[Ch15] Adding a synchronized solution to the problem

package ch15.exercise1;

import java.util.ArrayList;

import java.util.List;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class SynchronizedCollection {

  public static void main(String[] args) {

    ExecutorService threadPool = Executors.newFixedThreadPool(2);

    Data data = new DataSynchronized();

    threadPool.execute(new AddLowerCaseJob(data));

    threadPool.execute(new AddUpperCaseJob(data));

    threadPool.shutdown();

  }

}

class DataSynchronized implements Data {

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

  public List<String> getLetters() {

    return letters;

  }

  public synchronized void addLetter(char letter) {

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

  }

}