Java iterator pattern example 2017-01-16 01:02

The best example of iterator pattern are source code of util package in JDK. If you want to learn more, I recommend you read the source code of JDK. (Including but not limited to:Iterator, ArrayList and HashMap). In this page I will make an easy example to show how iterator works. The structure of classes is like following.

Aggregate and ArrayAggregate are like following.

public interface Aggregate<T> {
    Iterator<T> createIterator();
}

public class ArrayAggregate<T> implements Aggregate<T> {

    private T[] items;

    public ArrayAggregate(T[] items) {
        this.items = items;
    }

    @Override
    public Iterator<T> createIterator() {
        return new ArrayIterator<>(this);
    }

    public T[] allItems() {
        return items;
    }
}

To make it clear I don't put iterator into ArrayAggregate. You can move the implement of iterator into aggregate like JDK does.

public interface Iterator<T> {
    T next();
    boolean hasNext();
}

public class ArrayIterator<T> implements Iterator<T> {
    private int index;
    private T[] items;

    public ArrayIterator(ArrayAggregate<T> tArrayAggregate) {
        items = tArrayAggregate.allItems();
    }

    @Override
    public T next() {
        return items[index++];
    }

    @Override
    public boolean hasNext() {
        return index <= items.length - 1;
    }
}

Use iterator like following.

public class Client {
    public static void main(String[] args) {
        ArrayAggregate<Integer> arrayAggregate = new ArrayAggregate<>(new Integer[]{1, 2, 3});
        Iterator<Integer> iterator = arrayAggregate.createIterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }
    }
}

The key of iterator pattern is "Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation."