[Ch15b] Now trying to create the simplest example that throws concurrent modification

package ch15;

import java.util.ArrayList;

import java.util.List;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class LotsOfReaders {

  public static void main(String[] args) {

    ChatHistory chatHistory = new ChatHistory();

    ExecutorService executor = Executors.newFixedThreadPool(4);

    for (int i = 0; i < 5; i++) {

      executor.execute(new Chatty(chatHistory));

      executor.execute(new ChatReader(chatHistory));

      executor.execute(new ChatReader(chatHistory));

    }

    executor.shutdown();

  }

}

class ChatReader implements Runnable {

  private ChatHistory chatHistory;

  public ChatReader(ChatHistory chatHistory) {

    this.chatHistory = chatHistory;

  }

  @Override

  public void run() {

    for (Chat chat : chatHistory.getHistory()) {

      System.out.println(chat);

    }

  }

}

class Chatty implements Runnable {

  private ChatHistory chatHistory;

  public Chatty(ChatHistory chatHistory) {

    this.chatHistory = chatHistory;

  }

  @Override

  public void run() {

    chatHistory.add("Hi there!");

  }

}

class ChatHistory {

  private final List<Chat> history = new ArrayList<>();

  public List<Chat> getHistory() {

    return history;

  }

  public void add(String message) {

    history.add(new Chat(message));

  }

}

class Chat {

  private String message;

  public Chat(String message) {

    this.message = message;

  }

  @Override

  public String toString() {

    return message;

  }

}