What functional interface do we work with in the map() method?

In the map() method of Java streams, we work with the functional interface java.util.function.Function.

// Example of using Function in the map() method
import java.util.List;
import java.util.stream.Collectors;

List<String> words = List.of("hello", "world");

// Function<String, Integer> converts String to Integer
List<Integer> lengths = words.stream()
                             .map(s -> s.length()) // Lambda expression implements Function<String, Integer>
                             .collect(Collectors.toList());

// Another way using method reference
List<Integer> lengthsMethodRef = words.stream()
                                      .map(String::length) // Method reference also implements Function<String, Integer>
                                      .collect(Collectors.toList());

The Function<T, R> interface takes one argument of type T and returns a result of type R. In the context of map(), T is the type of the input stream elements, and R is the type of the output stream elements.