enum Gender
{
    MALE, FEMALE
}

public class Person
{
    private String name;
    private String email;
    private Gender gender;
    private int age;

    public Person(String name, String email, Gender gender, int age)
    {
        super();
        this.name = name;
        this.email = email;
        this.gender = gender;
        this.age = age;
    }

    public String getName()
    {
        return name;
    }

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

    public String getEmail()
    {
        return email;
    }

    public void setEmail(String email)
    {
        this.email = email;
    }

    public Gender getGender()
    {
        return gender;
    }

    public void setGender(Gender gender)
    {
        this.gender = gender;
    }

    public int getAge()
    {
        return age;
    }

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

    @Override
    public String toString()
    {
        return "Person [name=" + name + ", gender=" + gender + ", age=" + age
                + "]";
    }

}

import java.util.ArrayList;
import java.util.List;

public class StreamMapDemo
{
    public static void main(String[] args)
    {
        List<Person> personList = new ArrayList<>();

        personList.add(new Person("Alice", "alice@gmail.com", Gender.FEMALE, 16));
        personList.add(new Person("Bob", "bob@gmail.com", Gender.MALE, 15));
        personList.add(new Person("Carol", "carol@gmail.com", Gender.FEMALE, 23));
        personList.add(new Person("David", "david@gmail.com", Gender.MALE, 19));
        personList.add(new Person("Eric", "eric@gmail.com", Gender.MALE, 26));

        /*
         * The map operation returns a new stream consisting of elements which
         * are the results of applying a given function to the elements of the
         * current stream. For example, converting a stream of Objects to a
         * stream of String or a stream of primitive numbers.
         *
         * The Stream API provides 4 methods for the map operation:
         *
         * map(): transforms a stream of objects of type T to a stream of
         * objects of type R.
         *
         * mapToInt(): transforms a stream of objects to a stream of int
         * primitives.
         *
         * mapToLong(): transforms a stream of objects to a stream of long
         * primitives.
         *
         * mapToDouble(): transforms a stream of objects to a stream of double
         * primitives.
         *
         */
        
        personList.stream()                     // Stream<Person>
                  .map(p -> p.getEmail())      //  Stream<String>
                  .forEach(System.out::println);

        System.out.println("\n----------------------\n");
        
        personList.stream()  // Stream<Person>
                  .map(p -> p.getName().toUpperCase()) //  Stream<String>
                  .forEach(System.out::println);

        System.out.println("\n----------------------\n");

        personList.stream()  // Stream<Person>
                  .mapToInt(p -> p.getAge()) //IntStream
                  .forEach(age -> System.out.println(age));

    }

}

alice@gmail.com
bob@gmail.com
carol@gmail.com
david@gmail.com
eric@gmail.com

----------------------

ALICE
BOB
CAROL
DAVID
ERIC

----------------------

16
15
23
19
26