Java 8 Stream reduce() Example





Hey guys in this post we will discuss about reduce() of Stream API which is introduced in Java 8.

Introduction


  • Many times we need to perform operations where a stream reduces to single resultant value, for example, maximum, minimum, sum, minus, product, divide etc. Reducing is the repeated process of combining all elements
  • sum(), min(), max(), count(), etc are some examples of reduce operations. reduce() explicitly asks you to specify how to reduce the data that made it through the stream.
  • Stream.reduce() is terminal method

Different variant of reduce() method


  • reduce(BinaryOperator accumulator)
  • reduce(T indentity, BinaryOperator accumulator)
  • reduce(U indentity, BiFunction accumulator, BinaryOperator combiner)

reduce(BinaryOperator accumulator)


  • It performs a reduction on the elements of the given stream, using given accumulation function
  • accumulator – accumulator is a function for combining two values

#1 reduce() method with accumulator

import java.util.Arrays;
import java.util.stream.Stream;

public class Example2 {
	
	public static void main(String[] args) {
		
		int numbers[] = {1,2,3,4,5}; 
		String[] list = {"HP", "Lenovo", "Dell", "Macbook", "Acer"};
		
		Arrays.stream(numbers)
				.reduce((a, b)->a+b).ifPresent(System.out::println);
		
		Stream.of(10,20,30,40)
			.reduce(Integer::max)
			.ifPresent(System.out::println);
		
		Arrays.stream(list)
			.reduce((a, b) -> a+ " | "+b)
			.ifPresent(System.out::println);

	}
}	
//15
//40
// HP | Lenovo | Dell | Macbook | Acer

reduce(T indentity, BinaryOperator accumulator)


  • indentity – the indentity value for the accumulating function
  • accumulator – accumulator is a function for combining two values

#2 Reduce() with identity and accumulator

import java.util.Arrays;
import java.util.stream.Stream;

public class Example2 {
	
	public static void main(String[] args) {
		
		Integer numbers[] = {1,2,3,4,5}; 
		String[] list = {"HP", "Lenovo", "Dell", "Macbook", "Acer"};
		
		Integer resultOne = Stream.of(numbers)
					.reduce(100,  (x, y)-> x+y);
		System.out.println(resultOne);
		
		Integer resultTwo = Arrays.stream(numbers)
					.reduce(100, Integer::max);
		System.out.println(resultTwo);
		
		String resultThree = Stream.of(list)
					.reduce("Programming languages:",  (x, y)->x+" | "+y);
		System.out.println(resultThree);

	}
}	

Reduce(U indentity, BiFunction accumulator, BinaryOperator combiner)


  • identity – the identity value for the combiner function
  • accumulator – accumulator is function for incorporating additional element into result
  • combiner – combiner is function for combining two values. combiner works with only parallelStream




About the author

Bushan Sirgur

Hey guys, I am Bushan Sirgur from Banglore, India. Currently, I am working as an Associate project in an IT company.

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