~8:53 why we are able to add List to Set?! allHashTags.addAll(treet.getHashTags());

Hi Andre... It's me again, I hope you are not getting bored with me... (just kidding)

Why are we able to add a List into a Set? Well the simplest answer I can give you is because they were part of Java Collection Framework. Both Set and List have the same superinterfaces: Collection<E>, Iteratable<E> (by <E> is a generic name for element of any specific object types).

But Set and List as you may have understand by watching the course video handle the collections in different manners. All classes that implement List interface allows same object to be inputted inside any kind of List more than once. Thus it will create multiple equal (true equal) objects inside a List. On the other hand Set (all kind of Set) handles collection so that no such multiple equals objects can happen. So all objects inside a Set will be unique to one another. Therefore when Craig wanted to see what hashtags is available in a tweet he uses Set because he did not need to see the same hashtags twice if the writer of that tweet accidentally doing so.

It is better if I show you with some code I guess: I wrote this code in Treehouse workspace so you can implemented it right away. It shows how we can directly transfer a List (in this case arrayList) to Sets (in this case HashSet and TreeSet):

import java.util.List;
import java.util.ArrayList;
import java.util.Set;
import java.util.HashSet;
import java.util.TreeSet;

public class TreeStory {
  public static void main(String[] args) {
    //now we making a list of fruits:
    List<String> stringOrder = new ArrayList<>();
    stringOrder.add("Guava");
    stringOrder.add("Apple");
    stringOrder.add("Grapes");
    stringOrder.add("Banana");
    stringOrder.add("Guava");
    System.out.println("This is printed using ArrayList:");
    for(String element : stringOrder){

      System.out.printf("element: %s%n", element);
    }

    //now we want to convert that List (arrayList) to be a Set specifically HashSet:
    Set<String> stringHash = new HashSet<>(stringOrder);
    System.out.println("This is printed using HashSet:");
    for(String element : stringHash){

      System.out.printf("element: %s%n", element);
    }

    //now we use convert ArrayList into TresSet:
    Set<String> stringTree = new TreeSet<>(stringOrder);
    System.out.println("This is printed using TreeSet:");
    for(String element : stringTree){

      System.out.printf("element: %s%n", element);
    }
  }

}

As you can see I just directly input an ArrayList stringOrder into HashSet and TreeSet. By now I guess you also know that HashSet will only sensor the duplicated objects while TreeSet will also sort them out alphabetically. So if you compile and run that code in Treehouse workspace the result will be like this:

Picked up JAVA_TOOL_OPTIONS: -Xmx128m

Picked up _JAVA_OPTIONS: -Xmx128m

Picked up JAVA_TOOL_OPTIONS: -Xmx128m

Picked up _JAVA_OPTIONS: -Xmx128m

This is printed using ArrayList:

element: Guava

element: Apple

element: Grapes

element: Banana

element: Guava

This is printed using HashSet:

element: Guava

element: Apple

element: Grapes

element: Banana

This is printed using TreeSet:

element: Apple

element: Banana

element: Grapes

element: Guava

As you can see when I made ArrayList of fruits there are two "Guava"s there. But when I passed that ArrayList called stringOrder to HashSet it has no duplicated "Guava". Then when I transfer it to TreeSet I have printed all fruits in alphabetical order. I did all of that without repeating myself inputting objects over and over again to build a Set (HashSet and TreeSet). This is one of the benefit of implementing interfaces since you can implement different approach but still accessible from another class that implements the same super interface.

I hope this can help you a little. I am sorry if it just makes you even confused. But please keep on going there are still a lot ahead of you and Java is a good place to learn Object Oriented Programming. Best of luck and happy coding