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Java 8 Stream flatmap method example.


In a very simple note, Stream.flatMap() is used to convert a Stream of Stream into a list of values.

The flatMap() operation has the effect of applying a one-to-many transformation to the elements of the stream, and then flattening the resulting elements into a new stream.

This page gives a simple example on getting count of distinct elements in a file. In this example, each line represents a stream, each steam(line) is broken into words which represents again a independent stream. Now all these multiple streams will be aggregated to a single stream using flatMap method.

package com.java2novice.streams;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Arrays;

public class StreamFlatMapEx {

	public static void main(String a[]) {

		try {
			long noOfWords = Files.lines(Paths.get("C:\testfile.txt"))
									.flatMap(l->Arrays.stream(l.split(" +")))
									.distinct()
									.count();
			System.out.println("noOfWords: "+noOfWords);
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

Output:
noOfWords: 14
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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
What is fail-fast in java?
A fail-fast system is nothing but immediately report any failure that is likely to lead to failure. When a problem occurs, a fail-fast system fails immediately. In Java, we can find this behavior with iterators. Incase, you have called iterator on a collection object, and another thread tries to modify the collection object, then concurrent modification exception will be thrown. This is called fail-fast.
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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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