aclabs ex threads java

import java.util.HashMap;
import java.util.Random;
import java.util.Scanner;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicReference;

public class Main {

    AtomicReference<HashMap<String, Integer>> accToAmount = new AtomicReference<>(new HashMap<>());
    boolean withdraw(String accountNumber, Integer amountToSubstract) {
        if (accToAmount.get().get(accountNumber) < amountToSubstract) {
            return false;
        }

        HashMap<String, Integer> currentMap, newMap;
        synchronized (this){
            currentMap = accToAmount.get();
            newMap = new HashMap<>(currentMap);
            newMap.put(accountNumber, currentMap.get(accountNumber) - amountToSubstract);
            accToAmount.set(newMap);
        };

        return true;
    }

    void add(String accountNumber, Integer amountToAdd) {
        HashMap<String, Integer> currentMap, newMap;
        synchronized (this){
            currentMap = accToAmount.get();
            newMap = new HashMap<>(currentMap);
            newMap.put(accountNumber, currentMap.get(accountNumber) - amountToAdd);
            accToAmount.set(newMap);
        };
    }

    public static void main(String[] args) {
        Main main = new Main();
        for (int i = 1; i <= 10; i++) {
            main.accToAmount.get().put("Account" + i, (int)1e6);
        }

        final int NUM_THREADS = 100;
        ExecutorService executorService = Executors.newFixedThreadPool(NUM_THREADS);
        for (int i = 0; i < NUM_THREADS; i++) {
            executorService.execute(() -> {
                //main.withdraw("Account1", 100);
                Random random = new Random();
                int acNr = random.nextInt(10) + 1;
                for (int j = 0; j < 10; j++) {
                    double op = random.nextDouble();
                    int val = random.nextInt(1000) + 1;
                    if (op >= 0.5) {
                        boolean ok = main.withdraw("Account" + acNr, val);
                        if (!ok) {
                            System.err.println("failed withdraw");
                        }
                    } else {
                        main.add("Account" + acNr, val);
                    }
                }

            });
        }
        executorService.shutdown();

        for (int i = 1; i <= 10; i++) {
            System.out.println("Account" + i + ": " + main.accToAmount.get().get("Account" + i));
        }
    }
}