Monads with Java 8

Question

In the interests of helping to understand what a monad is, can someone provide an example using java ? Are they possible ?

Lambda expressions are possible using java

Answer 1

Here's the thing about monads which is hard to grasp: monads are a pattern, not a specific type. Monads are a shape, they are an abstract interface (not in the Java sense) more than they are a concrete data structure. As a result, any example-driven tutorial is doomed to incompleteness and failure. [...] The only way to understand monads is to see them for what they are: a mathematical construct.

Monads are not metaphors by Daniel Spiewak

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Monads in Java SE 8

List monad

interface Person {
    List parents();

    default List greatGrandParents1() {
        List list = new ArrayList<>();
        for (Person p : parents()) {
            for (Person gp : p.parents()) {
                for (Person ggp : p.parents()) {

                    list.add(ggp);
                }
            }
        }
        return list;
    }

    //  Stream flatMap(Function> mapper)
    default List greatGrandParents2() {
        return Stream.of(parents())
                .flatMap(p -> Stream.of(p.parents()))
                .flatMap(gp -> Stream.of(gp.parents()))
                .collect(toList());
    }
}

Maybe monad

interface Person {
    String firstName();
    String middleName();
    String lastName();

    default String fullName1() {
        String fName = firstName();
        if (fName != null) {
            String mName = middleName();
            if (mName != null) {
                String lName = lastName();
                if (lName != null) {
                    return fName + " " + mName + " " + lName;
                }
            }
        }
        return null;
    }

    //  Optional flatMap(Function> mapper)
    default Optional fullName2() {
        return Optional.ofNullable(firstName())
                .flatMap(fName -> Optional.ofNullable(middleName())
                .flatMap(mName -> Optional.ofNullable(lastName())
                .flatMap(lName -> Optional.of(fName + " " + mName + " " + lName))));
    }
}

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Monad is a generic pattern for nested control flow encapsulation. I.e. a way to create reusable components from nested imperative idioms.

Important to understand that a monad is not just a generic wrapper class with a flat map operation. For example, ArrayList with a flatMap method won't be a monad. Because monad laws prohibit side effects.

Monad is a formalism. It describes the structure, regardless of content or meaning. People struggle with relating to meaningless (abstract) things. So they come up with metaphors which are not monads.

See also: conversation between Erik Meijer and Gilad Bracha.