/**
 * The maximum capacity, used if a higher value is implicitly specified
 * by either of the constructors with arguments.
 * MUST be a power of two <= 1<<30.
 */
static final int MAXIMUM_CAPACITY = 1 << 30;

/**
 * The load factor used when none specified in constructor.
 */
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
 * 该表在首次使用时初始化，并根据需要调整大小。 分配时，长度始终是2的幂。
 * （在某些操作中，我们还允许长度为零，以  * 允许使用当前不需要的引导机制。）
 */
transient Node<K, V>[] table;

/**
 * 对该HashMap进行结构修改的次数结构修改是指更改HashMap中的
 * 映射次数或以其他方式修改其内部结构（例如，重新哈希）的修改。
 * 此字段用于使HashMap的Collection-view上的迭代器快速失败。
 * （请参见ConcurrentModificationException）。
 */
transient int modCount;
/**
 * The next size value at which to resize (capacity * load factor).
 *  阀域值
 * @serial
 */
// (The javadoc description is true upon serialization.
// Additionally, if the table array has not been allocated, this
// field holds the initial array capacity, or zero signifying
// DEFAULT_INITIAL_CAPACITY.)
int threshold;

  /**
 * Associates the specified value with the specified key in this map.
 * If the map previously contained a mapping for the key, the old
 * value is replaced.
 *
 * @param key key with which the specified value is to be associated
 * @param value value to be associated with the specified key
 * @return the previous value associated with <tt>key</tt>, or
 *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
 *         (A <tt>null</tt> return can also indicate that the map
 *         previously associated <tt>null</tt> with <tt>key</tt>.)
 */
public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

/**
   * Implements Map.put and related methods
   *
   * @param hash hash for key
   * @param key the key
   * @param value the value to put
   * @param onlyIfAbsent if true, don't change existing value
   * @param evict if false, the table is in creation mode.
   * @return previous value, or null if none
   */
  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)
          //执行一次，第一次才会执行 ，这里会触发 【扩容】
          n = (tab = resize()).length;
      
      // 注意  tab[i = (n - 1) & hash]
      if ((p = tab[i = (n - 1) & hash]) == null)
          //如果key已经存在就不会进入
          tab[i] = newNode(hash, key, value, null);
      
      else {
          // key 已经存在才会进入
          Node<K,V> e; K k;
          
          if (p.hash == hash &&
              ((k = p.key) == key || (key != null && key.equals(k))))
              e = p;
          
          else if (p instanceof TreeNode)
              e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
          
          else {
              
              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
           //除非表太小，否则用给定的哈希替换索引中bin处bin中的所有链接节点，在这种情况下，将调整大小。
                          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);
              
              // 注意这里流程中断
              return oldValue;
          }
          
          
      }
      
      ++modCount;
      
      if (++size > threshold)
          resize();
      
      afterNodeInsertion(evict);
      
      return null;
  }

// Create a regular (non-tree) node
Node<K,V> newNode(int hash, K key, V value, Node<K,V> next) {
    return new Node<>(hash, key, value, next);
}

/**
 * Returns the value to which the specified key is mapped,
 * or {@code null} if this map contains no mapping for the key.
 *
 * <p>More formally, if this map contains a mapping from a key
 * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
 * key.equals(k))}, then this method returns {@code v}; otherwise
 * it returns {@code null}.  (There can be at most one such mapping.)
 *
 * <p>A return value of {@code null} does not <i>necessarily</i>
 * indicate that the map contains no mapping for the key; it's also
 * possible that the map explicitly maps the key to {@code null}.
 * The {@link #containsKey containsKey} operation may be used to
 * distinguish these two cases.
 *
 * @see #put(Object, Object)
 */
public V get(Object key) {
    Node<K,V> e;
    return (e = getNode(hash(key), key)) == null ? null : e.value;
}

/**
 * Implements Map.get and related methods
 *
 * @param hash hash for key
 * @param key the key
 * @return the node, or null if none
 */
final Node<K,V> getNode(int hash, Object key) {
    Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
    
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & hash]) != null) {
        
        //第一个元素的key 与给定的key是否相等
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        
        if ((e = first.next) != null) {
            
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
            
        }
    }
    return null;
}

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

/**
   * Initializes or doubles table size.  If null, allocates in
   * accord with initial capacity target held in field threshold.
   * Otherwise, because we are using power-of-two expansion, the
   * elements from each bin must either stay at same index, or move
   * with a power of two offset in the new table.
   *
   * @return the table
   */
  final Node<K,V>[] resize() {
      Node<K,V>[] oldTab = table;
      //旧的集合容量大小
      int oldCap = (oldTab == null) ? 0 : oldTab.length;
      //旧的阀域值为系统的阀域值
      // threshold 的默认值为 0
      int oldThr = threshold;
      
      int newCap, newThr = 0;
      
      if (oldCap > 0) {
          // 定义    static final int MAXIMUM_CAPACITY = 1 << 30;
          if (oldCap >= MAXIMUM_CAPACITY) {
              //扩容的阀域值
              threshold = Integer.MAX_VALUE;
              return oldTab;
          }
          else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                   oldCap >= DEFAULT_INITIAL_CAPACITY)
              //新的阀域值为旧的
              newThr = oldThr << 1; // double threshold
      }
      
      
      else if (oldThr > 0) // initial capacity was placed in threshold
          //新的容量值为旧的阀域值
          newCap = oldThr;
      
      else {   
           ////////////////第一次时这里会进来//////////////
          // 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);
      }
      
      // 第一次之后，这两个值均为 12
      threshold = newThr;
      
      @SuppressWarnings({"rawtypes","unchecked"})
          Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
      
      //新的table  =====> 系统table
      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);
                  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;
  }

static class Node<K,V> implements Map.Entry<K,V> {
      final int hash;
      final K key;
      V value;
      Node<K,V> next;

      Node(int hash, K key, V value, Node<K,V> next) {
          this.hash = hash;
          this.key = key;
          this.value = value;
          this.next = next;
      }

      public final K getKey()        { return key; }
      public final V getValue()      { return value; }
      public final String toString() { return key + "=" + value; }

      public final int hashCode() {
          return Objects.hashCode(key) ^ Objects.hashCode(value);
      }

      public final V setValue(V newValue) {
          V oldValue = value;
          value = newValue;
          return oldValue;
      }

      public final boolean equals(Object o) {
          if (o == this)
              return true;
          if (o instanceof Map.Entry) {
              Map.Entry<?,?> e = (Map.Entry<?,?>)o;
              if (Objects.equals(key, e.getKey()) &&
                  Objects.equals(value, e.getValue()))
                  return true;
          }
          return false;
      }
  }