Have you used functional interfaces in Java in your projects?

Yes, in my projects I have used functional interfaces in Java, especially after the introduction of lambda expressions in Java 8. Functional interfaces are interfaces with a single abstract method, which allow the use of lambdas and method references for more concise and expressive code.

Examples of usage:

• Passing behavior to methods — for example, using Predicate<T>, Function<T,R>, Consumer<T> for filtering, transforming, and processing collections.
• Event handling and callbacks — simplifying code with lambdas.
• Parallel data processing with Stream API — where functional interfaces form the basis.

Example of using Predicate for filtering a list:

import java.util.*;
import java.util.function.Predicate;

public class Example {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Anna", "Bob", "Alice", "Mike");
        Predicate<String> startsWithA = s -> s.startsWith("A");
        names.stream()
             .filter(startsWithA)
             .forEach(System.out::println); // Will print Anna and Alice
    }
}

Using functional interfaces allows writing more declarative and concise code.