public class Account
{
   // 封裝賬戶編號(hào)、賬戶余額兩個(gè)成員變量
   private String accountNo;
   private double balance;
   public Account(){}
   // 構(gòu)造器
   public Account(String accountNo , double balance)
   {
      this.accountNo = accountNo;
      this.balance = balance;
   }
   // accountNo的setter和getter方法
   public void setAccountNo(String accountNo)
   {
      this.accountNo = accountNo;
   }
   public String getAccountNo()
   {
      return this.accountNo;
   }
   // 因此賬戶余額不允許隨便修改，所以只為balance提供getter方法，
   public double getBalance()
   {
      return this.balance;
   }
   // 提供一個(gè)線程安全draw()方法來(lái)完成取錢操作
   public synchronized void draw(double drawAmount)
   {
      // 賬戶余額大于取錢數(shù)目
      if (balance >= drawAmount)
      {
        // 吐出鈔票
        System.out.println(Thread.currentThread().getName()
           + "取錢成功！吐出鈔票:" + drawAmount);
        try
        {
           Thread.sleep(1);
        }
        catch (InterruptedException ex)
        {
           ex.printStackTrace();
        }
        // 修改余額
        balance -= drawAmount;
        System.out.println("\t余額為: " + balance);
      }
      else
      {
        System.out.println(Thread.currentThread().getName()
           + "取錢失敗！余額不足！");
      }
   }
   // 下面兩個(gè)方法根據(jù)accountNo來(lái)重寫hashCode()和equals()方法
   public int hashCode()
   {
      return accountNo.hashCode();
   }
   public boolean equals(Object obj)
   {
      if(this == obj)
        return true;
      if (obj !=null
        && obj.getClass() == Account.class)
      {
        Account target = (Account)obj;
        return target.getAccountNo().equals(accountNo);
      }
      return false;
   }
}

public class DrawThread extends Thread
{
   // 模擬用戶賬戶
   private Account account;
   // 當(dāng)前取錢線程所希望取的錢數(shù)
   private double drawAmount;
   public DrawThread(String name , Account account
      , double drawAmount)
   {
      super(name);
      this.account = account;
      this.drawAmount = drawAmount;
   }
   // 當(dāng)多條線程修改同一個(gè)共享數(shù)據(jù)時(shí)，將涉及數(shù)據(jù)安全問(wèn)題。
   public void run()
   {
      // 直接調(diào)用account對(duì)象的draw方法來(lái)執(zhí)行取錢
      // 同步方法的同步監(jiān)視器是this，this代表調(diào)用draw()方法的對(duì)象。
      // 也就是說(shuō)：線程進(jìn)入draw()方法之前，必須先對(duì)account對(duì)象的加鎖。
      account.draw(drawAmount);
   }
}

public class DrawTest
{
   public static void main(String[] args)
   {
      // 創(chuàng)建一個(gè)賬戶
      Account acct = new Account("1234567" , 1000);
      // 模擬兩個(gè)線程對(duì)同一個(gè)賬戶取錢
      new DrawThread("甲" , acct , 800).start();
      new DrawThread("乙" , acct , 800).start();
   }
}