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Java 8 Stream reduce method example.


The Stream.reduce() method is a reduction operation. A reduction operation takes a sequence of input elements and combines them into a single summary result by repeated application of a combining operation. The Stream.reduce() method comes with three variations.

Stream.reduce() with Accumulator

It performs a reduction on the elements of the given stream, using given accumulation function.

package com.java2novice.streams;

import java.util.stream.Stream;

public class StreamReduceAccEx {

	public static void main(String[] args) {

		Stream.of(10,20,22,12,14).reduce((x,y)->x+y).ifPresent(System.out::println);

		Stream.of(10,20,22,12,14).reduce(Integer::sum).ifPresent(System.out::println);

		Stream.of("java", "c", "c#", "python").reduce((x,y)->x+" | "+y).ifPresent(System.out::println);
	}
}

Output:
78
78
java | c | c# | python

Stream.reduce() with Identity and Accumulator

It performs a reduction on the elements of the given stream, using given identity value and an accumulation function. Here the identity is a starting value.

package com.java2novice.streams;

import java.util.stream.Stream;

public class StreamReduceAccEx {

	public static void main(String[] args) {

		Integer arrSum = Stream.of(10,20,22,12,14).reduce(1000, Integer::sum);
		System.out.println(arrSum);

		arrSum = Stream.of(10,20,22,12,14).reduce(1000, (x,y)->x+y);
		System.out.println(arrSum);

		String result = Stream.of("java", "c", "c#", "python").reduce("Languages:", (x,y)->x+" | "+y);
		System.out.println(result);
	}
}

Output:
1078
1078
Languages: | java | c | c# | python

Stream.reduce() with Identity, Accumulator and Combiner

It performs a reduction on the elements of the given stream, using given identity value, an accumulation function and combining functions. The identity value must be an identity for the combiner function. Combiner is a function which aggregates results of the accumulator. Combiner is called only in a parallel mode to reduce results of accumulators from different threads.

package com.java2novice.streams;

import java.util.stream.Stream;

public class StreamReduceAccEx {

	public static void main(String[] args) {

		Integer arrSum = Stream.of(10,20,22,12,14).parallel().reduce(1000, (x,y)->x+y, (p,q)->{
			System.out.println("combiner called");
			return p+q;
		});
		System.out.println(arrSum);
	}
}

Output:
combiner called
combiner called
combiner called
combiner called
5078
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Java 8 Streams Examples

  1. How Java 8 Streams work?
  2. Java 8 Streams parallelism introduction.
  3. Explain non-interference behavior of Java 8 Streams.
  4. Create Java 8 Stream using Stream.of() method example.
  5. Create Java 8 Stream using List example.
  6. Create Java 8 Stream using Stream.generate() method.
  7. Java 8 Stream.filter() example.
  8. Java 8 Stream.map() example.
  9. Java 8 Stream flatmap method example.
  10. Java 8 Stream peek method example.
  11. Java 8 Stream distinct method example.
  12. Java 8 Stream sorted method example.
  13. Java 8 Stream limit method example.
  14. Java 8 Stream forEach method example.
  15. Java 8 Stream toArray method example.
  16. Java 8 Stream reduce method example.
  17. Java 8 Stream collect method example.
  18. Java 8 Stream concat method example.
  19. Java 8 Stream anyMatch(), allMatch() and noneMatch() example.
  20. Java 8 Stream findFirst(), findAny() example.
  21. Primitive type Stream example.
Knowledge Centre
Difference between Enumeration and Iterator
The functionality of Enumeration and the Iterator are same. You can get remove() from Iterator to remove an element, while while Enumeration does not have remove() method. Using Enumeration you can only traverse and fetch the objects, where as using Iterator we can also add and remove the objects. So Iterator can be useful if you want to manipulate the list and Enumeration is for read-only access.
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About Author

I'm Nataraja Gootooru, programmer by profession and passionate about technologies. All examples given here are as simple as possible to help beginners. The source code is compiled and tested in my dev environment.

If you come across any mistakes or bugs, please email me to [email protected].

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Reference: Java™ Platform Standard Ed. 7 - API Specification | Java™ Platform Standard Ed. 8 - API Specification | Java is registered trademark of Oracle.
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