How do I implement Iterable in a class that has an ArrayList

Question

I am going to have a class that has a ArrayList people field I want clients of this class to be able to iterate over those objects. How do I do this? Do I just return people.iterator(); ?

for example, in the client I want to be able to do this:

for( Person p : people ) {
    // do something
}

Is this what I need in my class?

public class People  implements Iterable<Person> {

private ArrayList<Person> people;

@Override
public Iterator<Person> iterator() {
    // TODO Auto-generated method stub
    return people.iterator();
}

Answer 1

If you want to prevent modification to the list itself via the iterator, just delegate iterator() to an unmodifiable list like this:

public class People implements Iterable<Person> {

    private ArrayList<Person> people;

    @Override
    public Iterator<Person> iterator() {
        return Collections.unmodifiableList(people).iterator();
    }

}

Otherwise, what you have is fine.

Note, however, that unless Person is immutable, there is nothing you can do to prevent a caller from modifying the Person objects themselves. If that's OK, then no big deal. If that's a problem, then you could create a deep copy of people and return an iterator to that.

If you want the returned iterator to reflect modifications to the Person s but not to allow changing of the Person s, or if a deep copy is too expensive or unnecessary, you could create an ImmutablePerson that extends Person and wraps a Person but does not allow modification via setters, and return a list of those wrapped around the actual Person objects.

Answer 2

Yes. The enhanced for loop requires an array or Iterable . Iterable is required to have iterator( that returns a reasonable iterator that has useful meaning in regard to your data.

You can now use the enhanced for loop and it is somewhat semantically correct. However, Iterator provides remove( so anyone using your iterator can remove things from your people list, which may not be what you want. Perhaps using new ArrayList(people).iterator() or Collctions.unmodifiableList(people).iterator() can work as you copy the list and get the iterator for it .

You can also provide getPeople by creating a getter that creates an ImmutableList or just a copy of your internal arrayList.

On another note, why not extend ArrayList if you need extra functionality? I haven't seen exactly what you're doing with it so I can only guess.

Answer 3

Simply provide a getter for your People list:

public ArrayList<Person> get getPeople() {
   return people;
}