Question

I am just grasping collections and found that the add() methods actually deep copy objects into the collections. Should they be called something like “copyInto” ?

import java.util.*;
class Test {
    public static void main(String[] args) {

        Set <Frog> setOfFrogs = new HashSet <Frog>();

        Frog frog1 = new Frog("Kermit");
        Frog frog2 = new Frog("Bob");

        setOfFrogs.add(frog1);
        setOfFrogs.add(frog2);

        frog1.name = "May";
        frog2.name = "Mary";

        for (Frog eachFrog : setOfFrogs){
            System.out.println(eachFrog.name);
        }
    }
}

This prints: “May” and “Mary”.

Sorry all it is a shallow copy. I need to get this “by reference” business into my thick head…

Answer 1

.add(..) simply adds the reference to the object in the collection. It does no deep copying. For example, if you change the object, the change will be reflected in the collection.