HashMap

HashMap

• 结构: 数组+链表(红黑树)

• 放数据:

  public V put(K key, V value) {
  return putVal(hash(key), key, value, false, true);
  }

• key的hash(): key==null,其hash为0,不为null其值为key的高16位异或key的低16位(h右移位16的值),这样会把这个hashcode的位的用上,因为接下来通过hash值判断数组的位置时使用的是01111这样的与运算,如果直接使用hashcode会使得hashcode的高位没有意义

  hashcode与equals的关系: equals相等则hashcode一定相等,反之不一定,因为两个key相同的键值对数据就是hashcode相同但是equals不同

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

• 链表节点:

  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;
  }

• 数组大小: 默认16,最大值1 << 30

  static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
  static final int MAXIMUM_CAPACITY = 1 << 30;

• 数组扩容: 条件: 当前数组的容量被使用了0.75倍之后进行2倍扩容

  static final float DEFAULT_LOAD_FACTOR = 0.75f;

• 链表转变: 当链表节点增加大于等于8时转换成红黑树,减少到6时有红黑树转换成普通链表

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

• putVal

  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)
  n = (tab = resize()).length;//通过resize()初始化或者加倍数组大小

  //节点为空
  if ((p = tab[i = (n - 1) & hash]) == null)//用01111与hash进行与运算(计算机执行比较快,目的与取模一样)
  tab[i] = newNode(hash, key, value, null);//当该数组位置没有元素,直接把Node节点赋给该数组位置

  //节点非空
  else {
  Node<K,V> e; K k;

  //添加元素的key == p.key
  if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))
  e = p;

  //添加元素的key != p.key,放入红黑树
  else if (p instanceof TreeNode)
  e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);

  //添加元素的key != p.key,放入普通链表
  else {
  for (int binCount = 0; ; ++binCount) {
  if ((e = p.next) == null) {
  p.next = newNode(hash, key, value, null);
  if (binCount >= TREEIFY_THRESHOLD - 1) // 节点大于等于8时由链表转换为红黑树
  treeifyBin(tab, hash);
  break;
  }
  if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k))))//遍历链表时,存在相同key节点
  break;
  p = e;
  }
  }
  //上面的for循环如果发现相同的key的节点就把e置为改节点,否则置为null
  if (e != null) { // existing mapping for key
  V oldValue = e.value;
  if (!onlyIfAbsent || oldValue == null)
  e.value = value;
  afterNodeAccess(e);
  return oldValue;
  }
  }
  ++modCount;
  if (++size > threshold)//扩容: size为数组所使用的大小,threshold为0.75*数组容量
  resize();
  afterNodeInsertion(evict);
  return null;

  为什么数组两倍扩容? ---->因为需要使用数组长度(n)-1进行与计算,需要生成后位都是1的数,初始容量为16(10000),两倍扩容后就可以产生这个效果(例:10000-1=01111,100000000-1=011111111)

• 初始化或者加倍数组大小

  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) { //已经达到最大数组,不扩容了
  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) {
  //遍历数组
  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;
  //拆分红黑树
  else if (e instanceof TreeNode)
  ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
  //拆分链表
  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) {//链表节点迁移只有两种可能,原地不动和往后移oldCap
  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;
  }

  数组扩容后节点迁移

  扩容前: 0x0101 & 01111 = 0101

  扩容后: 0x0101 & 11111 = x0101

  x只能为0或1,当为0时节点保持不动,当为1时,节点位置增加了10000也就是oldCap(扩容前数组的容量)

• 设置初始容量

  initialCapacity = (存储元素个数 / 负载因子) + 1

• null值(<<阿里巴巴Java开发手册>>)