java – I want to combine lists with stream

Question: Question:

[[a, a, a], [b, b, b], [c, c, c]]
of
[a, a, a, b, b, b, c, c, c]
I want to do this, but is there a way to do it with stream in one shot?

//普通にループするとこんな感じです。
List innerA = Arrays.asList("a","a","a");
List innerB = Arrays.asList("b","b","b");
List innerC = Arrays.asList("c","c","c");
List<List> outer = Arrays.asList(innerA,innerB,innerC);
System.out.println(outer);
//[[a, a, a], [b, b, b], [c, c, c]]

List innerABC = new ArrayList<>();
for(List inner : outer){
    innerABC.addAll(inner);
}
System.out.println(innerABC);
//[a,a,a,b,b,b,c,c,c]

Answer: Answer:

You may be able to use Stream # flatMap (…) .

import java.util.*;
import java.util.stream.*;
import java.util.function.*;

public class StreamTest {
  public static void main(String... args) {
    List<String> innerA = Arrays.asList("a", "a", "a");
    List<String> innerB = Arrays.asList("b", "b", "b");
    List<String> innerC = Arrays.asList("c", "c", "c");
    List<List<String>> outer = Arrays.asList(innerA, innerB, innerC);
    //List innerABC = new ArrayList<>();
    //for(List inner : outer){
    //    innerABC.addAll(inner);
    //}
    List<String> innerABC = outer.stream()
      .flatMap(lst -> lst.stream())
      .collect(Collectors.toList());

    System.out.println(innerABC);
  }
}