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;
 }
 public final int hash(Object k) {
  int h = hashSeed;
  if (0 != h && k instanceof String) {
   return sun.misc.Hashing.stringHash32((String) k);
  }
  h ^= k.hashCode();
  // This function ensures that hashCodes that differ only by
  // constant multiples at each bit position have a bounded
  // number of collisions (approximately 8 at default load factor).
  h ^= (h >>> 20) ^ (h >>> 12);
  return h ^ (h >>> 7) ^ (h >>> 4);
 }
 public final int hashCode() {
  return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue()) 
 }
 static int indexFor(int h, int length) {
  // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
  return h & (length-1);
 }

 public final Entry<K,V> getEntry(Object key) {
  if (size == 0) {
   return null;
  }
  int hash = (key == null) ? 0 : hash(key);
  for (Entry<K,V> e = table[indexFor(hash, table.length)];
    e != null;
    e = e.next) {
   Object k;
   if (e.hash == hash &&
    ((k = e.key) == key || (key != null && key.equals(k))))
    return e;
  }
  return null;
 }

 public final boolean equals(Object o) {
   if (!(o instanceof Map.Entry))
    return false;
   Map.Entry e = (Map.Entry)o;
   Object k1 = getKey();
   Object k2 = e.getKey();
   if (k1 == k2 || (k1 != null && k1.equals(k2))) {
    Object v1 = getValue();
    Object v2 = e.getValue();
    if (v1 == v2 || (v1 != null && v1.equals(v2)))
     return true;
   }
   return false;
  }

如果 HashMap的大小 > HashMap的容量(即Entry[] table的大小)*加载因子(经验值0.75)

 则 HashMap中的Entry[] table 的容量扩充为当前的一倍；然后重新将以前桶中的`Entry<Key,Value>`链表重新分配到各个桶中