HashMap 소스 를 읽 고 쓴 노트 입 니 다.

/**
*      。16
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
 ```

/**
 *    0.75    ,  . 3/4。     2 
 * The load factor used when none specified in constructor.
 */
static final float DEFAULT_LOAD_FACTOR = 0.75f;

static final int TREEIFY_THRESHOLD = 8;
static final int UNTREEIFY_THRESHOLD = 6;

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;
    this.threshold = tableSizeFor(initialCapacity);
}

  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    true       key 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) {
    /**
      *       HashMap     ：tab    。    Node（      ）
      */
    Node[] tab; Node p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    /**
      *  hash      index
      */
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node e; K k;
        if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        /**
         *   table[i]    treeNode， table[i]       ，
         *       ，           
         */
        else if (p instanceof TreeNode)
            e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
        else {
            /**
             *   table[i]，          TREEIFY_THRESHOLD(    8)，  8           ，
             *            ，           ；        key        value  ；
             */
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    /**
                      *      Node next         Node，      
                      */
                    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;
            }
        }
      /**
       *      ， key            value
       */
        if (e != null) { // existing mapping for key
            /**
            *value      ，      ，      
            */
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    
   /**
     *      ，            size         threshold，
     *     ，    
     */
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

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

  Node(int hash, K key, V value, Node 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;
  }
}

/**
 * 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[] resize() {
    Node[] 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;
            return oldTab;
        }
      /**
       *    *2       ，          （8） ，
       *    ，   2 
       */
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                oldCap >= DEFAULT_INITIAL_CAPACITY)
            newThr = oldThr << 1; // double threshold     2 
    }
    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[] newTab = (Node[])new Node[newCap];
    table = newTab;
    if (oldTab != null) {
        for (int j = 0; j < oldCap; ++j) {
            Node e;
            if ((e = oldTab[j]) != null) {
                oldTab[j] = null;
                if (e.next == null)
                    newTab[e.hash & (newCap - 1)] = e;
                /**
                 *       ，          。
                 *  UNTREEIFY_THRESHOLD=6     6     
                 */
                else if (e instanceof TreeNode)
                    ((TreeNode)e).split(this, newTab, j, oldCap);
                else { // preserve order
                    Node loHead = null, loTail = null;
                    Node hiHead = null, hiTail = null;
                    Node 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;
}

public Set> entrySet() {
    Set> es;
    return (es = entrySet) == null ? (entrySet = new EntrySet()) : es;
  }