Java - Stream Map Operation

Introduction

A map operation applies a function to each element of the input stream to produce another stream.

You can map a stream of T to a stream of type S, where T and S may be the same or different types.

You can map a stream of Person to a stream of Person ID.

You can apply the map operation on a stream using one of the following methods of the Stream<T> interface:

<R> Stream<R> map(Function<? super T,? extends R> mapper)
 DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper)
    IntStream mapToInt(ToIntFunction<? super T> mapper)
   LongStream mapToLong(ToLongFunction<? super T> mapper)

The map operation takes a function as an argument.

Each element from the input stream is passed to the function.

The following code creates an IntStream whose elements are integers from 1 to 5, maps the elements of the stream to their squares, and prints the mapped stream on the standard output.

IntStream.rangeClosed(1, 5)
         .map(n -> n * n)
         .forEach(System.out::println);

The following code maps the elements of a stream of people to their names and prints the mapped stream.

Person.persons()
      .stream()
      .map(Person::getName)
      .forEach(System.out::println);

Demo

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;

public class Main {
  public static void main(String[] args) {
    Person.persons()/*from w w w .  ja  v a2  s.  co  m*/
    .stream()
    .map(Person::getName)
    .forEach(System.out::println);
  }
}

class Person {
  // An enum to represent the gender of a person
  public static enum Gender {
    MALE, FEMALE
  }

  private long id;
  private String name;
  private Gender gender;
  private LocalDate dob;
  private double income;

  public Person(long id, String name, Gender gender, LocalDate dob, double income) {
    this.id = id;
    this.name = name;
    this.gender = gender;
    this.dob = dob;
    this.income = income;
  }

  public long getId() {
    return id;
  }

  public void setId(long id) {
    this.id = id;
  }

  public String getName() {
    return name;
  }

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

  public Gender getGender() {
    return gender;
  }

  public boolean isMale() {
    return this.gender == Gender.MALE;
  }

  public boolean isFemale() {
    return this.gender == Gender.FEMALE;
  }

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

  public LocalDate getDob() {
    return dob;
  }

  public void setDob(LocalDate dob) {
    this.dob = dob;
  }

  public double getIncome() {
    return income;
  }

  public void setIncome(double income) {
    this.income = income;
  }

  public static List<Person> persons() {
    Person ken = new Person(1, "Java", Gender.MALE, LocalDate.of(1970, Month.MAY, 4), 6020.0);
    Person jeff = new Person(2, "Jack", Gender.MALE, LocalDate.of(1970, Month.JULY, 15), 7320.0);
    Person donna = new Person(3, "Javascript", Gender.FEMALE, LocalDate.of(1972, Month.JULY, 29), 8720.0);
    Person chris = new Person(4, "XML", Gender.MALE, LocalDate.of(1993, Month.DECEMBER, 16), 5800.0);
    Person laynie = new Person(5, "Json", Gender.FEMALE, LocalDate.of(2002, Month.DECEMBER, 13), 0.0);
    Person lee = new Person(6, "Database", Gender.MALE, LocalDate.of(2011, Month.MAY, 9), 2400.0);

    // Create a list of persons
    List<Person> persons = Arrays.asList(ken, jeff, donna, chris, laynie, lee);

    return persons;
  }

  @Override
  public String toString() {
    String str = String.format("(%s, %s, %s, %s, %.2f)", id, name, gender, dob, income);
    return str;
  }
}

Result