[Ch15] Data structure Ryan and Monica does not do what we want it to do

package ch15.ryanmonica;

import java.util.HashMap;

import java.util.Map;

import java.util.concurrent.ConcurrentHashMap;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

// This does not do what you think it does

// the original problem is, of course, that the check for balance is separate from the write. That's still true

// here, even with an immutable data structure. What you want is a withdrawal method that lets you atomically read

// the balance from the map and update it.

public class RyanAndMonicaUnmodifiableTest {

  public static void main(String[] args) throws InterruptedException {

    long start = System.currentTimeMillis();

class BankAccountCollection {

  Map<String, Integer> accountBalances = new HashMap<>();

  ConcurrentHashMap<String, Integer> accountBalances = new ConcurrentHashMap<>();

  {

    accountBalances.put("checking", 100);

    accountBalances.put("savings", 500);

    accountBalances.computeIfPresent(accountName, (s, integer) -> integer - amount);

  }

  public void makeWithdrawal(int amount, String accountName, String withdrawnBy) {

  public synchronized void makeWithdrawal(int amount, String accountName, String withdrawnBy) {

    final Integer balance = accountBalances.get(accountName);

    if (balance >= amount) {

      System.out.println(withdrawnBy + " is about to withdraw. balance: " + balance);