A Function is a functional interface whose sole purpose is to return any result by working on a single input argument.

A Function interface represents a function that accepts one argument and returns a result.

Java Function Functional Interface Example

The following example demonstrates:
1. Convert centigrade to Fahrenheit
2. Convert String to Integer
3. Convert PersonEntity to PersonDTO

import java.util.function.Function;

public class FunctionExample {

 public static void main(String[] args) {
  
  // convert centigrade to fahrenheit
  Function<Integer,Double> centigradeToFahrenheitInt = x -> new Double((x*9/5)+32);
  System.out.println("Centigrade to Fahrenheit: "+centigradeToFahrenheitInt.apply(100));

  // String to an integer
  Function<String, Integer> stringToInt = x -> Integer.valueOf(x);
  System.out.println(" String to Int: " + stringToInt.apply("4"));
  
  // convert person entity to person DTO. 
  Function<PersonEntity, PersonDTO> function = (entity) -> {
   return new PersonDTO(entity.getName(), entity.getAge());
  };
  
  PersonDTO personDTO = function.apply(new PersonEntity("ramesh", 20));
  System.out.println(personDTO.getName());
  System.out.println(personDTO.getAge());
 }
}

class PersonEntity {
 private String name;
 private int age;

 public PersonEntity(String name, int age) {
  super();
  this.name = name;
  this.age = age;
 }

 public String getName() {
  return name;
 }

 public void setName(String name) {
  this.name = name;
 }

 public int getAge() {
  return age;
 }

 public void setAge(int age) {
  this.age = age;
 }
}

class PersonDTO {
 private String name;
 private int age;

 public PersonDTO(String name, int age) {
  super();
  this.name = name;
  this.age = age;
 }

 public String getName() {
  return name;
 }

 public void setName(String name) {
  this.name = name;
 }

 public int getAge() {
  return age;
 }

 public void setAge(int age) {
  this.age = age;
 }
}

Output:

Centigrade to Fahrenheit: 212.0
 String to Int: 4
ramesh
20

Reference

https://www.javaguides.net/2018/07/java-8-functional-interfaces.html