import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.function.Consumer;

public class List<T> {
    private Element<T> head, tail;

    private static class Element<T> {
        T value;
        Element<T> next;

        Element(T value) {
            this.value = value;
        }
    }

    public void push(T value) {
        Element<T> newElem = new Element<>(value);
        if (tail == null) {
            head = tail = newElem;
        } else {
            tail.next = newElem;
            tail = newElem;
        }
    }

    public Iterable<T> all() {
        return () -> new Iterator<>() {
            private Element<T> current = head;

            @Override
            public boolean hasNext() {
                return current != null;
            }

            @Override
            public T next() {
                if (current == null) throw new NoSuchElementException();
                T value = current.value;
                current = current.next;
                return value;
            }
        };
    }

    public static void main(String[] args) {
        List<Integer> list = new List<>();
        list.push(10);
        list.push(13);
        list.push(23);

        // Iterating over the elements
        for (Integer element : list.all()) {
            System.out.println(element);
        }

        // Collecting all elements to a list
        java.util.List<Integer> collected = new java.util.ArrayList<>();
        list.all().forEach(collected::add);
        System.out.println("all: " + collected);

        // Generating Fibonacci numbers
        Iterable<Integer> fib = genFib();
        for (Integer n : fib) {
            if (n >= 10) break;
            System.out.println(n);
        }
    }

    public static Iterable<Integer> genFib() {
        return () -> new Iterator<>() {
            private int a = 1, b = 1;

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

            @Override
            public Integer next() {
                int temp = a;
                a = b;
                b = temp + b;
                return temp;
            }
        };
    }
}