Java 8 flatMap example

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In Java 8, Stream can hold different data types, for examples:

Stream<String[]>
Stream<Set<String>>
Stream<List<String>>
Stream<List<Object>>

But, the Stream operations (filter, sum, distinct…) and collectors do not support it, so, we need flatMap() to do the following conversion :

Stream<String[]>-> flatMap ->Stream<String>
Stream<Set<String>>-> flatMap ->Stream<String>
Stream<List<String>>-> flatMap ->Stream<String>
Stream<List<Object>>-> flatMap ->Stream<Object>

How flatMap() works :

{ {1,2}, {3,4}, {5,6} } -> flatMap -> {1,2,3,4,5,6}

{ {'a','b'}, {'c','d'}, {'e','f'} } -> flatMap -> {'a','b','c','d','e','f'}
1. Stream + String[] + flatMap
1.1 The below example will print an empty result, because filter() has no idea how to filter a stream of String[].

TestExample1.java
package com.mkyong.java8;

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

public class TestExample1 {

public static void main(String[] args) {

String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};

//Stream<String[]>
Stream<String[]> temp = Arrays.stream(data);

//filter a stream of string[], and return a string[]?
Stream<String[]> stream = temp.filter(x -> "a".equals(x.toString()));

stream.forEach(System.out::println);

}

Output

//empty...
1.2 In above example, we should use flatMap() to convert Stream<String[]> to Stream<String>.

TestExample1.java
package com.mkyong.java8;

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

public class TestExample1 {

public static void main(String[] args) {

String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};

//Stream<String[]>
Stream<String[]> temp = Arrays.stream(data);

//Stream<String>, GOOD!
Stream<String> stringStream = temp.flatMap(x -> Arrays.stream(x));

Stream<String> stream = stringStream.filter(x -> "a".equals(x.toString()));

stream.forEach(System.out::println);

/*Stream<String> stream = Arrays.stream(data)
.flatMap(x -> Arrays.stream(x))
.filter(x -> "a".equals(x.toString()));*/

}

Output

a
2. Stream + Set + flatMap
2.1 A student POJO.

Student.java
package com.mkyong.java8;

import java.util.HashSet;
import java.util.Set;

public class Student {

private String name;
private Set<String> book;

public void addBook(String book) {
if (this.book == null) {
this.book = new HashSet<>();
}
this.book.add(book);
}
//getters and setters

}

2.2 flatMap() and Set example.

TestExample2.java
package com.mkyong.java8;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class TestExample2 {

public static void main(String[] args) {

Student obj1 = new Student();
obj1.setName("mkyong");
obj1.addBook("Java 8 in Action");
obj1.addBook("Spring Boot in Action");
obj1.addBook("Effective Java (2nd Edition)");

Student obj2 = new Student();
obj2.setName("zilap");
obj2.addBook("Learning Python, 5th Edition");
obj2.addBook("Effective Java (2nd Edition)");

List<Student> list = new ArrayList<>();
list.add(obj1);
list.add(obj2);

List<String> collect =
list.stream()
.map(x -> x.getBook()) //Stream<Set<String>>
.flatMap(x -> x.stream()) //Stream<String>
.distinct()
.collect(Collectors.toList());

collect.forEach(x -> System.out.println(x));
}

}

Output

Spring Boot in Action
Effective Java (2nd Edition)
Java 8 in Action
Learning Python, 5th Edition
Try comments the flatMap(x -> x.stream()) the Collectors.toList() will prompts a compiler error, because it has no idea how to collect a stream of Set object.

3. Stream + Primitive + flatMapToInt
3.1 For primitive type, you can use flatMapToInt.

TestExample3.java
package com.mkyong.java8;

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

public class TestExample3 {

public static void main(String[] args) {

int[] intArray = {1, 2, 3, 4, 5, 6};

//1. Stream<int[]>
Stream<int[]> streamArray = Stream.of(intArray);

//2. Stream<int[]> -> flatMap -> IntStream
IntStream intStream = streamArray.flatMapToInt(x -> Arrays.stream(x));

intStream.forEach(x -> System.out.println(x));

}

Output

1
2
3
4
5
6