Java Stream reduce
last modified July 8, 2023
Java Stream reduce tutorial shows how to perform reduction operations on Java 8 streams.
Java Stream
Java Stream is a sequence of elements from a source that supports aggregate operations. Streams do not store elements; the elements are computed on demand. Elements are consumed from data sources such as collections, arrays, or I/O resources.
Java Stream reduction
A reduction is a terminal operation that aggregates
a stream into a type or a primitive. The Java 8 Stream API contains a set of predefined
reduction operations, such as average, sum, min,
max, and count, which return one
value by combining the elements of a stream.
Java Stream reduce method
Stream.reduce is a general-purpose method for generating our
custom reduction operations.
Optional<T> reduce(BinaryOperator<T> accumulator)
This method performs a reduction on the elements of this stream, using an
associative accumulation function. It returns an Optional
describing the reduced value, if any.
T reduce(T identity, BinaryOperator<T> accumulator)
This method takes two parameters: the identity and the accumulator. The identity
element is both the initial value of the reduction and the default result if
there are no elements in the stream. The accumulator function takes two
parameters: a partial result of the reduction and the next element of the
stream. It returns a new partial result. The Stream.reduce
method returns the result of the reduction.
Java Stream built-in reductions
The following example uses two predefined reduction operations.
package com.zetcode;
import java.util.Arrays;
public class JavaReduceEx {
public static void main(String[] args) {
int vals[] = { 2, 4, 6, 8, 10, 12, 14, 16 };
int sum = Arrays.stream(vals).sum();
System.out.printf("The sum of values: %d%n", sum);
long n = Arrays.stream(vals).count();
System.out.printf("The number of values: %d%n", n);
}
}
We have an array of integers. We create a stream from the array with
Arrays.stream and perform two reductions: sum
and count.
The sum of values: 72 The number of values: 8
Java reduce Optional
The reduce method with one parameter returns an Optional, which
is a Java class for null safety.
package com.zetcode;
public class Car {
private final String name;
private final int price;
public Car(String name, int price) {
this.name = name;
this.price = price;
}
public int getPrice() {
return price;
}
@Override
public String toString() {
StringBuilder builder = new StringBuilder();
builder.append("Car{name=").append(name).append(", price=")
.append(price).append("}");
return builder.toString();
}
}
This is the Car class.
package com.zetcode;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;
public class JavaReduceEx2 {
public static void main(String[] args) {
List<Car> persons = Arrays.asList(new Car("Skoda", 18544),
new Car("Volvo", 22344),
new Car("Fiat", 23650),
new Car("Renault", 19700));
Optional<Car> car = persons.stream().reduce((c1, c2)
-> c1.getPrice() > c2.getPrice() ? c1 : c2);
car.ifPresent(System.out::println);
}
}
The example creates a list of car objects. We compute the most expensive car.
Optional<Car> car = persons.stream().reduce((c1, c2)
-> c1.getPrice() > c2.getPrice() ? c1 : c2);
From the list, we create a stream; the accumulator of the
reduce method compares the prices of the cars and returns the more
expensive one.
car.ifPresent(System.out::println);
If the returned reduction value is not null, we print it to the console.
Car{name=Fiat, price=23650}
The next example adds other use cases.
package com.zetcode;
public class MyUtil {
public static int add2Ints(int num1, int num2) {
return num1 + num2;
}
}
This is MyUtil class having a method that adds two
integers.
package com.zetcode;
import java.util.stream.IntStream;
public class JavaReduceEx3 {
public static void main(String[] args) {
IntStream.range(1, 10).reduce((x, y) -> x + y)
.ifPresent(s -> System.out.println(s));
IntStream.range(1, 10).reduce(Integer::sum)
.ifPresent(s -> System.out.println(s));
IntStream.range(1, 10).reduce(MyUtil::add2Ints)
.ifPresent(s -> System.out.println(s));
}
}
We create three different accumulator functions to compute the sum of 1..10 values.
IntStream.range(1, 10).reduce((x, y) -> x + y).ifPresent(s -> System.out.println(s));
In the first case, we have a lambda expression doing the addition.
IntStream.range(1, 10).reduce(Integer::sum).ifPresent(s -> System.out.println(s));
The second case uses a built in Integer::sum method.
IntStream.range(1, 10).reduce(MyUtil::add2Ints).ifPresent(s -> System.out.println(s));
Finally, we have a custom addition method.
Java reduce with identity
As we have already mentioned, the identity is both the initial value of the reduction and the default result if there are no elements in the stream.
package com.zetcode;
import java.time.LocalDate;
import java.time.chrono.IsoChronology;
public class User {
private String name;
private LocalDate dateOfBirth;
public User(String name, LocalDate dateOfBirth) {
this.name = name;
this.dateOfBirth = dateOfBirth;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public LocalDate getDateOfBirth() {
return dateOfBirth;
}
public void setDateOfBirth(LocalDate dateOfBirth) {
this.dateOfBirth = dateOfBirth;
}
public int getAge() {
return dateOfBirth.until(IsoChronology.INSTANCE.dateNow())
.getYears();
}
@Override
public String toString() {
StringBuilder builder = new StringBuilder();
builder.append("User{name=").append(name).append(", dateOfBirth=")
.append(dateOfBirth).append("}");
return builder.toString();
}
}
This is the User class. In addition to the usual attributes and
getters and setters, we have the getAge method that returns the age
of the user using Java 8 date API.
package com.zetcode;
import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;
public class JavaReduceEx4 {
public static void main(String[] args) {
List<User> users = new ArrayList<>();
users.add(new User("Frank", LocalDate.of(1979, 11, 23)));
users.add(new User("Peter", LocalDate.of(1985, 1, 18)));
users.add(new User("Lucy", LocalDate.of(2002, 5, 14)));
users.add(new User("Albert", LocalDate.of(1996, 8, 30)));
users.add(new User("Frank", LocalDate.of(1967, 10, 6)));
int maxAge = users.stream().mapToInt(User::getAge)
.reduce(0, (a1, a2) -> a1 > a2 ? a1 : a2);
System.out.printf("The oldest user's age: %s%n", maxAge);
}
}
In the example, we create a list of users. The example calculates the age of the oldest user.
int maxAge = users.stream().mapToInt(User::getAge)
.reduce(0, (a1, a2) -> a1 > a2 ? a1 : a2);
From the list we create a Java 8 stream. The stream is mapped to an IntStream
with a mapToInt method. Finally, the reduce method provides an
identity value (0) and an accumulator; the accumulator compares the age values and returns
the bigger one.
In this article we have have worked with Java Stream reduction operations.
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