Java 8 Stream Convert List<List<String>> to List<String>

As title, we can use flatMap to convert it.

package com.mkyong.test;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;
public class Java9Example1 {
    public static void main(String[] args) {
        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

package com.mkyong.test;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;
public class Java9Example2 {
    public static void main(String[] args) {
        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