How does this multithreaded Java code work?

Question

Given this Java code:

class Account {
        private Integer number = 0;
        public synchronized void setNumber(Integer number) {
            this.number = number;
        }

         public synchronized Integer getNumber() {
            return number;
        }
    }

    class Client extends Thread {
        Account account;
        public Client(Account account) {
            this.account = account;
        }
        public  void run() {
            for (int i = 1; i <= 1000; i++) {
            account.setNumber(account.getNumber() + 1);
             }
        }
    }

    public class Run {
        public static void main(String[] args) throws Exception {
            Account account = new Account();
            Client one = new Client(account);
            Client two = new Client(account);
            one.start();
            two.start();
            one.join();
            two.join();
           System.out.println("Exiting main");
       System.out.println("account number value: " +account.getNumber());        
        }
    }

What is the value of number when the main method completes? Is it 2000 or less than 2000? I am getting less than 2000. How can two threads call getNumer() or setNumber() from run() at the same time, given that each one is synchronized?

Answer 1

Think carefully about what happens in the following section.

account.setNumber(account.getNumber() + 1);

Even though both methods are separately synchronized, the operation as a whole isn't.