[Ch15b] Final exercise example, book version

package ch15b.exercise1;

import java.util.ArrayList;

import java.util.List;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.*;

import java.util.concurrent.*;

public class TwoThreadsWriting {

  public static void main(String[] args) {

      } catch (InterruptedException ignored) {}

    }

    System.out.println(Thread.currentThread().getName() + data.getLetters());

    System.out.println(Thread.currentThread().getName() + " getLetters().size() = " + data.getLetters().size());

    System.out.println(Thread.currentThread().getName()

                               + " size = " + data.getLetters().size());

  }

}

class Data {

final class Data {

  private final List<String> letters = new ArrayList<>();

  public List<String> getLetters() {

    return letters;

  }

  public List<String> getLetters() {return letters;}

  public void addLetter(char letter) {

    letters.add(String.valueOf(letter));