Partitioning Data
Description
Partitioning is a special case of grouping.
Grouping data is based on the keys returned from a function. There could be many groups.
Partitioning only takes care of two groups based on a predicate. The value evaluated to true is one group, false is another group.
partitioningBy() method, which collects data in a Map whose keys are always of the type Boolean, is overloaded in two versions.
partitioningBy(Predicate<? super T> predicate)
partitioningBy(Predicate<? super T> predicate, Collector<? super T,A,D> downstream)
Example
The first version of the partitionedBy() method returns a collector that performs the partitioning based on the predicate.
The following code shows how to partition employee based on gender.
import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
Map<Boolean, List<Employee>> partionedByMaleGender =
Employee.persons()/*from w ww .jav a2 s . c om*/
.stream()
.collect(Collectors.partitioningBy(Employee::isMale));
System.out.println(partionedByMaleGender);
}
}
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 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 second version allows us to specify another collector that can perform a reduction operation.
The following code shows how to partition employee by gender, and collects their names in a comma-separated string:
import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
Map<Boolean,String> partionedByMaleGender = Employee.persons()
.stream()//from ww w .jav a2 s .c o m
.collect(Collectors.partitioningBy(Employee::isMale,
Collectors.mapping(Employee::getName, Collectors.joining(", "))));
System.out.println(partionedByMaleGender);
}
}
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.
