Iterators

The enhanced for loop, or the for each loop, allows us to easily iterate over objects in some data structure without having to index each explicitly:

Set<Integer> javaset = new HashSet<>();
javaset.add(5);
javaset.add(23);
javaset.add(42);
for (int i : javaset) {
    System.out.println(i);
}

However, this doesn’t work by default on any of our own, custom classes. Let’s strip away the magic so we can build our own classes that also support this enhanced for loop.

The enhanced for loop uses an iterator that is obtained by calling a .iterator() method on the object, which should return an object of type Iterator<T>. The Iterator interface contains the hasNext method, which tells us if there are more values, and next, which gets the next value:

Set<Integer> javaset = new HashSet<>();
javaset.add(5);
javaset.add(23);
javaset.add(42);
Iterator<Integer> seer = javaset.iterator();
while (seer.hasNext()) {
    System.out.println(seer.next());
}