Java - Stream Join Operation

Introduction

joining() method from the Collectors class concatenates the elements of a stream of CharSequence and returns the result as a String.

The concatenation occurs in the encounter order.

The joining() method is overloaded and it has three versions:

joining()
joining(CharSequence delimiter)
joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)

Demo

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

public class Main {
  public static void main(String[] args) {
    List<Person> persons = Person.persons();
    String names = persons.stream()
                          .map(Person::getName)
                          .collect(Collectors.joining());

    String delimitedNames = persons.stream()
                                   .map(Person::getName)
                                   .collect(Collectors.joining(", "));

    String prefixedNames = persons.stream()
            .map(Person::getName)/*from   w  ww  . ja  v  a 2s.c  o m*/
            .collect(Collectors.joining(", ", "Hello ", ". Goodbye."));

    System.out.println("Joined names: " + names);
    System.out.println("Joined, delimited names: " + delimitedNames);
    System.out.println(prefixedNames);

  }
}
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;
  }
}

Java - Stream Join Operation

Introduction

Demo

Result