哈希对 HashSet 是如何工作的？

Question

I have just read and understood how hashing works with HashMap: how it uses a hashtable, makes hashcodes with a hash function etc.

My question is: If the same principles apply to HashSet? Well obviously it cannot compute a hashcode from HashSet's key, because it doesn't have one. But does it compute the hashcode from the value of the HashSet then?

Answer 1

Actually, a HashSet is backed by HashMap that maps all its keys to a single constant object. The key-set of this map acts as the set. See for yourself by looking at the HashSet source . So yes, the mechanisms by which the two operate are very similar.

Answer 2

Iс the same principles for HashSet's?

Yes, HashSet uses the same exact general principle: the keys inserted into the set are arranged according to their hash codes, using the equals method to resolve collisions.

Obviously, it cannot compute a hashcode from HashSet 's key, because it doesn't have one

Of course it has, and it can! In fact, hash keys is all the HashSet has -- just keys, no values.

In a HashMap the "value" is passive: that's something stored at the key; hash maps never look at values; in particular, maps never check values for equality, or compute their hash code. You can think of a hash set as a hash map that maps everything to null .

Answer 3

HashSet uses HashMap internally to store it's objects. Whenever you create a HashSet object, one HashMap object associated with it is also created. This HashMap object is used to store the elements you enter in the HashSet. The elements you add into HashSet are stored as keys of this HashMap object. The value associated with those keys will be a constant. Whenever you insert an element into HashSet using add() method, it actually creates an entry in the internally backing HashMap object with element you have specified as it's key and constant called “PRESENT” as it's value. This “PRESENT” is defined in the HashSet class as below.

// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();
public boolean add(E e)
{
        return map.put(e, PRESENT)==null;
}

You can notice that, add() method of HashSet class internally calls put() method of backing HashMap object by passing the element you have specified as a key and constant “PRESENT” as it's value.