Java Streams - Java Stream Collectors








To group data in a Stream we can use collect() method of the Stream<T> interface.

The collect() method is overloaded with two versions: 

<R> R collect(Supplier<R> supplier, BiConsumer<R,? super T> accumulator, BiConsumer<R,R> combiner)
<R,A> R collect(Collector<?  super T,A,R> collector)

The first version of the collect() method takes three arguments:

  • A supplier that supplies a mutable container to store the results.
  • An accumulator that accumulates the results into the mutable container.
  • A combiner that combines the partial results when used in parallel.

We use the following steps to collect the employee names in an ArrayList<String>.

First, create a supplier which returns an ArrayList<String> using either of the following statements to create the supplier:

Supplier<ArrayList<String>> supplier = () -> new ArrayList<>();

or

Supplier<ArrayList<String>> supplier = ArrayList::new;

Second, create an accumulator that receives two arguments which is the container returned from the supplier(ArrayList<String> in this case). and the element of the stream.

The accumulator adds the names to the list. 

BiConsumer<ArrayList<String>, String>  accumulator = (list, name)  ->  list.add(name);
BiConsumer<ArrayList<String>, String>  accumulator = ArrayList::add;

Finally, a combiner combines the results into one ArrayList<String>.

The combiner is used only in a parallel stream.

The following code shows how to use the collect() method to collect the names of all employee in a list. 

import java.time.LocalDate;
import java.time.Month;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
/*from w  w  w.ja v a 2  s  . co m*/
public class Main {
  public static void main(String[] args) {
    List<String> names = Employee.persons()
        .stream()
        .map(Employee::getName)
        .collect(ArrayList::new,  ArrayList::add, ArrayList::addAll);
    System.out.println(names);

  }
}

class Employee {
  public static enum Gender {
    MALE, FEMALE
  }

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

  public Employee(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 String getName() {
    return name;
  }
  public static List<Employee> persons() {
    Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
        Month.JANUARY, 1), 2343.0);
    Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
        Month.JULY, 21), 7100.0);
    Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
        Month.MAY, 29), 5455.0);
    Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
        Month.OCTOBER, 16), 1800.0);
    Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
        Month.DECEMBER, 13), 1234.0);
    Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
        Month.JUNE, 9), 3211.0);

    List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);

    return persons;
  }
}

The code above generates the following result.





Collector interface

The second version of the collect() takes an instance of the Collector interface as an argument.

Streams API provides a utility class called Collectors that provides out-of-box implementations for commonly used collectors.

Three of the most commonly used methods of the Collectors class are toList(), toSet(), and toCollection().

  • The toList() method returns a Collector that collects the data in a List.
  • The toSet() method returns a Collector that collects data in a Set.
  • The toCollecton() takes a Supplier that returns a Collection to be used to collect data.

The following code collects all names of employee in a List<String>. 

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
//  w  ww  .  jav  a2s .com
public class Main {
  public static void main(String[] args) {
    List<String> names = Employee.persons()
        .stream()
        .map(Employee::getName)
        .collect(Collectors.toList());
     System.out.println(names);

  }
}

class Employee {
  public static enum Gender {
    MALE, FEMALE
  }

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

  public Employee(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 String getName() {
    return name;
  }
  public static List<Employee> persons() {
    Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
        Month.JANUARY, 1), 2343.0);
    Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
        Month.JULY, 21), 7100.0);
    Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
        Month.MAY, 29), 5455.0);
    Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
        Month.OCTOBER, 16), 1800.0);
    Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
        Month.DECEMBER, 13), 1234.0);
    Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
        Month.JUNE, 9), 3211.0);

    List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);

    return persons;
  }
}

The code above generates the following result.





Example 2

The following code collects all names in a Set<String> which keeps only unique elements. 

Set<String> uniqueNames  = Person.persons()
                        .stream()
                        .map(Person::getName)
                        .collect(Collectors.toSet());
System.out.println(uniqueNames);

Example 3

We can collect names in a sorted set using the toCollection() method as follows: 

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.SortedSet;
import java.util.TreeSet;
import java.util.stream.Collectors;
// ww w .  jav a2s  . co m
public class Main {
  public static void main(String[] args) {
    SortedSet<String> uniqueSortedNames=   Employee.persons()
        .stream()
        .map(Employee::getName)
        .collect(Collectors.toCollection(TreeSet::new));
        System.out.println(uniqueSortedNames);
    System.out.println(uniqueSortedNames);
  }
}

class Employee {
  public static enum Gender {
    MALE, FEMALE
  }

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

  public Employee(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 String getName() {
    return name;
  }
  public static List<Employee> persons() {
    Employee p1 = new Employee(1, "Jake", Gender.MALE, LocalDate.of(1971,
        Month.JANUARY, 1), 2343.0);
    Employee p2 = new Employee(2, "Jack", Gender.MALE, LocalDate.of(1972,
        Month.JULY, 21), 7100.0);
    Employee p3 = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1973,
        Month.MAY, 29), 5455.0);
    Employee p4 = new Employee(4, "Jode", Gender.MALE, LocalDate.of(1974,
        Month.OCTOBER, 16), 1800.0);
    Employee p5 = new Employee(5, "Jeny", Gender.FEMALE, LocalDate.of(1975,
        Month.DECEMBER, 13), 1234.0);
    Employee p6 = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1976,
        Month.JUNE, 9), 3211.0);

    List<Employee> persons = Arrays.asList(p1, p2, p3, p4, p5, p6);

    return persons;
  }
}

The code above generates the following result.