Is it incorrect work Collectors.toSet() in JDK 8?

Question

If I run this code:
public class Main {
    public static void main(String[] args) {
        Collection<String> set = new ArrayList<String>();
        set.add("b");
        set.add("a");
        set.add("c");
        set.add("c");
        set.stream().collect(Collectors.toSet());
        System.out.println(set);
I will get [b,a,c,c]. But if I right, previous code should be short form of expression:
public class Main {
    public static void main(String[] args) {
        Collection<String> set = new ArrayList<>();
        set.add("b");
        set.add("a");
        set.add("c");
        set.add("c");
        Collection<String> set2 = new HashSet<>(set);
        System.out.println(set2);
And here, I got [a,b,c]
So, in example 1. Is it incorrect result or I do something wrong?

Answer 1

But, now I think I was wrong and do not understand, when we need use Collectors.toSet(). Before that, I think the toSet() method, need for returned without duplicates and completing sorting form [a,b,c], and after I thought I will get [a,b,c] in ArrayList.
Thanks for explaining what you were confused about. Now it is very simple.
This line of code WILL produce a set and that set will NOT contain duplicates.
set.stream().collect(Collectors.toSet());
But you are NOT assigning that new set to any variable. If you want to replace the 'set' variable will the new set do an assignment:
set = set.stream().collect(Collectors.toSet());
To see the difference add a couple more lines to your code:
        Collection<String> set = new ArrayList<String>();
        Collection<String> set1 = new ArrayList<String>(); // new collection
        set.add("b");
        set.add("a");
        set.add("c");
        set.add("c");
        set.toArray();
        set.stream().collect(Collectors.toSet());
        set1 = set.stream().collect(Collectors.toSet());
        System.out.println(set);
        System.out.println(set1);
What does that second 'println' produce?
That code assigns the new set to a new collection.
Were you trying to follow and understand the Java Tutorials trail for 'The Set Interface'?
http://docs.oracle.com/javase/tutorial/collections/interfaces/set.html
The example there contains code just like yours:
Here's a simple but useful Set idiom. Suppose you have a Collection, c, and you want to create another Collection containing the same elements but with all duplicates eliminated. The following one-liner does the trick.
Collection<Type> noDups = new HashSet<Type>(c); 
It works by creating a Set (which, by definition, cannot contain duplicates), initially containing all the elements in c. It uses the standard conversion constructor described in the The Collection Interface section.
Or, if using JDK 8 or later, you could easily collect into a Set using aggregate operations:
c.stream().collect(Collectors.toSet()); // no duplicates 
Unfortunately that code has the same flaw yours does. It does, indeed, show that you can 'easily collect into a Set using aggregate operations'. But it doesn't capture that new set anywhere. That might give the impression that the 'c' collection somehow gets modified; but it doesn't.