Java 8 flatMap Example
May 31, 2016
According to the Java Doc, Stream.flatMap() method signature is as given below.
<R> Stream<R> flatMap(Function<? super T,? extends Stream<? extends R>> mapper)
public <U> Optional<U> flatMap(Function<? super T,Optional<U>> mapper)
Contents
Stream flatMap with List
Here we have aList of writers. Each writer has list of books. Using Stream.flatMap() we will get stream of books from all writers. And then we will find the book with highest price. We will understand it step wise.
1. Stream of writers.
{
{"Mohan",
{
{10,"AAA"}, {20,"BBB"}
}
},
{"Sohan",
{
{30,"XXX"}, {15,"ZZZ"}
}
}
}
2. After flatMap(writer -> writer.getBooks().stream()), find the stream of books.
{
{10,"AAA"},
{20,"BBB"},
{30,"XXX"},
{15,"ZZZ"}
}
Here the result has been flattened by flatMap().
3. After
max(new BookComparator()), find the book with maximum price.
{30,"XXX"}
Now find the code.
FlatmapWithList.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class FlatmapWithList {
public static void main(String[] args) {
List<Book> books = Arrays.asList(new Book(10, "AAA"), new Book(20, "BBB"));
Writer w1 = new Writer("Mohan", books);
books = Arrays.asList(new Book(30, "XXX"), new Book(15, "ZZZ"));
Writer w2 = new Writer("Sohan", books);
List<Writer> writers = Arrays.asList(w1, w2);
Book book = writers.stream().flatMap(writer -> writer.getBooks().stream())
.max(new BookComparator()).get();
System.out.println("Name:"+book.getName()+", Price:"+ book.getPrice() );
}
}
package com.concretepage;
import java.util.List;
public class Writer {
private String name;
private List<Book> books;
public Writer(String name, List<Book> books) {
this.name = name;
this.books = books;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Book> getBooks() {
return books;
}
public void setBooks(List<Book> books) {
this.books = books;
}
}
package com.concretepage;
public class Book {
private int price;
private String name;
public Book(int price, String name) {
this.price = price;
this.name = name;
}
public int getPrice() {
return price;
}
public void setPrice(int price) {
this.price = price;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
package com.concretepage;
import java.util.Comparator;
public class BookComparator implements Comparator<Book> {
@Override
public int compare(Book b1, Book b2) {
if (b1.getPrice() > b2.getPrice()) {
return 1;
} else if (b1.getPrice() == b2.getPrice()) {
return 0;
} else {
return -1;
}
}
}
Name:XXX, Price:30
Stream flatMap with List of Lists
Here we will useflatMap with list of lists. We are creating two lists and each list is containing the objects of Book. Finally I am adding these two lists in a third list. We will find out the book with minimum price.
FlatmapWithListOfList.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class FlatmapWithListOfList {
public static void main(String[] args) {
List<Book> list1 = Arrays.asList(new Book(10, "AAA"), new Book(20, "BBB"));
List<Book> list2 = Arrays.asList(new Book(30, "XXX"), new Book(15, "ZZZ"));
List<List<Book>> finalList = Arrays.asList(list1, list2);
Book book = finalList.stream().flatMap(list -> list.stream()).min(new BookComparator()).get();
System.out.println("Name:"+book.getName()+", Price:"+ book.getPrice() );
}
}
Name:AAA, Price:10
Stream flatMap with Array
Here we will useflatMap with array. I am creating a two dimensional array with integer data. Finally we will find out even numbers.
1. Sample Array
{{1,2},{3,4},{5,6}}
2. After flatMap(row -> Arrays.stream(row))
{1,2,3,4,5,6}
3. After filter(num -> num%2 == 0)
{2,4,6}
Now find the example.
FlatMapWithArray.java
package com.concretepage;
import java.util.Arrays;
public class FlatMapWithArray {
public static void main(String[] args) {
Integer[][] data = {{1,2},{3,4},{5,6}};
Arrays.stream(data).flatMap(row -> Arrays.stream(row)).filter(num -> num%2 == 0).
forEach(System.out::println);
}
}
2 4 6
Stream flatMap with Array of Objects
Here we will provide the example offlatMap with array of objects. We will create two dimensional array of Writer. This class will contain list of books. We will find the book with maximum price.
FlatMapWithArrayOfObject.java
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class FlatMapWithArrayOfObject {
public static void main(String[] args) {
List<Book> books = Arrays.asList(new Book(10, "AAA"), new Book(20, "BBB"));
Writer w1 = new Writer("Mohan", books);
books = Arrays.asList(new Book(30, "CCC"), new Book(15, "DDD"));
Writer w2 = new Writer("Sohan", books);
books = Arrays.asList(new Book(45, "EEE"), new Book(25, "FFF"));
Writer w3 = new Writer("Vikas", books);
books = Arrays.asList(new Book(5, "GGG"), new Book(15, "HHH"));
Writer w4 = new Writer("Ramesh", books);
Writer[][] writerArray = {{w1,w2},{w3,w4}};
Book book = Arrays.stream(writerArray).flatMap(row -> Arrays.stream(row)).
flatMap(writer -> writer.getBooks().stream()).max(new BookComparator()).get();
System.out.println("Name:"+book.getName()+", Price:"+ book.getPrice() );
}
}
Name:EEE, Price:45
Stream flatMap with Files.lines()
Files.lines() has been introduced in Java 8. It reads all the lines of the file as a stream. Here in our example we have a file with some lines. We will store all the words in a list and print it out.
info.txt
My name is Mohan Country is India
package com.concretepage;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Stream;
public class FlatMapWithFile {
public static void main(String[] args) {
Stream<String> lines = null;
try {
lines = Files.lines(Paths.get("D:/cp/info.txt"), StandardCharsets.UTF_8);
} catch (IOException e) {
e.printStackTrace();
}
Stream<String> stream = lines.flatMap(line -> Stream.of(line.split(" +")));
List<String> words = new ArrayList<>();
stream.forEach(w->words.add(w));
words.forEach(w -> System.out.println(w));
}
}
My name is Mohan Country is India
Optional flatMap
Optional has been introduced in Java 8. It behaves like a container that may keep non-null value. It handles NullPointerException. flatMap is applied only if value is present. Find the example.
OptionalflatMap.java
package com.concretepage;
import java.util.Optional;
public class OptionalflatMap {
public static void main(String[] args) {
Optional<PrimeMinister> primeMinister = Optional.of(new PrimeMinister("Narendra Modi", 65));
Optional<Country> country = Optional.of(new Country(primeMinister));
Optional<Person> person = Optional.of(new Person(country));
String pmName= person.flatMap(Person::getCountry).flatMap(Country::getPrimeMinister)
.map(PrimeMinister::getName).orElse("None");
System.out.println(pmName);
}
}
package com.concretepage;
import java.util.Optional;
public class Country {
Optional<PrimeMinister> primeMinister;
public Country(){}
public Country(Optional<PrimeMinister> primeMinister){
this.primeMinister = primeMinister;
}
public Optional<PrimeMinister> getPrimeMinister() {
return primeMinister;
}
public void setPrimeMinister(Optional<PrimeMinister> primeMinister) {
this.primeMinister = primeMinister;
}
}
package com.concretepage;
import java.util.Optional;
public class Person {
Optional<Country> country;
public Person(){}
public Person(Optional<Country> country){
this.country = country;
}
public Optional<Country> getCountry() {
return country;
}
public void setCountry(Optional<Country> country) {
this.country = country;
}
}
Narendra Modi
