Java Streams - Java Parallel Streams

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Streams can be sequential or parallel.

Operations on a sequential stream are processed in serial by one thread.

Operations on a parallel stream are processed in parallel using multiple threads.

Most of the methods in the Streams API produce sequential streams by default. To create a parallel stream from a collection such as a List or a Set, call the parallelStream() method of the Collection interface.

Use the parallel() method on a stream to convert a sequential stream into a parallel stream.

Use the sequential() method on a stream to convert a parallel stream into a sequential stream.

Example

The following code shows how to do serial processing of the stream:

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
//from  ww  w .j a v a2 s  . com
public class Main {
  public static void main(String[] args) {
    String names = Employee.persons()              
        .stream()                  
        .filter(Employee::isMale)  
        .map(Employee::getName)    
        .collect(Collectors.joining(", ")); 
    System.out.println(names);

    names = Employee.persons()                 
        .parallelStream()    
        .filter(Employee::isMale)
        .map(Employee::getName)  
        .collect(Collectors.joining(", ")); 
    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 boolean isMale() {
    return this.gender == Gender.MALE;
  }
  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;
  }

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

The code above generates the following result.

Example 2

The following code shows how to mix serial and parallel streams:

import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
//from   w w w.  j  a v a 2  s  .  co m
public class Main {
  public static void main(String[] args) {
    String names = Employee.persons()                // The data source
        .stream()                  // Produces a  sequential  stream
        .filter(Employee::isMale)   // Processed in serial
        .parallel()               // Produces a  parallel  stream
        .map(Employee::getName)       // Processed in parallel
        .collect(Collectors.joining(", "));  // Processed in parallel
    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 boolean isMale() {
    return this.gender == Gender.MALE;
  }
  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.

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