Iteration

We saw that Java allows us to iterate through Lists using a convenient shorthand syntax sometimes called the “foreach” or “enhanced for” loop.

For example,

List<Integer> friends =
new ArrayList<Integer>();
friends.add(5);
friends.add(23);
friends.add(42);
for (int x : friends) {
    System.out.println(x);
}

Let’s strip away the magic so we can build our own classes that support this.

The key here is an object called an iterator.

For our example, in List.java we might define an iterator() method that returns an iterator object.

public Iterator<E> iterator();

Now, we can use that object to loop through all the entries in our list:

List<Integer> friends = new ArrayList<Integer>();
...
Iterator<Integer> seer = friends.iterator();

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

This code behaves identically to the foreach loop version above.

There are three key methods in our iterator approach:

First, we get a new iterator object with Iterator<Integer> seer = friends.iterator();

Next, we loop through the list with our while loop. We check that there are still items left with seer.hasNext(), which will return true if there are unseen items remaining, and false if all items have been processed.

Last, seer.next() does two things at once. It returns the next element of the list, and here we print it out. It also advances the iterator by one item. In this way, the iterator will only inspect each item once.

Implementing Iterators

In this section, we are going to talk about how to build a class to support iteration.

Let's start by thinking about what the compiler need to know in order to successfully compile the following iterator example:

List<Integer> friends = new ArrayList<Integer>();
Iterator<Integer> seer = friends.iterator();

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

We can look at the static types of each object that calls a relevant method. friends is a List, on which iterator() is called, so we must ask:

  • Does the List interface have an iterator() method?

seer is an Iterator, on which hasNext() and next() are called, so we must ask:

  • Does the Iterator interface have next/hasNext() methods?

So how do we implement these requirements?

The List interface extends the Iterable interface, inheriting the abstract iterator() method. (Actually, List extends Collection which extends Iterable, but it's easier to codethink of this way to start.)

public interface Iterable<T> {
    Iterator<T> iterator();
}
public interface List<T> extends Iterable<T>{
...
}

Next, the compiler checks that Iterators have hasNext() and next(). The Iterator interface specifies these abstract methods explicitly:

package java.util;
public interface Iterator<T> {
    boolean hasNext();
    T next();
}

Specific classes will implement their own iteration behaviors for the interface methods. Let's look at an example. (Note: if you want to build this up from the start, follow along with the live coding in the video.)

We are going to add iteration through keys to our ArrayMap class. First, we write a new class called KeyIterator, nested inside of ArrayMap:

public class KeyIterator {
    private int ptr;
    public KeyIterator() {
        ptr = 0;
    }

    public boolean hasNext() {
        return (ptr != size);
    }

    public K next() {
        K returnItem = keys[ptr];
        ptr = ptr + 1;
        return returnItem;
    }
}

This KeyIterator implements a hasNext() method, and a next() method, using a ptr variable to keep track of its position in the array of keys. For a different data structure, we might implement these two methods differently.

Thought Excercise: How would you design hasNext() and next() for a linked list?

Now that we have the appropriate methods, we can use a KeyIterator to iterate through an ArrayMap:

ArrayMap<String, Integer> am = new ArrayMap<String, Integer>();
am.put("hello", 5);
am.put("syrups", 10);
ArrayMap.KeyIterator ami = am.new KeyIterator();

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

There's an interesting line in the code above: am.new KeyIterator();. This construction allows us to instantiate a non-static nested class, meaning a class that needs to be associated with a particular instance of the enclosing class. It wouldn't make sense to have a KeyIterator not associated with a particular ArrayMap -- what would it iterate through? So, we must use dot notation with a specific ArrayMap instance to create a new KeyIterator associated with that instance.

Now we have a KeyIterator, and it can loop through an ArrayMap. We still want to be able to support the enhanced for loop, though, to make our calls cleaner. So, we need to make ArrayMap implement the Iterable interface. The essential method of the Iterable interface is iterator(), which returns an Iterator object for that class:

public class ArrayMap<K, V> implements Iterable<K> 
    @Override
    public Iterator<T> iterator() { 
        return new KeyIterator();
    }
}

We override that iterator() method to return the KeyIterator that we just wrote.

There's one more step before the code will compile: we have to tell Java that KeyIterator is an Iterator. To make that happen, KeyIterator must implement Iterator. This way, we can return a KeyIterator in our iterator() method above, and successfully implement the Iterator methods:

public class KeyIterator implements Iterator<K> {
    private int ptr;
    public KeyIterator() { ptr = 0; }
    public boolean hasNext() { return (ptr != size); } 
    public K next() { ... }
}

Now we can use our enhanced for loop construction with an ArrayMap:

ArrayMap<String, Integer> am = new ArrayMap<String, Integer>();
for (String s : am) {
    System.out.println(s);
}

Here we've seen Iterable, the interface that makes a class able to be iterated on, and requires the method iterator(), which returns an Iterator object. And we've seen Iterator, the interface that defines the object with methods to actually do that iteration. You can think of an Iterator as a machine that you put onto an iterable that facilitates the iteration. Any iterable is the object on which the iterator is performing.

With these two components, you can make fancy for loops for your classes!

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