A very simple template of a simple array based list for teaching purposes.

ArrayList.java

import java.util.function.BiFunction;
import java.util.function.Function;

public class FunctionalList<E> {

    private E[] elements; 
    private int size;

    @SuppressWarnings("unchecked")
    public FunctionalList(int capacity) {
        this.elements = (E[])new Object[capacity];
        this.size = 0;
    }

    public void add(E e){
        if (size == elements.length) {
            throw new IllegalStateException("List is full");
        }
        elements[size] = e;
        size++; 
    }

    public E get(int index){
        if (index < 0 || index >= size) {
            throw new IndexOutOfBoundsException("Index out of bounds");
        }
        return elements[index];
    }

    public <Y> Y foldLeft(Y startValue, BiFunction<Y, E, Y> f){ TODO }

    public <Y> FunctionalList<Y> map(Function<E, Y> f){ TODO }

    public static void main(String[] args) {
        FunctionalList<String> l = new FunctionalList<>(10);
        l.add("a");
        l.add("bb");
        l.add("ccc");
        l.add("asdf");
        FunctionalList<Integer> lengths = l.map(e -> e.length());
        System.out.println(lengths.foldLeft("", (a, b) -> a + " " + b));

        Integer totalLength = l.foldLeft(0, (a, b) -> a + b.length());
        System.out.println(totalLength); 
    }
}