can't get access to ArrayLists of ArrayList (generic)

Question

I wrote class that represent a Set. in the Set i have a field "elements" of ArrayList. all the methods in the class are generics.

first method i wrote is a method that get another Set as argument and returns new Set that hold the union between this.Set to the argument. here is the method:

   public Set<E> union(Set<E> s){
      Set<E> toReturn = new Set<>();

      for(E toAdd : this.elements) {
        toReturn.addToSet(toAdd);
      }
      for(E toAdd : s.elements)
        toReturn.addToSet(toAdd);

      return toReturn;
   }

Now i want to write another method that get as argument ArrayList> and returns new Set that represent the union between this.Set to all the Sets inside the argument ArrayList. but i can't get access to the Sets inside the ArrayList. here is my code:

   public Set<E> union(ArrayList<Set<E>> s) {
      Set<E> toReturn = new Set<>();

      for(E el : s) {
               toReturn.addToSet(s.union(el));
      }

      return toReturn;
   }

The compiler tell me "The method union(E) is undefined for the type ArrayList>.

I'll be happy for some help. Thanks

Why do you pass in an ArrayList of a Set<E>? Why not simply pass in a List<E>? — sekky
– sekky, Commented May 12, 2019 at 18:44
Share the code of the class with the attributs to avoid misunderstanding please — azro
– azro, Commented May 12, 2019 at 18:55
Are you sure you are no confusing java.util.ArrayList and java.util.Set with your code ? — NoDataFound
– NoDataFound, Commented May 12, 2019 at 18:56
Did you name your class Set ? You should never use a build-in name — azro
– azro, Commented May 12, 2019 at 18:59
@azro Since the class that the OP designed represents exactly a Set, Set should be its name. — crow
– crow, Commented May 14, 2019 at 3:45

Jeremy Then · Accepted Answer · 2019-05-12 19:38:06Z

1

You are using the wrong variable in the for loop, s is the ArrayList, and toReturn is the set that you want to calculate the union of, with the addAll(otherSet); method. You may be using the Guava's Set library if so, you can keep using the union(otherSet) method.

Where you have:

for(E el : s) {
               toReturn.addToSet(s.union(el));
      }

Better put:

for(Set<E> el : s) {
               toReturn.addToSet(toReturn.union(el));
      }

edited May 12, 2019 at 19:38

answered May 12, 2019 at 18:47

Jeremy Then

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2 Comments

Ohadyk Over a year ago

yes i called my class "Set" because i have to realize the Set class. i changed the line to "toReturn.addToSet(toReturn.addAll(el));" but its still tell me "the method addAll(int) is undefined for the type Set<E>. I think i have to add cast to "toReturn". but i dont know how...

Jeremy Then Over a year ago

Then try: for(Set<E> el : s) {... So the compiler knows that el is a set.

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