Streams Reduce without default value
Description
The third version of the reduce() listed as follows method is used to perform a reduction operation without default value.
reduce(BinaryOperator<T> accumulator)
If the stream is empty, we cannot use default value as 0.
Optional<T> is used to wrap the result or the absence of a result.
Example
The following code shows how to compute the maximum of integers in a stream:
import java.util.Optional;
import java.util.stream.Stream;
//w ww .j ava 2 s . c om
public class Main {
public static void main(String[] args) {
Optional<Integer> max = Stream.of(1, 2, 3, 4, 5).reduce(Integer::max);
if (max.isPresent()) {
System.out.println("max = " + max.get());
} else {
System.out.println("max is not defined.");
}
max = Stream.<Integer> empty().reduce(Integer::max);
if (max.isPresent()) {
System.out.println("max = " + max.get());
} else {
System.out.println("max is not defined.");
}
}
}
The code above generates the following result.
Example 2
The following code prints the details of the highest earner in the employee's list.
import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;
//from w w w . j a va2 s . c o m
public class Main {
public static void main(String[] args) {
Optional<Employee> person = Employee
.persons()
.stream()
.reduce((p1, p2) -> p1.getIncome() > p2.getIncome() ? p1 : p2);
if (person.isPresent()) {
System.out.println("Highest earner: " + person.get());
} else {
System.out.println("Could not get the highest earner.");
}
}
}
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 double getIncome() {
return income;
}
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.
