如果key不存在，新建key进行存储；如果key存在，value为null则删除此节点，不为null替换节点value并返回此value。