Java - Stream Adapt Operation

Introduction

You can adapt the result of the collector to a different type.

Such a collector is returned by using the collectingAndThen() method of the Collectors class.

The method signature is

collectingAndThen(Collector<T,A,R> downstream, Function<R,RR> finisher)

downstream is a collector that collects the data.

finisher is free to modify the result, including its type.

The return type of such a collector is the return type of the finisher.

The following code returns an unmodifiable list of person names:

List<String> names = Person.persons()
                        .stream()
                        .map(Person::getName)
                        .collect(Collectors.collectingAndThen(Collectors.toList(),
                                result -> Collections.unmodifiableList(result)));

Demo

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;

public class Main {
  public static void main(String[] args) {
    List<String> names = Person.persons()
        .stream()/*from   w  ww.ja v  a  2  s.com*/
        .map(Person::getName)
        .collect(Collectors.collectingAndThen(Collectors.toList(),
                result -> Collections.unmodifiableList(result)));
    System.out.println(names);
  }
}

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

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