這兩天在復習JAVA的知識點，想更深層次的了解一下JAVA，所以就看了看JAVA的源碼，把自己的分析寫在這里，也當做是筆記吧，方便記憶。寫的不對的地方也請大家多多指教。

　　JDK1.6中HashMap采用的是位桶+鏈表的方式，即我們常說的散列鏈表的方式，而JDK1.8中采用的是位桶+鏈表/紅黑樹的方式，也是非線程安全的。當某個位桶的鏈表的長度達到某個閥值的時候，這個鏈表就將轉換成紅黑樹。

基本的數據結構：

 1 //鏈表節點 2 static class Node<K,V> implements Map.Entry<K,V> { 3         final int hash; 4         final K key; 5         V value; 6         Node<K,V> next; 7      //省略  8 } 9 //紅黑樹節點10 static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {11         TreeNode<K,V> parent;  // red-black tree links12         TreeNode<K,V> left;13         TreeNode<K,V> right;14         TreeNode<K,V> PRev;    // needed to unlink next upon deletion15         boolean red;16         TreeNode(int hash, K key, V val, Node<K,V> next) {17             super(hash, key, val, next);18         }19         //省略  20 }21 // HashMap的主要屬性22 public class HashMap<K,V> extends AbstractMap<K,V>23     implements Map<K,V>, Cloneable, Serializable {24     // 槽數組，Node<K,V>類型，TreeNode extends LinkedHashMap.Entry<K,V>，所以可以存放TreeNode來實現Tree bins25     transient Node<K,V>[] table;26     27     transient Set<Map.Entry<K,V>> entrySet;28 29     transient int size;30     // 去掉了volatile的修飾符31     transient int modCount;32 33     int threshold;34 35     final float loadFactor;36 37     ...38 39 } 　　

//計算key的hash

static final int hash(Object key) {        int h;        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); }

get(key) 函數

 1 public V get(Object key) { 2         Node<K,V> e; 3         return (e = getNode(hash(key), key)) == null ? null : e.value; 4     } 5      final Node<K,V> getNode(int hash, Object key) { 6             Node<K,V>[] tab;  7             Node<K,V> first, e;  8             int n; K k; 9             //hash & length-1 定位數組下標10             if ((tab = table) != null && (n = tab.length) > 0 &&11                 (first = tab[(n - 1) & hash]) != null) 12             {13                 if (first.hash == hash && // always check first node14                     ((k = first.key) == key || (key != null && key.equals(k))))15                     return first;16                 if ((e = first.next) != null) {17                     /*第一個節點是TreeNode,則采用位桶+紅黑樹結構，18                      * 調用TreeNode.getTreeNode(hash,key),19                      *遍歷紅黑樹，得到節點的value20                      */21                     if (first instanceof TreeNode)22                         return ((TreeNode<K,V>)first).getTreeNode(hash, key);23                     do {24                         if (e.hash == hash &&25                             ((k = e.key) == key || (key != null && key.equals(k))))26                             return e;27                        } while ((e = e.next) != null);28                 }29             }30             return null;31         }32      final TreeNode<K,V> getTreeNode(int h, Object k) {33              //找到紅黑樹的根節點并遍歷紅黑樹34          return ((parent != null) ? root() : this).find(h, k, null);35      }36      /*37       *通過hash值的比較，遞歸的去遍歷紅黑樹，這里要提的是compareableClassFor(Class k)這個函數的作用，在某些時候38       *如果紅黑樹節點的元素are of the same "class C implements Comparable<C>" type 39       *利用他們的compareTo()方法來比較大小，這里需要通過反射機制來check他們到底是不是屬于同一個類,是不是具有可比較性.40       */41      final TreeNode<K,V> find(int h, Object k, Class<?> kc) {42          TreeNode<K,V> p = this;43          do {44              int ph, dir; K pk;45              TreeNode<K,V> pl = p.left, pr = p.right, q;46              if ((ph = p.hash) > h)47                  p = pl;48              else if (ph < h)49                  p = pr;50              else if ((pk = p.key) == k || (k != null && k.equals(pk)))51                  return p;52              else if (pl == null)53                  p = pr;54              else if (pr == null)55                  p = pl;56              else if ((kc != null ||57                        (kc = comparableClassFor(k)) != null) &&58                       (dir = compareComparables(kc, k, pk)) != 0)59                  p = (dir < 0) ? pl : pr;60              else if ((q = pr.find(h, k, kc)) != null)61                  return q;62              else63                  p = pl;64          } while (p != null);65          return null;66      }      

put(K key,V value)函數

 1 //put(K key,V value)函數  2     public V put(K key, V value) { 3             return putVal(hash(key), key, value, false, true); 4         } 5      6     final V putVal(int hash, K key, V value, boolean onlyIfAbsent, 7             boolean evict) { 8          Node<K,V>[] tab;  9          Node<K,V> p; 10          int n, i;11          //如果table為空或者長度為0，則resize()12          if ((tab = table) == null || (n = tab.length) == 0)13              n = (tab = resize()).length;14          //找到key值對應的槽并且是第一個，直接加入15          if ((p = tab[i = (n - 1) & hash]) == null)16              tab[i] = newNode(hash, key, value, null);17          else {18                  Node<K,V> e;19                  K k;20                  //第一個node的hash值即為要加入元素的hash21                  if (p.hash == hash &&22                      ((k = p.key) == key || (key != null && key.equals(k)))){23                       e = p;24                  }else if (p instanceof TreeNode)//第一個節點是TreeNode，即tree-bin25                     /*Tree version of putVal.26                      *final TreeNode<K,V> putTreeVal(HashMap<K,V> map, Node<K,V>[] tab,int h, K k, V v)27                      */28                      e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);29                      else {30                          //不是TreeNode,即為鏈表,遍歷鏈表31                          for (int binCount = 0; ; ++binCount) {32                              /*到達鏈表的尾端也沒有找到key值相同的節點，33                               *則生成一個新的Node,并且判斷鏈表的節點個數是不是到達轉換成紅黑樹的上界34                               *達到，則轉換成紅黑樹35                               */36                              if ((e = p.next) == null) {37                                  p.next = newNode(hash, key, value, null);38                                  if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st39                                      treeifyBin(tab, hash);40                                  break;41                              }42                              if (e.hash == hash &&43                                  ((k = e.key) == key || (key != null && key.equals(k))))44                                  break;45                              p = e;46                          }47                      }48                  if (e != null) { // existing mapping for key49                      V oldValue = e.value;50                      if (!onlyIfAbsent || oldValue == null)51                          e.value = value;52                      afterNodeaccess(e);53                      //返回舊的value值54                      return oldValue;55                  }56          }57          ++modCount;58          if (++size > threshold)59              resize();60          afterNodeInsertion(evict);61          return null;62 }