I have solved the task, but I don't understand the difference between 2 lines of code

Question about the task

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Hello, so the point was to finish the method that converts a map to a set. Initially I tried to initiate a Set and then put the values of the map in the set, like this:

Set<Cat> catSet = new HashSet<>();
catSet = map.values();

However, this would prompt an error that I am trying to put Collections (if I remember correctly) into a Set object. So then, after a bit of research, I saw the following code:

Set<Cat> catSet = new HashSet<>(map.values());

I don't understand the difference between the two types of code. Can someone please explain? Thank you!

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Nouser

Level 36 , Germany

16 November 2020, 09:28

useful

values() returns a Collection object that holds all values of a map and is backed by this map. Delete an entry in that Collection view and it's gone in the map as well. Read more here. If you give the values() object to a HashSet constructor (it has a matching one: HashSet(Collection<? extends E> c)), then a copy of that Collection view is created. The set is independent of the map.

Andrei

Level 41

16 November 2020, 11:34

OK, let me see if I understood this correctly. In the first type of code, I am basically putting a reference of map's values in the catSet. So any value modified from the catSet will be reflected in the values of the map. Here comes a question from this: why is there a problem if values are modified from this reference, existing in the catSet? Is it because of this: "If the map is modified while an iteration over the collection is in progress (except through the iterator's own remove operation), the results of the iteration are undefined." ? Meaning, the program does not want to risk throwing an error if I am to iterate over this collection in the future? Then, for your second paragraph, you are saying that by putting the map.values() as the argument in the creation of the new set, it creates a new individual set of those values, which, if modified, would not affect the original values of the map. Did I understand correctly?

Nouser

Level 36 , Germany

16 November 2020, 11:58

useful

Collection set = map.values();

Here the set is using a part of the map object, the values. both objects, map and set share the same data.

Set newSet = new Hashset(map.values());

A new set is created and the Collection view of map values is copied into newSet, meaning a new object with an independent dataset is created. Here comes a question from this: why is there a problem if values are modified from this reference, existing in the catSet? There is no problem if this is what you want. And yes, if you remove elements from a collection in a loop, this may lead to problems. Having dependent collections may cause more confusion. On the other side this can be very useful. Did I understand correctly? I'd say yes ;)

Andrei

Level 41

16 November 2020, 12:49

OK, I understand better now. :) What is still unclear though, is why the error? Because that is what I want but the compiler throws an error. :-/

Nouser

Level 36 , Germany

16 November 2020, 13:28

useful

Answered in my first sentence ;) ... cause the values() method returns a Collection object and not a Set. You'd have to cast to Set like:

Set<Cat> catSet =  (HashSet<Cat>) map.values();

Andrei

Level 41

16 November 2020, 13:38

Got it now, thank you! :)