Collectors groupingBy(Function classifier, Supplier mapFactory, Collector downstream) example
Description
Collectors groupingBy(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
returns a Collector implementing a cascaded "group by" operation on input elements of type T, grouping elements according to a classification function, and performing a reduction operation on the values associated with a given key using the downstream Collector.
Syntax
groupingBy
has the following syntax.
public static <T,K,D,A,M extends Map<K,D>> Collector<T,?,M> groupingBy(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
Example
The following example shows how to use groupingBy
.
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;
// w w w . j av a2 s . com
public class Main {
public static void main(String[] args) {
Map<Gender, Map<Object, String>> personsByGenderAndDobMonth
= Employee.persons()
.stream()
.collect(Collectors.groupingBy(Employee::getGender,
Collectors.groupingBy(p -> p.getDob().getMonth(),
Collectors.mapping(Employee::getName,
Collectors.joining(", ")))));
System.out.println(personsByGenderAndDobMonth);
}
}
enum Gender {
MALE, FEMALE
}
class Employee {
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 Gender getGender() {
return gender;
}
public LocalDate getDob() {
return dob;
}
public static List<Employee> persons() {
Employee ken = new Employee(1, "Jame", Gender.MALE, LocalDate.of(1970,
Month.MAY, 4), 6123.0);
Employee jeff = new Employee(2, "Jeff", Gender.MALE, LocalDate.of(1971,
Month.JULY, 5), 7112.0);
Employee donna = new Employee(3, "Jane", Gender.FEMALE, LocalDate.of(1972,
Month.JULY, 9), 8712.0);
Employee chris = new Employee(4, "Jodd", Gender.MALE, LocalDate.of(1973,
Month.DECEMBER, 6), 1823.0);
Employee laynie = new Employee(5, "Janey", Gender.FEMALE, LocalDate.of(1974,
Month.DECEMBER, 3), 1234.0);
Employee lee = new Employee(6, "Jason", Gender.MALE, LocalDate.of(1975, Month.MAY,
8), 2412.0);
// Create a list of persons
List<Employee> persons = Arrays.asList(ken, jeff, donna, chris, laynie, lee);
return persons;
}
}
The code above generates the following result.