how to remove a non-string object from a set?

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I'm having difficulty getting very far with this task. All of the previous examples for removing an element from a set (and also every single result that comes up in Google searches) only uses string objects, which allows you to do:

set.add("string1");
set.remove("string1");

And importantly, you can add the string in one method or constructor or wherever, and set.remove("string1") will work anywhere after that. But if I am creating other objects and storing those in the set, how do I refer to them outside of the function they're created in? createCats is where the Cat objects are created and stored in the HashSet which is returned. For example:

Cat cat1 = new Cat();
cats.add(cat1);

But when it comes time to remove something from cats in main, this does not work:

cats.remove(cat1);

because it no longer knows what cat1 is.

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Guadalupe Gagnon

Level 37 , Tampa, United States

14 July 2021, 15:57

Removing 1 object from a set is very simple. You are not able to access any of the individual contained objects without having a reference to the object in the method, as you have realized in main; however you can very easily use a loop to loop through the items in the list, save a reference to one of them, then remove that reference. I solved this simply by creating an iterator, calling iterator.next(), then iterator.remove(). Do make sure to check that the Set actually contains objects before doing this or you will get an exception thrown. This works for this task because it doesn't say to remove a specific Cat object, just any single Cat object witll do.

Thomas

Level 41 , Bayreuth, Germany

14 July 2021, 15:34

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

class Dog {
    String name;

    public Dog(String name) { this.name = name; }

    @Override
    public String toString() { return "{" + name + "}"; }
}

public class Solution {
    public static void main(String[] args) {
        Set<Dog> dogs = new HashSet<>();
        dogs.add(new Dog("Butcher"));
        dogs.add(new Dog("Cooper"));
        dogs.add(new Dog("Daisy"));

        System.out.println(dogs);

        Iterator<Dog> it = dogs.iterator();
        while (it.hasNext())
            if (it.next().name.equals("Daisy"))
                it.remove();
        System.out.println(dogs);

        dogs.removeIf(e -> e.name.equals("Cooper"));
        System.out.println(dogs);
    }
}

Justin Smith

Level 41 , Greenfield, USA, United States

14 July 2021, 15:56

I thought about assigning a name variable to the Cat class and doing something like this. But they don't tell you to assign any variables in the Cat constructor, and I'm pretty familiar at this point with failing validation for doing things they don't ask you to do. I'm assuming they just want an empty constructor like:

public class Cat
{
    public Cat()
    {
    }
}

Guadalupe Gagnon

Level 37 , Tampa, United States

14 July 2021, 16:04

solution

Using Thomas's example, this is what I described in my above comment:

public static void main(String[] args) {
   Set<Dog> dogs = new HashSet<>();
   dogs.add(new Dog("Butcher"));
   dogs.add(new Dog("Cooper"));
   dogs.add(new Dog("Daisy"));
   System.out.println("Before removing: " + dogs);

   // method 1: get first dog object through loop
   for (Dog x: dogs) {
      dogs.remove(x); // remove object
      break; // loop only runs 1 time
   }
   System.out.println("After removing 1 from for loop: " + dogs);

   // OR method 2: use an Iterator to remove 1 object
   Iterator<Dog> iter = dogs.iterator();
   if(iter.hasNext()){ // check if objects are in the Set first
      iter.next();
      iter.remove();
   } // removes first object
   System.out.println("After removing 1 from Iterator: " + dogs);
}

Either one of those methods will work to remove 1 item. In the first method it is very important that if you remove an item while looping over the Set that you break it immediately after doing so. Allowing the loop to continue will result in a ConcurrentModificationException being thrown.

Justin Smith

Level 41 , Greenfield, USA, United States

14 July 2021, 16:13

Thank you! Very interesting solution. I think I prefer the second method just because it seems more intuitive to use if rather than for when I know I only want it to find one element.

Guadalupe Gagnon

Level 37 , Tampa, United States

14 July 2021, 16:19

The second method is superior in my opinion too because it allows you to remove any amount of items from a Collection; using a for loop restricts you to only being able to remove only 1 item, after that if the loop continues to the next cycle an Exception will be thrown.

Thomas

Level 41 , Bayreuth, Germany

14 July 2021, 16:58

If you just want to remove any element of the list (and you do not mind the overhead) then you could create a temporary collection (here I use an ArrayDeque cause it has getFirst and getLast) - or peek, peekFirst depending on what implementation you prefer) and then get an element using that collection. The reference to that element you can use as parameter of the sets remove method.

dogs.remove(new ArrayDeque<>(dogs).getLast());