[Ch15] Creating code that can incorrectly fill a not-thread-safe collection

package ch15;

import java.util.ArrayList;

import java.util.List;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class NotThreadSafeCollection {

  public static void main(String[] args) {

    ExecutorService threadPool = Executors.newFixedThreadPool(2);

    threadPool.execute(new JobOne());

    threadPool.execute(new JobTwo());

    threadPool.shutdown();

  }

}

class Data {

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

  private final static Data instance = new Data();

  private Data() {

  }

  public static Data getInstance() {

    return instance;

  }

  public List<String> getLetters() {

    return letters;

  }

  public void addLetter(char letter) {

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

  }

}

class JobOne implements Runnable {

  Data a = Data.getInstance();

  char letter = 'A';

  public void run() {

    for (int x = 0; x < 26; x++) {

      a.addLetter(letter++);

      try {

        Thread.sleep(50);

      } catch (InterruptedException ignored) {

      }

    }

    System.out.println("one " + a.getLetters());

    System.out.println("a.getLetters().size() = " + a.getLetters().size());

  }

}

class JobTwo implements Runnable {

  Data a = Data.getInstance();

  char letter = 'a';

  public void run() {

    for (int x = 0; x < 26; x++) {

      a.addLetter(letter++);

      try {

        Thread.sleep(50);

      } catch (InterruptedException ignored) {

      }

    }

    System.out.println("two " + a.getLetters());

    System.out.println("a.getLetters().size() = " + a.getLetters().size());

  }

}