Collect with Collectors
Description
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.
Example
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;
/* www . j av a 2s.c om*/
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.
