HashMap和HashTable底层原理以及常见面试题

/**
The table, resized as necessary. Length MUST Always be a power of two.
**/
transient Entry[] table;
static class Entry<K,V> implements Map.Entry<K,V>{
   final K key;
   V value;
    Entry<K,V> next;
    final int hash;
    ...
 }

public V put(K key, V value) {
 //HashMap允许存放null键和null值。
 //当key为nu11时， 调用putForNullKey方法，将valve放置在数组第一个位置。
 if (key = null)
 return putForNullKey(value);
 //根据key的keyCode重新计算hash值。
 int hash = hash(key .hashCode());
 //搜索指定hash值在对应table中的索
 int i = indexFor(hash, table . length);
 //如果i索引处的Entry不为null,通过循环不断遍历e元素的下一个元素。
 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;
  }
 }
 //如果读引处的Entry为null，表明此处还没有Entry
 omodCount++;
 //将key、 value添加到索引处。
 addEntry(hash, key, value, i);
 return null;
}

void addEntry(int hash, K key, V value, int bucketIndex) {
 //获取指定bucketIndex 索引处的Entry
 Entry<K,V> e = table[bucketIndex];
 //将新创健的Entry 放入bucketIndex索引处，并让新的Entry 指向原来的Entry
 table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
 //如果Hap中的key-value对的数里超过了极限
 if (size++ >= threshold)
 //把table对象的长度扩充到原理的2倍
  resize(2*table.length);
}

 void resize(int newCapacity) {
    Entry[] oldTable = table;
    int oldCapacity = oldTable.length;
    if (oldCapacity == MAXIMUM_CAPACITY) {
      threshold = Integer.MAX_VALUE;
      return;
    }
    Entry[] newTable = new Entry[newCapacity];
    transfer(newTable, initHashSeedAsNeeded(newCapacity));
    table = newTable;
    threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
  }

void transfer(Entry[] newTable, boolean rehash) {
    int newCapacity = newTable.length;
    for (Entry<K,V> e : table) {
      while(null != e) {
        Entry<K,V> next = e.next;
        if (rehash) {
          e.hash = null == e.key ? 0 : hash(e.key);
        }
        int i = indexFor(e.hash, newCapacity);
        e.next = newTable[i];
        newTable[i] = e;
        e = next;
      }
    }
  }

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 int hash(int h){
 h ^ =(h>>>20)^ (h>>>12);
 return h ^ (h>>>7) ^ (h>>>4);
}

public V get(object key) {
   if (key = null)
   return getForNullKey();
   int hash ”hash(key.hashCode());
   for (Entry<K,V> e = table[indexFor(hash, table. length)];
    e!=null;
    e = e.next) {
    0bject k;
    if (e.hash=hash && ((k=e.key)==key II key.equals(k)){
    return e.value;
    }
   return null;
}

int hash = hash(key.hashcode0);
int i= indexFor(hash, table.length);
static int hash(Objectx) {
 int h = x.hashCode();
 h += ~(h<<9);h^= (h>>> 14);
 h+=(h<< 4);
 h ^= (h>>> 10);
 returm h;
}

static int indexFor(int h, int length) {
 return h & (length-1); //hashmap的表长永远是2^n。
}

int hash = key.hashCode(); //注意区分2者的hash值!! int index = (hash & 0x7FFFFFFF) % tab.length;