How can you create and use functional interfaces in Java?

A functional interface in Java is an interface with exactly one abstract method. Such interfaces are used to create lambda expressions and method references.

To create a functional interface, you can use the @FunctionalInterface annotation, which guarantees that the interface contains only one abstract method.

Example of a functional interface and its usage:

@FunctionalInterface
public interface Calculator {
    int calculate(int a, int b);
}

public class Main {
    public static void main(String[] args) {
        Calculator sum = (a, b) -> a + b;
        Calculator multiply = (a, b) -> a * b;

        System.out.println(sum.calculate(5, 3));       // 8
        System.out.println(multiply.calculate(5, 3));  // 15
    }
}

Thus, functional interfaces allow writing more concise and expressive code using lambda expressions.