Java streams flatmap example

Just call flatMap() method to convert List> to List.

import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; /** * * List> to List * * @author Java8Example * */ public class Java8StreamFlatMapExample1 < public static void main(String[] args) < Listlist1 = Arrays.asList("One", "Two", "Three"); List list2 = Arrays.asList("Four", "Five", "Six", "Seven"); List list3 = Arrays.asList("Eight", "Nine"); List> listOfLists = Arrays.asList(list1, list2, list3); System.out.println("List of Lists before flattening (List>) : " + listOfLists); System.out.println("List of Lists before flattening size : " + listOfLists.size()); Stream flatmapStream = listOfLists.stream().flatMap(list -> list.stream()); List output = flatmapStream.collect(Collectors.toList()); System.out.println("After Flattening flatMap() output " + output); System.out.println("After Flattening flatMap() output size " + output.size()); > >

List of Lists before flattening (List>) : [[One, Two, Three], [Four, Five, Six, Seven], [Eight, Nine]] List of Lists before flattening size : 3 After Flattening flatMap() output [One, Two, Three, Four, Five, Six, Seven, Eight, Nine] After Flattening flatMap() output size 9

6. Stream flatMap() Example: Converting Arrays of Arrays integers into a single Array

Exampel program to covnert Integer[][] into List .

Integer[][] is having nested arrays. This is the simple way of collecting all values into a single list instead of iterating all over the Integer[][] array.

import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; /** * * Integer[][] to List * * @author Java8Example * */ public class Java8StreamFlatMapExampleArrays2 < public static void main(String[] args) < // Create array. Integer[][] intArray = new Integer[][] < < 1, 2, 3 >, < 4, 5 >, < 6, 7, 8, 9 >>; // converting array to Stream Stream stream = Arrays.stream(intArray); // stream using flatMap() Stream intStream = stream.flatMap(array -> Arrays.stream(array)); List output = intStream.collect(Collectors.toList()); System.out.println("After Flattening flatMap() output " + output); > >

After Flattening flatMap() output [1, 2, 3, 4, 5, 6, 7, 8, 9]

7. Stream + flatMap() -> List

If you want to do any comparisons or validations using filter() method then you should use flatMap() first then apply filter() method.

If you do not use flatMap() then it will find the match to the given filter(Predicate p).

import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; /** * * String[][] to String * * @author Java8Example * */ public class Java8StreamFlatMapExampleArraysString < public static void main(String[] args) < // Create array. String[][] strArray = new String[][] < < "100", "200", "300" >, < "400", "500" >, < "600", "700", "800", "900" >>; // converting array to Stream Stream strStream = Arrays.stream(strArray); // stream using flatMap() Stream flatMapStream = strStream.flatMap(array -> Arrays.stream(array)); // filtering based on a condition. Stream filteredStream = flatMapStream.filter(value -> value.endsWith("00")); List output = filteredStream.collect(Collectors.toList()); System.out.println("After Flattening flatMap() output " + output); > > 

After Flattening flatMap() output [100, 200, 300, 400, 500, 600, 700, 800, 900]

8. Stream >> + flatMap() -> Set

Let us filter only Employee designation by «Developer» using flatmap() from all the employees of Apple and Samsung.

import java.util.HashSet; import java.util.List; import java.util.Set; import java.util.stream.Collectors; import com.java.w3schools.blog.java8.stream.Employee; /** * * String[][] to String * * @author Java8Example * */ public class Java8StreamFlatMapSetEmployee < public static void main(String[] args) < Employee appleEmp1 = new Employee(100, "Jira Sunait", "Developer", "Apple"); Employee appleEmp2 = new Employee(200, "Suman Jaran", "Lead", "Apple"); Employee appleEmp3 = new Employee(300, "Riyad Paul", "Developer", "Apple"); SetappleSet = new HashSet<>(); appleSet.add(appleEmp1); appleSet.add(appleEmp2); appleSet.add(appleEmp3); Employee samsungEmp1 = new Employee(400, "Baiym Chpal", "Developer", "Samsung"); Employee samsungEmp2 = new Employee(500, "Atthat Gates", "Lead", "Samsung"); Set samsungSet = new HashSet<>(); appleSet.add(samsungEmp1); appleSet.add(samsungEmp2); Set allEmps = new HashSet<>(); allEmps.add(appleSet); allEmps.add(samsungSet); Set developers = allEmps.stream().flatMap(set -> set.stream()) .filter(emp -> emp.getDesgnation().equals("Developer")).collect(Collectors.toSet()); System.out.println("Filterting only developers from all companies as Set : " + developers); > >

Filterting only developers from all companies as Set : [Employee [id=300, fullName=Riyad Paul, age=0, desgnation=Developer, company=Apple], Employee [id=400, fullName=Baiym Chpal, age=0, desgnation=Developer, company=Samsung], Employee [id=100, fullName=Jira Sunait, age=0, desgnation=Developer, company=Apple]]

9. Stream >> + flatMap() -> List

Collecting the employees desingnation Developer as List.

List developerslist = allEmps.stream().flatMap(set -> set.stream()) .filter(emp -> emp.getDesgnation().equals("Developer")).collect(Collectors.toList()); System.out.println("Filterting only developers from all companies as List : " + developerslist);

This code produces output as the above program.

10. Stream IllegalStateException

if the Stream is closed and invoked any options on it then it will throw «java.lang.IllegalStateException: stream has already been operated upon or closed«. This is a common exception in java 8 streams.

Exception in thread "main" java.lang.IllegalStateException: stream has already been operated upon or closed at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:229) at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578) at com.java.w3schools.blog.java.program.to.java8.stream.Java8StreamFlatMapExampleArraysString.main(Java8StreamFlatMapExampleArraysString.java:33)

11. Conclusion

In this article, we’ve seen many examples of the Stream API flatMap() method.

As usual, Examples are shown in this article are over GitHub.

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Java Stream flatMap()

The Stream flatMap() method is used to flatten a Stream of collections to a Stream of objects. The objects are combined from all the collections in the original Stream.

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.

Stream.flatMap() helps in converting Stream> to Stream .

flatMap() = Flattening + map()

In very layman’s terms, flattening is referred to as merging multiple collections/arrays into one. Consider the following example.

In this example, we have an array of 3 arrays. After the flattening effect, we will have one result array with all the items from the 3 arrays.

Before flattening : [[1, 2, 3], [4, 5], [6, 7, 8]] After flattening : [1, 2, 3, 4, 5, 6, 7, 8]

In the following example, lines is a stream of lines in the file. Each line consists of multiple words. The words stream is a fattened version of all streams into a single stream – consisting of all the words in all the lines.

Stream lines = Files.lines(path, StandardCharsets.UTF_8); Stream words = lines.flatMap(line -> Stream.of(line.split(" +")));

The stream flatMap() method has the following syntax.

 Stream flatMap(Function> mapper)

• R represents the element type of the new stream.
• mapper is a non-interfering, stateless function to apply to each element which produces a stream of new values.
• The method returns a new stream of objects of type R .

Stream interface has three more similar methods which produce IntStream , LongStream and DoubleStream respectively after the flatMap() operation. If the streams created after flatMap() operations return above given types then consider using these functions directly.

IntStream flatMapToInt(Function mapper) LongStream flatMapToLong(Function mapper) DoubleStream flatMapToDouble(Function mapper)

• flatMap() is an intermediate operation and return a new Stream.
• It returns a Stream consisting of the results of replacing each element of the given stream with the contents of a mapped stream produced by applying the provided mapping function to each element.
• The mapper function used for transformation in flatMap() is a stateless function and returns only a stream of new values.
• Each mapped Stream is closed after its contents have been placed into new Stream.
• flatMap() operation flattens the stream; opposite to map() operation which does not apply flattening.

3. Stream flatMap() Examples

Example 1: Converting Nested Lists into a Single List

Java 8 example of Stream.flatMap() function to get a single List containing all elements from a list of lists.

This program uses flatMap() operation to convert List> to List .

List list1 = Arrays.asList(1,2,3); List list2 = Arrays.asList(4,5,6); List list3 = Arrays.asList(7,8,9); List listOfLists = Arrays.asList(list1, list2, list3); List listOfAllIntegers = listOfLists.stream() .flatMap(x -> x.stream()) .collect(Collectors.toList()); System.out.println(listOfAllIntegers);

Example 2: Collecting Nested Arrays into a Sinle List

Java 8 example of Stream.flatMap() function to get a single List containing all elements from an array of arrays.

String[][] dataArray = new String[][], , , >; List listOfAllChars = Arrays.stream(dataArray) .flatMap(x -> Arrays.stream(x)) .collect(Collectors.toList()); System.out.println(listOfAllChars);

Drop me your questions related to Stream flatMap() method in Java Stream API.

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