Java线程同步

public class Accout {

    private static Account account = new Account();
    
    public static void main(String[] args) {
        ExecutorService executor = Executors.newCachedThreadPool();

        for (int i = 0; i < 1000; i++) {
            executor.execute(new AddAPennyTask());
        }
        executor.shutdown();
        
        while (!executor.isTerminated()){
            
        }
        System.out.println("账户余额：" + account.getBalance());
    }
    
    private static class Account{
        private int balance = 0;
        
        public int getBalance(){
            return balance;
        }
        
        public void deposit(int amount){
            balance = balance + amount;
            try {
                Thread.sleep(10);
            }catch (Exception e ){
                
            }
        }
    }
    
    private static class AddAPennyTask implements Runnable{

        @Override
        public void run() {
            account.deposit(1);
        }
    }
}

结果不可预测：账户余额：980 ? 970? 990?