java - java 中的同步给出了意外的输出

Question

我正在翻阅一本书，看到一段代码，上面写着下面的代码是线程同步的:

//账户类别

 public class Account {

 private double balance;

 public double getBalance() {
 return balance;
}

 public void setBalance(double balance) {
 this.balance = balance;
}


 public synchronized void addAmount(double amount) {
 double tmp=balance;
 try {
 Thread.sleep(10);
 } catch (InterruptedException e) {
   e.printStackTrace();
 }
 tmp+=amount;
 balance=tmp;
}


public synchronized void subtractAmount(double amount) {
  double tmp=balance;
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
tmp-=amount;
balance=tmp;
}

}

//银行类

public class Bank implements Runnable {

 private Account account;
 public Bank(Account account) {
  this.account=account; 
}

@Override

public void run() {

for (int i=0; i<1000; i++){

     account.subtractAmount(1000);
  }
 }
}

//公司类

public class Company implements Runnable {

 private Account account;

 public Company(Account account) {
    this.account=account;
}

@Override

 public void run() {
   for (int i=0; i<1000; i++){
   account.addAmount(1000);
   }
  }
}

//主类

 public class Main {
 public static void main(String[] args) {
 Account account=new Account();
 account.setBalance(1000);
 Company company=new Company(account);
 Thread companyThread=new Thread(company);
 Bank bank=new Bank(account);
 Thread bankThread=new Thread(bank);
 System.out.printf("Account : Initial Balance: %f\n",account.
 getBalance());
 companyThread.start();
 bankThread.start();

 try {
   companyThread.join();
   bankThread.join();
   System.out.printf("Account : Final Balance: %f\n",account.
   getBalance());
 } 
    catch (InterruptedException e) {
    e.printStackTrace();
  }
 }
}

这里两个不同对象的两个线程正在使用addAmount和subtractAmount函数同时更改余额变量，但它总是给出正确的输出，因为初始和最终余额相同，即1000.000。我的意思是他们应该给我一些问题，因为两个线程同时处理同一个变量。

请纠正我哪里错了。 提前致谢

Answer 1

这就是同步的全部意义。它确保一次只有一个线程可以执行该函数。