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Java Lambdas Unleashed: A Beginner's Guide to Functional Programming

Updated: Dec 16, 2023


Java Lambdas

Java Lambdas Unleashed: Java introduced lambda expressions and functional programming features in Java 8, which was a significant enhancement to the language. These features allow you to write more concise and expressive code when working with functions and collections. Here's an overview of Java lambda expressions and functional programming concepts:

What is Functional Programming in Java?

Java's adoption of lambda expressions and functional interfaces enables you to embrace functional programming principles, such as Immutability, Pure Functions, Function composition, Higher-Order Functions and declarative code.

Functional programming in Java refers to a programming paradigm that emphasizes the use of functions (or methods) as first-class citizens and treats computation as the evaluation of mathematical functions. In Java, functional programming features were introduced in Java 8 with the introduction of lambda expressions and the Stream API. Here are some key aspects of functional programming in Java:

Lambda Expressions: Lambda expressions allow you to define small, inline, anonymous functions. They are often used to represent single abstract methods of functional interfaces (interfaces with a single abstract method), which are also known as SAM (Single Abstract Method) types. A lambda expression in Java is a concise way to represent an anonymous function, essentially a block of code that can be passed around, stored in a variable, or passed as an argument to other functions.For example: (parameter(s)) -> expression or { statements }

Let's break down the components of a lambda expression:

  • (parameter(s)): This represents the input parameters (if any) of the function.

  • ->: This is known as the arrow operator and separates the parameter list from the body of the function.

  • expression or { statements }: This is the code that gets executed when the lambda expression is called. It can be a single expression or a block of statements.

Example: (a, b) -> a + b Lambda expressions are often used with functional interfaces.


Core Java Programming

Java Back-End Development

Functional Interfaces: A functional interface is an interface that has only one abstract method. Lambda expressions can be used to implement the abstract method of a functional interface. Java provides several built-in functional interfaces in the java.util.function package, such as Consumer, Predicate, Function, and Supplier.

Example: Predicate<Integer> isEven = (num) -> num % 2 == 0;

Method References: Method references are a shorthand notation for a lambda expression to call a method. They can be used when a lambda expression simply calls an existing method. There are four types of method references:

  • Static method reference: ClassName::staticMethodName

  • Instance method reference of a particular object: object::instanceMethodName

  • Instance method reference of an arbitrary object of a particular type: ClassName::instanceMethodName

  • Constructor reference: ClassName::new

Example: List<String> names = Arrays.asList("Alice", "Bob", "Charlie");

names.forEach(System.out::println);

Stream API: The Stream API is a powerful feature introduced in Java 8 for working with sequences of elements, such as collections. It allows you to perform various operations like filtering, mapping, reducing, and more on data in a functional and concise manner. Streams can be created from collections or generated dynamically.

Example: List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);

int sum = numbers.stream()

.filter(n -> n % 2 == 0)

.mapToInt(Integer::intValue)

.sum();

Functional Programming Concepts: Java's adoption of lambda expressions and functional interfaces enables you to embrace functional programming principles, such as immutability, pure functions, and declarative code. These concepts can lead to more readable and maintainable code.

Example: List<String> fruits = Arrays.asList("apple", "banana", "cherry");

List<String> upperCaseFruits = fruits.stream()

.map(String::toUpperCase)

.collect(Collectors.toList());


Java's support for lambda expressions and functional programming makes it more expressive and concise for certain types of tasks, especially when working with collections and data transformations. It encourages a more functional and declarative style of programming, which can lead to more readable and maintainable code.

Pure Functions: Pure functions are a fundamental concept in functional programming, including in the context of Java's functional programming capabilities. A pure function is a function that, given the same input, will always produce the same output and has no side effects.

Immutability It refers to the property of an object that, once created, cannot be changed. In the context of functional programming, immutability is a crucial principle. It ensures that data remains constant, which simplifies code, enhances predictability, and reduces the potential for errors. Higher-Order Functions in Java Functional Programming

A higher-order function is a function that takes one or more functions as arguments or returns a function as its result. Higher-order functions are a key concept in functional programming, and Java introduced support for higher-order functions with lambda expressions and functional interfaces.


Function composition

Function composition is a powerful concept in functional programming that allows you to create new functions by combining and chaining existing functions. In Java, you can achieve function composition using lambda expressions, functional interfaces, and the Stream API.

// Function composition using Java's Stream API, to play long songs from the playlist

List<String> longSongs = playList.getSongs().stream()

.filter(isLongSong)

.map(songDescription)

.peek(playSong)

.collect(Collectors.toList());


Core Java Programming

Java Back-End Development


Difference between OOP and Functional programming in Java

Object-Oriented Programming (OOP) and Functional Programming (FP) are two different programming paradigms, and Java, as a language, supports both. Here are the key differences between OOP and FP in the context of Java:

Paradigm Focus:

  • OOP: Object-oriented programming is centered around objects, which are instances of classes. In OOP, you model your software as a collection of objects, each with its own state (attributes) and behavior (methods). You emphasize the encapsulation of data and the interactions between objects.

  • FP: Functional programming is centered around functions. It treats computation as the evaluation of mathematical functions and avoids changing state and mutable data. FP emphasizes immutability, pure functions, and the use of higher-order functions (functions that take other functions as arguments or return them as results).

State Management:

  • OOP: OOP involves the management of mutable objects with internal state. Objects can change their state over time, and methods may modify object attributes.

  • FP: FP promotes immutability, meaning once data is created, it cannot be changed. Functions do not have side effects and should not modify any external state. Instead, they create new data structures or transform existing ones.

Core Constructs:

  • OOP: Core constructs in OOP include classes, objects, inheritance, and polymorphism. Java, being an object-oriented language, provides strong support for these concepts.

  • FP: Core constructs in FP include functions, higher-order functions, and immutability. Java introduced functional programming features like lambda expressions and the Stream API starting with Java 8.

Data Transformation:

  • OOP: OOP often involves iterating over collections of objects and using object methods to perform operations on those objects.

  • FP: FP often uses higher-order functions like map, filter, and reduce to transform and process data. Java's Stream API provides a functional way to work with collections.

Code Style:

  • OOP: OOP code tends to be more verbose due to the need for class definitions and object creation.

  • FP: FP code can be more concise and expressive, especially when using lambda expressions and functional constructs.

Modularity:

  • OOP: OOP encourages modularization through classes and interfaces.

  • FP: FP encourages modularity through the composition of pure functions and the use of higher-order functions.

In Java, you can use both OOP and FP paradigms, and it's common to see code that combines elements of both. The choice between OOP and FP often depends on the specific problem you are trying to solve and your coding style preferences. Java's support for functional programming features introduced in Java 8 and subsequent versions makes it more versatile in this regard.

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