Java Stream - 如何使用flatMap

import java.util.Arrays;
import java.util.List;

public class Main {

  public static void main(String[] args) throws Exception{
    // map
    List<String> words = Arrays.asList("Hello", "World");

    words.stream()
    .flatMap((String line) -> Arrays.stream(line.split("")))
    .distinct()
    .forEach(System.out::println);

  }

}