Iterable and Iterator

Solution

Bag<T> must implement Iterable<T>.

Solution

Iterator<String> it = words.iterator();
while (it.hasNext()) {
  String w = it.next();
  System.out.println(w);
}

Solution

A data structure implements Iterable<T> to say it can be iterated over. Its iterator() method returns an Iterator<T>. An Iterator<T> is the object that returns the elements one at a time, through hasNext() and next().

Exercise

nums is a Set<Integer>. This code prints every ordered pair of its elements:

for (Integer a : nums) {
  for (Integer b : nums) {
    System.out.println(a + ", " + b);
  }
}

The inner loop runs through the whole set once for each element of the outer loop. Why does that not disturb the outer loop’s progress through the set?

Solution

Each loop calls nums.iterator() and gets its own iterator, with its own position. The inner loop advances only its own iterator, so the outer iterator’s position does not change.