代码如下：
（代码图片，没有想到这个排版是这个样子）

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);
    }
}

}
上述代码是为了探究线程并发性的问题，但是输出如下图：
主要不理解的就是输出中前两句是第一个对象的线程在执行，然后就行sleep，那么这个时候就应该是另一个线程执行，那么这个第二个执行的为什么不是从头开始执行，而是从makeWithdrawl方法中间的部分开始执行。