《Head First Java》中银行账户示例：为何多个线程操作同一账户会导致Monica线程从中间开始执行？

class bankaccount {
    private int balance = 100;

    public int getbalance() {
        return balance;
    }

    public void withdraw(int amount) {
        balance = balance - amount;
    }
}

public class ryanandmonicajob implements runnable {
    private bankaccount account = new bankaccount();

    public static void main(string[] args) {
        ryanandmonicajob thejob = new ryanandmonicajob();
        thread one = new thread(thejob);
        thread two = new thread(thejob);
        one.setname("ryan");
        two.setname("monica");
        one.start();
        two.start();
    }

    public void run() {
        for (int x = 0; x < 10; x++) {
            makewithdrawal(10);
            if (account.getbalance() < 0) {
                system.out.println("overdrawn!");
            }
        }
    }

    private void makewithdrawal(int amount) {
        string currentthread = thread.currentthread().getname();
        if (account.getbalance() >= amount) {
            system.out.println(currentthread + " is about to withdraw");
            try {
                system.out.println(currentthread + " is going to sleep");
                thread.sleep(500);
            } catch (interruptedexception ex) {
                ex.printstacktrace();
            }
            system.out.println(currentthread + " woke up.");
            account.withdraw(amount);
            system.out.println(currentthread + " completes the withdrawl");
        } else {
            system.out.println("sorry, not enough for " + currentthread);
        }
    }
}

Ryan is about to withdraw
Monica is about to withdraw
Ryan is going to sleep
Monica is going to sleep
Monica woke up.
Sorry, not enough for Monica
Ryan woke up.
Ryan completes the withdrawl
Monica is about to withdraw
...