[App A] Replacing Thread with newSingleThreadExecutor.

import java.io.ObjectOutputStream;

import java.net.Socket;

import java.util.*;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class BeatBoxFinal {

  public static final int NUMBER_OF_BEATS = 16;

  private static final int NUMBER_OF_BEATS = 16;

  private final List<BeatInstrument> instruments;

  private final Map<BeatInstrument, List<JCheckBox>> instrumentCheckboxes = new TreeMap<>();

  private final Map<BeatInstrument, List<JCheckBox>> instrumentCheckboxes = new TreeMap<>();

  private JList<String> messages;

  private JTextField userMessage;

  private int nextNum;

  private final Vector<String> incomingMessages = new Vector<>();

  private String userName;

  private final Vector<String> incomingMessages = new Vector<>();

  private ObjectOutputStream out;

  private ObjectInputStream in;

  private final HashMap<String, boolean[]> otherSeqsMap = new HashMap<>();

  public void startUp(String name) {

    userName = name;

    // open connection to the server

    try {

      Socket sock = new Socket("127.0.0.1", 4242);

      out = new ObjectOutputStream(sock.getOutputStream());

      in = new ObjectInputStream(sock.getInputStream());

      Thread remote = new Thread(new RemoteReader());

      remote.start();

      ExecutorService executor = Executors.newSingleThreadExecutor();

      executor.submit(new RemoteReader());

    } catch (IOException ex) {

      System.out.println("Couldn't connect-you'll have to play alone.");

    }