define List Monad for java.util.List

Monadic.java

import java.util.*;
import java.util.stream.*;
import java.util.function.*;

public class  Monadic {
        // define List Monad
        public static class ListMonad<T> {
                public List<T> unit(T x) {
                        return List.of(x);
                }
                public <U> List<U> bind(List<T> xs, Function<T, List<U>> fun) {
                        return xs.stream().flatMap(x -> fun.apply(x).stream()).collect(Collectors.toList());
                }
        }
        public static void main(String[] args) {
                //create List Monad
                var listMonad = new ListMonad<Integer>();

                // use List Monad
                // for { x <- List(1, 2, 3)
                //       y <- List(4, 5, 6) } yield (x + y)
                var result1 = listMonad.bind(
                                List.of(1, 2, 3),
                                x -> listMonad.bind(
                                        List.of(4, 5, 6),
                                        y -> listMonad.unit(x + y)
                                )
                );
                System.out.println(result1);

        }
}