Parallel Streams
Description
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;
//w w w .j ava2 s . c o m
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;
/* www. j a v a2s .com*/
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.
