全面解析Java中的HashMap类

static class Entry<K,V> implements Map.Entry<K,V> {
 final K key;
 V value;
 Entry<K,V> next;
 int hash;
 Entry(int h, K k, V v, Entry<K,V> n) {
  value = v;
  next = n;
  key = k;
  hash = h;
 }
 //key, value的get与set方法省略，get与set操作会在后面的迭代器中用到
 ...
 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;
 }
 //此处将Key的hashcode与Value的hashcode做亦或运算得到Entry的hashcode
 public final int hashCode() {
  return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
 }
 public final String toString() {
  return getKey() + "=" + getValue();
 }
 /**
  * This method is invoked whenever the value in an entry is
  * overwritten by an invocation of put(k,v) for a key k that's already
  * in the HashMap.
  */
 void recordAccess(HashMap<K,V> m) {
 }
 /**
  * This method is invoked whenever the entry is
  * removed from the table.
  */
 void recordRemoval(HashMap<K,V> m) {
 }
 }

public V get(Object key) {
 //key是null的情况
 if (key == null)
  return getForNullKey();
 //根据key查找Entry
 Entry<K,V> entry = getEntry(key);
 return null == entry ? null : entry.getValue();
 }

private V getForNullKey() {
 if (size == 0) {
  return null;
 }
 //遍历冲突链
 for (Entry<K,V> e = table[0]; e != null; e = e.next) {
  if (e.key == null)
  return e.value;
 }
 return null;
 }

final Entry<K,V> getEntry(Object key) {
 if (size == 0) {
  return null;
 }
 int hash = (key == null) ? 0 : hash(key);
 //通过hash得到table中的索引位置，然后遍历冲突链表找到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 V put(K key, V value) {
 //空表table的话，根据size的阈值填充
 if (table == EMPTY_TABLE) {
  inflateTable(threshold);
 }
 //填充key为Null的Entry
 if (key == null)
  return putForNullKey(value);
 //生成hash，得到索引Index的映射
 int hash = hash(key);
 int i = indexFor(hash, table.length);
 //遍历当前索引的冲突链，找是否存在对应的key
 for (Entry<K,V> e = table[i]; e != null; e = e.next) {
  Object k;
  //如果存在对应的key， 则替换oldValue并返回oldValue
  if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
  V oldValue = e.value;
  e.value = value;
  e.recordAccess(this);
  return oldValue;
  }
 }
 //冲突链中不存在新写入的Entry的key
 modCount++;
 //插入一个新的Entry
 addEntry(hash, key, value, i);
 return null;
 }

void addEntry(int hash, K key, V value, int bucketIndex) {
 //插入新Entry前，先对当前HashMap的size和其阈值大小的判断，选择是否扩容
 if ((size >= threshold) && (null != table[bucketIndex])) {
  resize(2 * table.length);
  hash = (null != key) ? hash(key) : 0;
  bucketIndex = indexFor(hash, table.length);
 }
 createEntry(hash, key, value, bucketIndex);
 }
 void createEntry(int hash, K key, V value, int bucketIndex) {
 Entry<K,V> e = table[bucketIndex];
 //头插法，新写入的entry插入当前索引位置的冲突链第一个Entry的前面
 table[bucketIndex] = new Entry<>(hash, key, value, e);
 size++;
 }

final Entry<K,V> removeEntryForKey(Object key) {
 if (size == 0) {
  return null;
 }
 //根据key计算hash值，获取索引
 int hash = (key == null) ? 0 : hash(key);
 int i = indexFor(hash, table.length);
 //链表的删除，定义两个指针，pre表示前驱
 Entry<K,V> prev = table[i];
 Entry<K,V> e = prev;
 //遍历冲突链，删除所有为key的Enrty
 while (e != null) {
  Entry<K,V> next = e.next;
  Object k;
  //找到了
  if (e.hash == hash &&
  ((k = e.key) == key || (key != null && key.equals(k)))) {
  modCount++;
  size--;
  //找到第一个结点就是要删除的结点
  if (prev == e)
   table[i] = next;
  else
   prev.next = next;
  e.recordRemoval(this);
  return e;
  }
  prev = e;
  e = next;
 }
 return e;
 }

 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).
 //为了降低冲突的几率
 h ^= (h >>> 20) ^ (h >>> 12);
 return h ^ (h >>> 7) ^ (h >>> 4);
 }

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);
 }

 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
 static final int MAXIMUM_CAPACITY = 1 << 30;
 static final float DEFAULT_LOAD_FACTOR = 0.75f;

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;
 threshold = initialCapacity;
 init();
 }

private void inflateTable(int toSize) {
 // Find a power of 2 >= toSize
 int capacity = roundUpToPowerOf2(toSize);
 threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
 table = new Entry[capacity];
 initHashSeedAsNeeded(capacity);
 }

private static int roundUpToPowerOf2(int number) {
 // assert number >= 0 : "number must be non-negative";
 return number >= MAXIMUM_CAPACITY
  ? MAXIMUM_CAPACITY
  : (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1;
 }

void resize(int newCapacity) {
 Entry[] oldTable = table;
 int oldCapacity = oldTable.length;
 //哈希表已达到最大容量，1 << 30
 if (oldCapacity == MAXIMUM_CAPACITY) {
  threshold = Integer.MAX_VALUE;
  return;
 }
 Entry[] newTable = new Entry[newCapacity];
 //将oldTable中的Entry转移到newTable中
 //initHashSeedAsNeeded的返回值决定是否重新计算hash值
 transfer(newTable, initHashSeedAsNeeded(newCapacity));
 table = newTable;
 //重新计算threshold
 threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
 }
void transfer(Entry[] newTable, boolean rehash) {
 int newCapacity = newTable.length;
 //遍历oldTable
 for (Entry<K,V> e : table) {
  //遍历冲突链
  while(null != e) {
  Entry<K,V> next = e.next;
  if (rehash) {
   //重新计算hash值
   e.hash = null == e.key ? 0 : hash(e.key);
  }
  int i = indexFor(e.hash, newCapacity);
  //将元素插入到头部，头插法
  e.next = newTable[i];
  newTable[i] = e;
  e = next;
  }
 }
 }

private abstract class HashIterator<E> implements Iterator<E> {
 Entry<K,V> next; // next entry to return
 int expectedModCount; // For fast-fail
 int index;  // current slot，table index
 Entry<K,V> current; // current entry
 HashIterator() {
  expectedModCount = modCount;
  //在哈希表中找到第一个Entry
  if (size > 0) { 
  Entry[] t = table;
  while (index < t.length && (next = t[index++]) == null)
   ;
  }
 }
 public final boolean hasNext() {
  return next != null;
 }
 final Entry<K,V> nextEntry() {
  //HashMap是非线程安全的，遍历时仍然先判断是否有表结构的修改
  if (modCount != expectedModCount)
  throw new ConcurrentModificationException();
  Entry<K,V> e = next;
  if (e == null)
  throw new NoSuchElementException();
  if ((next = e.next) == null) {
  //找到下一个Entry
  Entry[] t = table;
  while (index < t.length && (next = t[index++]) == null)
   ;
  }
  current = e;
  return e;
 }
 public void remove() {
  if (current == null)
  throw new IllegalStateException();
  if (modCount != expectedModCount)
  throw new ConcurrentModificationException();
  Object k = current.key;
  current = null;
  HashMap.this.removeEntryForKey(k);
  expectedModCount = modCount;
 }
 }