[Java] Java源码解析HashMap的resize函数

/**
 * 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;
  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;
  }
  else if ((newcap = oldcap << 1) < maximum_capacity &&
     oldcap >= default_initial_capacity)
    newthr = oldthr << 1; // double threshold
  }
  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];
  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;
}