Iterating over sets of sets

Question

I am attempting to write a program that iterates over a set of sets. In the example code below, I am getting an error that stating that iter.next() is of type object rather than a set of strings. I am having some other more mysterious issues with iterating over sets of sets as well. Any suggestions?

            Set<String> dogs= new HashSet<String>();
            dogs.add("Irish Setter");
            dogs.add("Poodle");
            dogs.add("Pug");
            dogs.add("Beagle");

            Set<String> cats = new HashSet<String>();
            cats.add("Himalayan");
            cats.add("Persian");

            Set<Set<String>> allAnimals = new HashSet<Set<String>>();                   
            allAnimals.add(cats);
            allAnimals.add(dogs);

            Iterator iter = allAnimals.iterator();            
            System.out.println(allAnimals.size());

            while (iter.hasNext()) 
            {
                System.out.println(iter.next().size());
            }   

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A related question with the same setup (minus the loop).
The code fragment below results in a final output that includes tildes. But I don't want to change allAnimals as I go! How can I edit extension without affecting the larger set (allAnimals).

 for (Set<String> extension : allAnimals) 
                {
                    System.out.println("Set size: " + extension.size());
                    extension.add("~");     
                    System.out.println(extension);
                }   

                System.out.println(allAnimals);

Answer 1

Your allAnimals variable is of type Set<Set<String>>, however, when you ask its Iterator you "forget" the type information. According to the compiler, your iterator just contains Objects. Change the line where you get the Iterator to this

Iterator<Set<String>> iter = allAnimals.iterator();

and all should be fine.

Answer 2

Use an enhanced for loop for traversing the sets, is easier than using an iterator:

for (Set<String> names : allAnimals) {
    System.out.println(names.size());
}

For example, to traverse all the animal's names:

for (Set<String> names : allAnimals) {
    for (String name : names) {
        System.out.println(name);
    }
}

Answer 3

You do not mention the type on which your iterator is defined. So as far as it is concerned it expects an object as next.

Answer 4

I would just use a (nested) foreach loop:

for(Set<String> animals : allAnimals) {
    int size = animals.size(); // if you want it
    for (String animal : animals) {
        // do something with the name
    }
}