I completed the first three tasks, but I don't know how to complete task 4. How to provide code protection for threads?

10 public class Account { 11 12 private volatile int balance; private final int id; 13 14 15 16 17 18 public Account(int id, int initialBalance) { this.id = id; this.balance = initialBalance; } 19 20 public synchronized int getBalance() { return balance; } 21 22 23 24 25 26 27 28 29 30 public synchronized boolean withdraw(int amount) { if (amount