[Ch15] Creating an example that uses CopyOnWriteArrayList to correct store all change

package ch15;

import java.util.List;

import java.util.concurrent.CopyOnWriteArrayList;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class ThreadSafeCollection {

  public static void main(String[] args) {

    ExecutorService threadPool = Executors.newFixedThreadPool(2);

    threadPool.execute(new AddUpperCaseJob());

    threadPool.execute(new AddLowerCaseJob());

    threadPool.shutdown();

  }

}

class DataSafe {

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

  private final static DataSafe instance = new DataSafe();

  private DataSafe() {

  }

  public static DataSafe getInstance() {

    return instance;

  }

  public List<String> getLetters() {

    return letters;

  }

  public void addLetter(char letter) {

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

  }

}

class AddUpperCaseJob implements Runnable {

  DataSafe a = DataSafe.getInstance();

  char letter = 'A';

  public void run() {

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

      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 AddLowerCaseJob implements Runnable {

  DataSafe a = DataSafe.getInstance();

  char letter = 'a';

  public void run() {

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

      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());

  }

}