Java 8 Stream – Convert List of List Strings to List Strings

As title, we can use flatMap to convert it.

Java9Example1.java

package com.favtuts.java8.stream;

import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;

public class StreamFlatMapList {

    public static void main(String[] args) {
        buildListOfListString();        
    }

    static void buildListOfListString() {
        List<String> numbers = Arrays.asList("1", "2", "A", "B", "C1D2E3");

        List<List<String>> collect = numbers.stream()
                .map(x -> new Scanner(x).findAll("\\D+")
                        .map(m -> m.group())
                        .collect(Collectors.toList()))
                .collect(Collectors.toList());

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

}

Output

[]
[]
[A]
[B]
[C, D, E]

Solution

Java9Example2.java

package com.favtuts.java8.stream;

import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;

public class StreamFlatMapList {

    public static void main(String[] args) {
        flatMapListOfListString();
    }

    static void flatMapListOfListString() {
        List<String> numbers = Arrays.asList("1", "2", "A", "B", "C1D2E3");

        List<String> collect = numbers.stream()
                .map(x -> new Scanner(x).findAll("\\D+")
                        .map(m -> m.group())
                        .collect(Collectors.toList())) // List<List<String>>
                .flatMap(List::stream) // List<String>
                .collect(Collectors.toList());

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

Output

A
B
C
D
E

Download Source Code

$ git clone https://github.com/favtuts/java-core-tutorials-examples

$ cd java-basic/java8/stream

References

• Java Regular Expression Examples
• Scanner.findAll JavaDocs