Java Stream - 如何减少大整数列表到字符串

import java.util.ArrayList;
import java.util.List;

public class Main {

  public static void main(String[] args) {

    List<Integer> l = getDatas();

    System.out.println(
        l.stream().map((a) -> (a.toString()))
        .reduce((a, b) -> (a + b ))
        .orElse("ko"));

  }

  public static List<Integer> getDatas() {

    List<Integer> l = new ArrayList(100);

    for (int i = 0; i < 99; i++) {
      l.add(i);
    }
    return l;
  }

}