Дан пример как flatMap ведет себя в stream API:

import java.util.Iterator;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class FlatMap {
    public static void main(String[] args) {
        List<List<Integer>> data = List.of(
                List.of(1, 2, 3),
                List.of(4, 5, 6),
                List.of(7, 8, 9)
        );
        List<Integer> flat =
                data.stream()
                        .flatMap(List::stream)
                        .collect(Collectors.toList());
        System.out.println(flat);
    }

}

Эта пример как ведет себя flatmap, в задании нужно создать подобное поведение.

Сама задача :

import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Collections;

public class FlatMap<T> implements Iterator<T> {
    private final Iterator<Iterator<T>> data;
    private Iterator<T> cursor = Collections.emptyIterator();
    
    public FlatMap(Iterator<Iterator<T>> data) {
        this.data = data;
    }

    @Override
    public boolean hasNext() {
        return false;
    }

    @Override
    public T next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        return cursor.next();
    }

    public static void main(String[] args) {
        Iterator<Iterator<Integer>> data = List.of(
                List.of(1, 2, 3).iterator(),
                List.of(4, 5, 6).iterator(),
                List.of(7, 8, 9).iterator()
        ).iterator();
        FlatMap<Integer> flat = new FlatMap<>(data);
        while (flat.hasNext()) {
            System.out.println(flat.next());
        }
    }
}

И Тесты на нее :

import static org.junit.Assert.assertThat;
import static org.hamcrest.Matchers.is;
import org.junit.Test;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

public class FlatMapTest {
    @Test
    public void whenDiffNext() {
        Iterator<Iterator<Integer>> data = List.of(
                List.of(1).iterator(),
                List.of(2, 3).iterator()
        ).iterator();
        FlatMap<Integer> flat = new FlatMap<>(data);
        assertThat(flat.next(), is(1));
        assertThat(flat.next(), is(2));
        assertThat(flat.next(), is(3));
    }
    
    @Test
    public void whenSeqNext() {
        Iterator<Iterator<Integer>> data = List.of(
                List.of(1, 2, 3).iterator()
        ).iterator();
        FlatMap<Integer> flat = new FlatMap<>(data);
        assertThat(flat.next(), is(1));
        assertThat(flat.next(), is(2));
        assertThat(flat.next(), is(3));
    }

    @Test
    public void whenMultiHasNext() {
        Iterator<Iterator<Integer>> data = List.of(
                List.of(1).iterator()
        ).iterator();
        FlatMap<Integer> flat = new FlatMap<>(data);
        assertThat(flat.hasNext(), is(true));
        assertThat(flat.hasNext(), is(true));
    }

    @Test
    public void whenHasNextFalse() {
        Iterator<Iterator<Integer>> data = List.of(
                List.of(1).iterator()
        ).iterator();
        FlatMap<Integer> flat = new FlatMap<>(data);
        assertThat(flat.next(), is(1));
        assertThat(flat.hasNext(), is(false));
    }

    @Test(expected = NoSuchElementException.class)
    public void whenEmpty() {
        Iterator<Iterator<Object>> data = List.of(
                List.of().iterator()
        ).iterator();
        FlatMap<Object> flat = new FlatMap<>(data);
        flat.next();
    }

    @Test
    public void whenSeveralEmptyAndNotEmpty() {
         Iterator<Iterator<?>> it = List.of(
              List.of().iterator(), 
              List.of().iterator(), 
              List.of().iterator(), 
              List.of(1).iterator()
         ).iterator();
         FlatMap flat = new FlatMap(it);
         assertTrue(flat.hasNext());
         assertThat(1, is(flat.next()));
    }
    
    @Test
    public void whenSeveralEmptyThenReturnFalse() {
         Iterator<Iterator<Object>> it = List.of(
              List.of().iterator(),
              List.of().iterator(),
              List.of().iterator(),
              List.of().iterator()
         ).iterator();
         FlatMap flat = new FlatMap(it);         
         assertFalse(flat.hasNext());
    }
}

Задание:

1. Допишите методы next и hasNext.

Помогите разобраться!!!!

Зачем они показали пример с flatMap co stream ???