Hashmap and how this works behind the scene [duplicate]

问题

quick question to be sure I understood well how HashMap in java works.

Here is an example of code:

//String key = new String("key");
//String val = new String("value");
String key = "key";
String val = "value";

HashMap map = new HashMap();
map.put(key, val);

System.out.println("hashmap object created. Its key hashcode = "+key.hashCode());
// the hashcode is 106079
System.out.println("hashmap object value for key = "+map.get(key));

// Let's store using a key with same hashcode
Integer intkey = new Integer(106079);
val = "value2";
map.put(intkey, val);
System.out.println("hashmap object created. Its intkey hashcode = "+intkey.hashCode());
// this returns 106079 once again. So both key and intkey have the same hashcode

// Let's get the values
System.out.println("hashmap object value for intkey = "+map.get(intkey));
System.out.println("hashmap object value for key = "+map.get(key));

the returned values are as expected. I read that behind the scene, HashMap works as follows:

get the key/value.
make a hashcode from the key.
store in bucket the key and value objects (in my case bucket number 106079)

same again for the second one.

stores it within the same bucket but as this is at this point a LinkedList, I suppose store it at "next available allocation".

To get it:

pick up the key, get hashcode,
look at the bucket,
then look at the key from the first element in the LinkedList,
then check if key passed and key from element match, if not then continue to the next, and so on until value can be retrieved.

Am I understanding this concept correctly?

Many thanks!

EDIT:

many thanks, so to complete: - How does Java HashMap store entries internally - How does a Java HashMap handle different objects with the same hash code?

And:

There shouldn't be that many "buckets"
A bucket is not the same as an entry. A bucket is a logical collection of all entries sharing the same bucket# (which in the Java case is a function of the key's hashCode()). As stated in other answers, bucket overflow could be implemented several ways, not necessarily in a List.
There are other existing collision resolutions: http://en.wikipedia.org/wiki/Hash_table#Collision_resolution
it uses Object's equals method to compare and retrieve objects from the same bucket.

回答1:

Please also note, there are several ways HashMap can implement hash codes collision resolution, not only utilizing linked list as you mentioned

Java's HashMap implementation does not only use LinkedList strategy to handle key-values with same key.hashCode() values.

Also, you may want to read this article

回答2:

Yes, your understanding is correct. Just note that a single bucket is assigned many hashcodes: in a fresh HashMap there are altogether 16 buckets, each assigned a total of 2³²/16 = 2²⁸ hashcodes.

回答3:

Your understandings are OK, but take into account that there are several implementations. The actual hashcode HashMap uses to store the value may not be 106079. Here's one implementation (java-6-openjdk):

public V put(K key, V value) {
    if (key == null)
        return putForNullKey(value);
    int hash = hash(key.hashCode());
    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;
}

Notice the hash method, which consists of the following:

/**
 * Applies a supplemental hash function to a given hashCode, which
 * defends against poor quality hash functions.  This is critical
 * because HashMap uses power-of-two length hash tables, that
 * otherwise encounter collisions for hashCodes that do not differ
 * in lower bits. Note: Null keys always map to hash 0, thus index 0.
 */
static int hash(int h) {
    // 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);
}

So in this JVM's example, it does not use 106079 as a hash, since HashMap recreates a hash to 'harden' it.

I hope that helps

来源：https://stackoverflow.com/questions/19644626/hashmap-and-how-this-works-behind-the-scene

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