在Java8与Java7中HashMap源码实现的对比

//代码块1
 public HashMap(int initialCapacity,float loadFactor) {
 if (initialCapacity < 0)
  throw new IllegalArgumentException("Illegal initial capacity: " +
      initialCapacity);
 if (initialCapacity > MAXIMUM_CAPACITY)
  initialCapacity = MAXIMUM_CAPACITY;
 if (loadFactor <= 0 || Float.isNaN(loadFactor))
  throw new IllegalArgumentException("Illegal load factor: " +loadFactor);

 this.loadFactor = loadFactor;
 threshold = initialCapacity;
 init();
 }

代码块2
public V put(K key,V value) {
 if (table == EMPTY_TABLE) {
  inflateTable(threshold);
 }
 if (key == null)
  return putForNullKey(value);
 int hash = hash(key);
 int i = indexFor(hash,table.length);
 for (Entry<K,V> e = table[i]; e != null; e = e.next) {
  Object k;
  if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
  V oldValue = e.value;
  e.value = value;
  e.recordAccess(this);
  return oldValue;
  }
 }

 modCount++;
 addEntry(hash,key,value,i);
 return null;
 }

//addEntry方法中会检查当前table是否需要resize
 void addEntry(int hash,K key,V value,int bucketIndex) {
 if ((size >= threshold) && (null != table[bucketIndex])) {
  resize(2 * table.length); //当前map中的size 如果大于threshole的阈值，则将resize将table的length扩大2倍。
  hash = (null != key) ? hash(key) : 0;
  bucketIndex = indexFor(hash,table.length);
 }

 createEntry(hash,bucketIndex);
 }

//代码块3 --JDK7中HashMap.resize()方法
void resize(int newCapacity) {
 Entry[] oldTable = table;
 int oldCapacity = oldTable.length;
 if (oldCapacity == MAXIMUM_CAPACITY) {
  threshold = Integer.MAX_VALUE;
  return;
 }

 Entry[] newTable = new Entry[newCapacity];
 transfer(newTable,initHashSeedAsNeeded(newCapacity));
 table = newTable;
 threshold = (int)Math.min(newCapacity * loadFactor,MAXIMUM_CAPACITY + 1);
 }

 /**
 * 将当前table的Entry转移到新的table中
 */
 void transfer(Entry[] newTable,boolean rehash) {
 int newCapacity = newTable.length;
 for (Entry<K,V> e : table) {
  while(null != e) {
  Entry<K,V> next = e.next;
  if (rehash) {
   e.hash = null == e.key ? 0 : hash(e.key);
  }
  int i = indexFor(e.hash,newCapacity);
  e.next = newTable[i];
  newTable[i] = e;
  e = next;
  }
 }
 }

代码块4 -- JDK8中HashMap中常量定义
 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; 
 static final int TREEIFY_THRESHOLD = 8; // 是否将list转换成tree的阈值
 static final int UNTREEIFY_THRESHOLD = 6; // 在resize操作中，决定是否untreeify的阈值
 static final int MIN_TREEIFY_CAPACITY = 64; // 决定是否转换成tree的最小容量
 static final float DEFAULT_LOAD_FACTOR = 0.75f; // default的加载因子

代码块5 --JDK8 HashMap.put方法
 public V put(K key,V value) {
 return putVal(hash(key),false,true);
 }

 final V putVal(int hash,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; //table为空的时候，n为table的长度
 if ((p = tab[i = (n - 1) & hash]) == null)
  tab[i] = newNode(hash,null); // (n - 1) & hash 与Java7中indexFor方法的实现相同，若i位置上的值为空，则新建一个Node，table[i]指向该Node。
 else {
  // 若i位置上的值不为空，判断当前位置上的Node p 是否与要插入的key的hash和key相同
  Node<K,V> e; K k;
  if (p.hash == hash &&
  ((k = p.key) == key || (key != null && key.equals(k))))
  e = p;//相同则覆盖之
  else if (p instanceof TreeNode)
  // 不同，且当前位置上的的node p已经是TreeNode的实例，则再该树上插入新的node。
  e = ((TreeNode<K,V>)p).putTreeVal(this,tab,hash,value);
  else {
  // 在i位置上的链表中找到p.next为null的位置，binCount计算出当前链表的长度，如果继续将冲突的节点插入到该链表中，会使链表的长度大于tree化的阈值，则将链表转换成tree。
  for (int binCount = 0; ; ++binCount) {
   if ((e = p.next) == null) {
   p.next = newNode(hash,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;
  afterNodeAccess(e);
  return oldValue;
  }
 }
 ++modCount;
 if (++size > threshold)
  resize();
 afterNodeInsertion(evict);
 return null;
 }

 代码块6 -- JDK8的resize方法
 inal Node<K,V>[] resize() {
 Node<K,V>[] oldTab = table;
 int oldCap = (oldTab == null) ? 0 : oldTab.length;
 int oldThr = threshold;
 int newCap,newThr = 0;
 if (oldCap > 0) {
  if (oldCap >= MAXIMUM_CAPACITY) {
  threshold = Integer.MAX_VALUE;//如果超过最大容量，无法再扩充table
  return oldTab;
  }
  else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
   oldCap >= DEFAULT_INITIAL_CAPACITY)
  newThr = oldThr << 1; // threshold门槛扩大至2倍
 }
 else if (oldThr > 0) // initial capacity was placed in threshold
  newCap = oldThr;
 else {  // zero initial threshold signifies using defaults
  newCap = DEFAULT_INITIAL_CAPACITY;
  newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
 }
 if (newThr == 0) {
  float ft = (float)newCap * loadFactor;
  newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
   (int)ft : Integer.MAX_VALUE);
 }
 threshold = newThr;
 @SuppressWarnings({"rawtypes","unchecked"})
  Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];// 创建容量为newCap的newTab，并将oldTab中的Node迁移过来，这里需要考虑链表和tree两种情况。
 table = newTab;
 if (oldTab != null) {
  for (int j = 0; j < oldCap; ++j) {
  Node<K,V> e;
  if ((e = oldTab[j]) != null) {
   oldTab[j] = null;
   if (e.next == null)
   newTab[e.hash & (newCap - 1)] = e;
   else if (e instanceof TreeNode)
   ((TreeNode<K,V>)e).split(this,newTab,j,oldCap); 
   // split方法会将树分割为lower 和upper tree两个树，
如果子树的节点数小于了UNTREEIFY_THRESHOLD阈值，则将树untreeify，将节点都存放在newTab中。
   else { // preserve order
   Node<K,V> loHead = null,loTail = null;
   Node<K,V> hiHead = null,hiTail = null;
   Node<K,V> next;
   do {
    next = e.next;
    if ((e.hash & oldCap) == 0) {
    if (loTail == null)
     loHead = e;
    else
     loTail.next = e;
    loTail = e;
    }
    else {
    if (hiTail == null)
     hiHead = e;
    else
     hiTail.next = e;
    hiTail = e;
    }
   } while ((e = next) != null);
   if (loTail != null) {
    loTail.next = null;
    newTab[j] = loHead;
   }
   if (hiTail != null) {
    hiTail.next = null;
    newTab[j + oldCap] = hiHead;
   }
   }
  }
  }
 }
 return newTab;
 }

 // 代码块7 
 //MIN_TREEIFY_CAPACITY 的值为64，若当前table的length不够，则resize（）
 final void treeifyBin(Node<K,V>[] tab,int hash) {
 int n,index; Node<K,V> e;
 if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
  resize();
 else if ((e = tab[index = (n - 1) & hash]) != null) {
  TreeNode<K,V> hd = null,tl = null;
  do {
  TreeNode<K,V> p = replacementTreeNode(e,null);
  if (tl == null)
   hd = p;
  else {
   p.prev = tl;
   tl.next = p;
  }
  tl = p;
  } while ((e = e.next) != null);
  if ((tab[index] = hd) != null)
  hd.treeify(tab);
 }
 }
// putVal 的tree版本 
 final TreeNode<K,V> putTreeVal(HashMap<K,V> map,Node<K,int h,K k,V v) {
  Class<?> kc = null;
  boolean searched = false;
  TreeNode<K,V> root = (parent != null) ? root() : this;
  for (TreeNode<K,V> p = root;;) {
  int dir,ph; K pk;
  if ((ph = p.hash) > h)
   dir = -1;
  else if (ph < h)
   dir = 1;
  else if ((pk = p.key) == k || (k != null && k.equals(pk)))
   return p;
  else if ((kc == null &&
    (kc = comparableClassFor(k)) == null) ||
    (dir = compareComparables(kc,k,pk)) == 0) {
   if (!searched) {
   TreeNode<K,V> q,ch;
   searched = true;
   if (((ch = p.left) != null &&
    (q = ch.find(h,kc)) != null) ||
    ((ch = p.right) != null &&
    (q = ch.find(h,kc)) != null))
    return q;
   }
   dir = tieBreakOrder(k,pk);
  }
  TreeNode<K,V> xp = p;
  if ((p = (dir <= 0) ? p.left : p.right) == null) {
   Node<K,V> xpn = xp.next;
   TreeNode<K,V> x = map.newTreeNode(h,v,xpn);
   if (dir <= 0)
   xp.left = x;
   else
   xp.right = x;
   xp.next = x;
   x.parent = x.prev = xp;
   if (xpn != null)
   ((TreeNode<K,V>)xpn).prev = x;
   moveRootToFront(tab,balanceInsertion(root,x));
   return null;
  }
  }
 }