(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。