Stream.reduce() method code examples in Java

We can perform a reduction operation on elements of a Java Stream using the Stream.reduce() method that returns an Optional describing the reduced object or the reduced value itself. This post will discuss a few simple examples of the Stream.reduce() method.

1. Find maximum value of a field among custom objects

Suppose we have a Person class with name and age as its fields. We also have a list of Person objects, and the goal is to find the person having maximum age.

The idea is to create a custom method that takes two Person objects as input and returns the Person object with the maximum age. Then we create a stream of Person objects via the List.stream() method and pass the reference of the custom method to the reduce() method for the reduction operation.

import java.util.Arrays;

import java.util.List;

// `Person` class having `name` and `age` private fields

class Person

{

private String name;

private Integer age;

public Person(String name, Integer age)

{

this.name = name;

this.age = age;

}

public Integer getAge() {

return age;

}

// other getters and setters

@Override

public String toString() {

return "[" + name + ", " + String.valueOf(age) + "]";

}

public static Person max(Person x, Person y) {

return x.getAge() > y.getAge() ? x : y;

}

// Program to find the maximum value of a field among custom objects

// using `Stream.reduce()` method

class Main

{

public static void main(String[] args)

{

Person p1 = new Person("George", 15);

Person p2 = new Person("Tom", 10);

Person p3 = new Person("Mike", 12);

List<Person> users = Arrays.asList(p1, p2, p3);

// get a person with the maximum age

Person user = users.stream()

.reduce(Person::max)

.get();

System.out.println("Person with maximum age is " + user);

}

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Output:

Person with maximum age is [George, 15]

We can also directly pass a lambda function instead of using a custom method, as shown below. It takes two parameters – a partial result of the reduction (in this example, an object with the maximum age of all processed objects so far) and the next element of the stream (in this example, a Person object). It returns a new value every time it processes an element of a stream.

// get a person with the maximum age

Person user = users.stream()

.reduce((x, y) -> (x.getAge() > y.getAge()) ? x : y)

.get();

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There is another overloaded version of the reduce() method, which also takes an identity value along with the associative accumulation method and performs a reduction on the stream elements.

The identity element is both the initial value of the reduction and the default result if there are no elements in the stream.

// get a person with the maximum age

Person user = users.stream()

.reduce(new Person("Dummmy", Integer.MIN_VALUE),

(x, y) -> x.getAge() > y.getAge() ? x : y);

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2. Find maximum element from a list of Integer

Suppose we have a list of Integer, and the goal is to find the maximum element in the list.

The idea is to create a stream of Integer and pass the method reference of Integer.max() to the reduce() method for the reduction operation, which then returns an Optional describing the maximum value.

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

// Program to find a maximum element from a list using `Stream.reduce()`

// method in Java 8 and above

class Main

{

public static Integer getMax(List<Integer> list)

{

return list.stream() // Stream<Integer>

.reduce(Integer::max) // Optional<Integer>

.get(); // Integer

}

public static void main(String[] args)

{

List<Integer> list = Arrays.asList(4, 2, 6, 5, 9, 1);

System.out.println("Maximum element is " + getMax(list));

}

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Here’s how we can use the overloaded version of the reduce() method by passing the identity element.

// Method to find a maximum element from a list of Integer in Java 8 and above

public static Integer getMax(List<Integer> list)

{

return list.stream()

.reduce(Integer.MIN_VALUE, Integer::max);

}

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That’s all about the Stream.reduce() method in Java with code examples.

Exercise:

1. Find the minimum value of a field among custom objects.
2. Find the minimum element from a list of Integer.

Reference: Reduction (The Java™ Tutorials > Collections > Aggregate Operations)

Stream.reduce() method code examples in Java

1. Find maximum value of a field among custom objects

2. Find maximum element from a list of Integer

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