What is a functional programming paradigm?

The functional programming paradigm is a style of building software where the main principle is the use of functions as the primary building blocks. It emphasizes:

• Pure functions: Functions that always return the same result given the same inputs and have no side effects (do not modify external state).
• Immutability of data: Data cannot be changed after creation. Any modification creates a new copy of the data.
• First-class functions: Functions can be passed as arguments, returned from other functions, and assigned to variables.
• Absence of mutable state: The program aims to minimize or completely eliminate the use of mutable variables.

Advantages:

• Simplifies debugging and testing due to the absence of side effects.
• Improves parallelism and multithreading, as there is no need for locks when working with immutable data.
• More readable and predictable code.

In Java, functional features were introduced with Java 8 through:

• Lambda expressions.
• Functional interfaces.
• Stream API.
• Method references.

Example of using Stream API for filtering and transforming:

// Import necessary classes
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

// Example list of numbers
List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

// Using Stream API to filter even numbers and multiply them by 2
List<Integer> processedNumbers = numbers.stream()
                                        .filter(n -> n % 2 == 0) // Filter even numbers
                                        .map(n -> n * 2)         // Multiply by 2
                                        .collect(Collectors.toList()); // Collect result into a new list

// Output the result
// System.out.println(processedNumbers); // Will output: [4, 8, 12, 16, 20]