在Java8与Java7中HashMap源码实现的对比

java;">//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2public 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, key, value, 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), key, value, false, true); } 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; //table为空的时候，n为table的长度 if ((p = tab[i = (n - 1) & hash]) == null)  tab[i] = newNode(hash, key, value, 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, key, 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, 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;  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,V>[] tab,     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, k, kc)) != null) ||    ((ch = p.right) != null &&    (q = ch.find(h, k, 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, k, 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;  }  } }