为什么需要迭代Java中的HashSet？

Question

I understand that in Java, the HashMap class implements the Map interface and the HashSet class implements the Set interface.

I also understand that both those classes are implementations of a hash table.

That said, why is it necessary to iterate through a HashSet while a HashMap has a get() method? Isn't the point of having a hash table being able to fetch an element through a value associated to it?

I've looked up the docs and done some searching and could not find an answer.

Thank you in advance.

Answer 1

A get() method of HashSet (or any Set implementation) would be redundant, since if set.contains(x) returns true , you already have a reference ( x ) to an instance which is deemed equal to the corresponding element of the Set .

In the Map interface you need map.get(key) in order to obtain the value associated with the key. In the Set interface, there is only the key, with no (meaningful) value associated to it.

I also understand that both those classes are implementations of a hash table.

If by that statement you meant that HashSet is implemented with a backing HashMap instance, that's just an implementation detail. The values corresponding to the keys of this backing HashMap are all references to the same dummy object. There's no reason to ever call get() for the backing HashMap .

Answer 2

I also understand that both those classes are implementations of a hash table.

I think you're guilty of assuming too much about the implementation. As with all objects, the key is the public interface the designer chooses to expose to you.

The Set interface does not include a get method to access individual objects directly. The semantics are different from Map , by design.

Answer 3

I think there is no get method because, hashset are internally backed by hashMap and all your values of set are actually stored as keys of this HashMap and values is just a constant field called "PRESENT". So if you have a get() it will cause a problem.

Please refer below link for more details http://www.java67.com/2014/01/how-hashset-is-implemented-or-works-internally-java.html