属性

  1.  private static final long serialVersionUID = 362498820763181265L;
  2. //初始化容量
  3.  static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
  5.  static final int MAXIMUM_CAPACITY = 1 << 30; 
  6.  static final float DEFAULT_LOAD_FACTOR = 0.75f;
  7. //数组转树阈值
  8.  static final int TREEIFY_THRESHOLD = 8; 
  9. //
  10.  static final int UNTREEIFY_THRESHOLD = 6;
  11. //hashtable容量大于64才会数组转红黑树，否则会先扩容数组
  12.  static final int MIN_TREEIFY_CAPACITY = 64;
  13. //
  14.  transient Node<K,V>[] table;
  16.  transient int modCount;
  17. //The next size value at which to resize (capacity * load factor).
  18.  int threshold;
  19.  final float loadFactor;
  20.  transient Set<Map.Entry<K,V>> entrySet;
  21. //The number of key-value mappings contained in this map.
  22.  transient int size;

构造方法

  1.  //initialCapacity初始化容量
  2.  //loadFactor负载因子
  3.  //threshold阈值
  4.  public HashMap(int initialCapacity, float loadFactor) {
  5.         if (initialCapacity < 0)
  6.             throw new IllegalArgumentException("Illegal initial capacity: " +
  7.                                                initialCapacity);
  8.         if (initialCapacity > MAXIMUM_CAPACITY)
  9.             initialCapacity = MAXIMUM_CAPACITY;
  10.         if (loadFactor <= 0 || Float.isNaN(loadFactor))
  11.             throw new IllegalArgumentException("Illegal load factor: " +
  12.                                                loadFactor);
  13.         this.loadFactor = loadFactor;
  14.         this.threshold = tableSizeFor(initialCapacity);
  15.     }
  17.   public HashMap(int initialCapacity) {
  18.         this(initialCapacity, DEFAULT_LOAD_FACTOR);
  19.     }
  21.   public HashMap() {
  22.        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
  23.     }
  25.    public HashMap(Map<? extends K, ? extends V> m) {
  26.         this.loadFactor = DEFAULT_LOAD_FACTOR;
  27.         putMapEntries(m, false);
  28.     }
  29.    //找到大于或等于cap的最小2的幂
  30.    //无符号位运算>>>
  31.   static final int tableSizeFor(int cap) {
  32.           int n = cap - 1;
  33.           n |= n >>> 1;
  34.           n |= n >>> 2;
  35.           n |= n >>> 4;
  36.           n |= n >>> 8;
  37.           n |= n >>> 16;
  38.           return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
  39.       }

put方法

  1.  public V put(K key, V value) {
  2.         return putVal(hash(key), key, value, false, true);
  3.     }
  5.  static final int hash(Object key) {
  6.         int h;
  7.         return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
  8.     }
  10. final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
  11.                    boolean evict) {
  12.         Node<K,V>[] tab; Node<K,V> p; int n, i;
  13.     //如果当前table没有初始化（引用=null并且table长度=0）
  14.         if ((tab = table) == null || (n = tab.length) == 0)
  15.             //初始化table容量
  16.             n = (tab = resize()).length;
  17.     //找到要插入元素的table下标，判断为null则直接插入
  18.         if ((p = tab[i = (n - 1) & hash]) == null)
  19.             tab[i] = newNode(hash, key, value, null);
  20.      //否则
  21.         else {
  22.            //如果hash相同并且key也相同
  23.             Node<K,V> e; K k;
  24.             if (p.hash == hash &&
  25.                 ((k = p.key) == key || (key != null && key.equals(k))))
  26.                 //先用Node<K,V> e保存p
  27.                 e = p;
  28.             //红黑树插入
  29.             else if (p instanceof TreeNode)
  30.                 e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
  31.             else {
  32.                 //数组元素是链表
  33.                 for (int binCount = 0; ; ++binCount) {
  34.                     //p.next如果为null，直接插入
  35.                     if ((e = p.next) == null) {
  36.                         p.next = newNode(hash, key, value, null);
  37.                         if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
  38.                             treeifyBin(tab, hash);
  39.                         break;
  40.                     }
  41.                     //如果哈希碰撞就退出循环
  42.                     if (e.hash == hash &&
  43.                         ((k = e.key) == key || (key != null && key.equals(k))))
  44.                         break;
  45.                     p = e;
  46.                 }
  47.             }
  48.             //处理哈希碰撞
  49.             if (e != null) { // existing mapping for key
  50.                 V oldValue = e.value;
  51.                 if (!onlyIfAbsent || oldValue == null)
  52.                     e.value = value;
  53.                 afterNodeAccess(e);
  54.                 return oldValue;
  55.             }
  56.         }
  57.     //操作数加一
  58.         ++modCount;
  59.     //扩容处理
  60.         if (++size > threshold)
  61.             resize();
  62.         afterNodeInsertion(evict);
  63.         return null;
  64.     }

resize方法

  1. final Node<K,V>[] resize() {
  2.         Node<K,V>[] oldTab = table;
  3.         int oldCap = (oldTab == null) ? 0 : oldTab.length;
  4.         int oldThr = threshold;
  5.         int newCap, newThr = 0;
  6.     //oldTab只初始化没有数据
  7.         if (oldCap > 0) {
  8.             if (oldCap >= MAXIMUM_CAPACITY) {
  9.                 threshold = Integer.MAX_VALUE;
  10.                 return oldTab;
  11.             }
  12.             else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
  13.                      oldCap >= DEFAULT_INITIAL_CAPACITY)
  14.                 newThr = oldThr << 1; // double threshold
  15.         }
  16.         else if (oldThr > 0) // initial capacity was placed in threshold
  17.             newCap = oldThr;
  18.         else {               // zero initial threshold signifies using defaults
  19.             newCap = DEFAULT_INITIAL_CAPACITY;
  20.             newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
  21.         }
  22.         if (newThr == 0) {
  23.             float ft = (float)newCap * loadFactor;
  24.             newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
  25.                       (int)ft : Integer.MAX_VALUE);
  26.         }
  27.         threshold = newThr;
  28.         @SuppressWarnings({"rawtypes","unchecked"})
  29.         Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
  30.         table = newTab;
  31.         //oldTab有数据
  32.         if (oldTab != null) {
  33.             //扩容，整个数组移动，数组下标是根据数组容量和key的hashcode决定的
  34.             for (int j = 0; j < oldCap; ++j) {
  35.                 Node<K,V> e;
  36.                 //如果指定下标的数组元素有值，需要先置空
  37.                 if ((e = oldTab[j]) != null) {
  38.                     oldTab[j] = null;
  39.                     //桶只有一个元素则直接存放到数组新下标处的位置
  40.                     if (e.next == null)
  41.                         newTab[e.hash & (newCap - 1)] = e;
  42.                     //如果是树结构
  43.                     else if (e instanceof TreeNode)
  44.                         ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
  45.                     //是链表结构
  46.                     else { // preserve order
  47.                         Node<K,V> loHead = null, loTail = null;
  48.                         Node<K,V> hiHead = null, hiTail = null;
  49.                         Node<K,V> next;
  50.                         do {
  51.                             next = e.next;
  52.                             //容量扩容2幂次，假设为两倍，数组分为原长度+新增的一倍长度
  53.                             //在原来位置（低位）
  54.                             if ((e.hash & oldCap) == 0) {
  55.                                 if (loTail == null)
  56.                                     loHead = e;
  57.                                 else
  58.                                     loTail.next = e;
  59.                                 loTail = e;
  60.                             }
  61.                             //数组位置（高位）
  62.                             else {
  63.                                 if (hiTail == null)
  64.                                     hiHead = e;
  65.                                 else
  66.                                     hiTail.next = e;
  67.                                 hiTail = e;
  68.                             }
  69.                         } while ((e = next) != null);
  70.                         if (loTail != null) {
  71.                             loTail.next = null;
  72.                             newTab[j] = loHead;
  73.                         }
  74.                         if (hiTail != null) {
  75.                             hiTail.next = null;
  76.                             newTab[j + oldCap] = hiHead;
  77.                         }
  78.                     }
  79.                 }
  80.             }
  81.         }
  82.         return newTab;
  83.     }