How lambda expression in Java 8 enables functional programming?

Question

How lambda expressions in Java 8 enable functional programming? Because lambda expression type is represented as interface and which is object-oriented class type.

How normal method call differs from lambda expression statement?

Answer 1

Lambda does not enable functional programming. It allows you to process data in more functional style.

Java is an OOP language, where first-class citizens are objects, not functions. Therefore, even when you declare a lambda expression you have to assign it to some Functional Interface, which as you said is an object oriented class type.

However, in Java 8 you can use a lambda expression, which is an anonymous function, like a first-class citizen to process data in a functional style. E.g. you can pass an anonymous function as a method parameter.

accounts.stream()
        .map(account -> account.getEmail())
        .filter(email -> email.endsWith("gmail.com"))

This is a valid Java 8 code, where you pass as method arguments, not values, but functions. Of course, under the hood these lambdas become instances of classes that implement functional interface Function<Account,String>, and Predicate<String> accordingly, but still it allows you to process data in more functional style than in old Java versions.

The whole idea of processing data in a functional style involves also Streams, Functional Interfaces, and Method Reference, so you should consider them all when you think about functional programming in Java.

Answer 2

In the Functional Programming world, functions are objects too and can be treated as traditional objects.

Lambdas itself are simply behaviours without name handles. If you write x -> x + 1, you just created a value that represents an incrementation (or a concatenation of two strings).

This option allows to not only compose classical objects together but also to compose behaviours/actions - which is critical for Functional Programming which utilizes declarative programming a lot - in order to be able to achieve this, you need to be able to pass a behaviour in some way.

But, the presence of Lambda Expressions does not make any language functional automatically - there is much more to it, e.g. Immutability, Algebraic Data Types, lack of side-effects, functors, data types... and so on.

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So, the relation between a lambda and a method is somehow similar to a relation between an anonymous value and a class field.

Answer 3

Lets imagine pure OOP without class fields, setters and getters. only classes and methods... and we got a pure functional programming. As fact, OOP - extends Functional Programming by adding state. State adds reusability, save the time, by storing input parameters, incoming from class constructor.