synchronized(obj)
{
  //此處代碼塊就是同步代碼塊。
}

public class Account
{
   // 封裝賬戶編號(hào)、賬戶余額的兩個(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;
   }
   // 此處省略了accountNo和balance的setter和getter方法
   // accountNo的setter和getter方法
   public void setAccountNo(String accountNo)
   {
      this.accountNo = accountNo;
   }
   public String getAccountNo()
   {
      return this.accountNo;
   }
   // balance的setter和getter方法
   public void setBalance(double balance)
   {
      this.balance = balance;
   }
   public double getBalance()
   {
      return this.balance;
   }
   // 下面兩個(gè)方法根據(jù)accountNo來(lái)重寫(xiě)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(qián)線程所希望取的錢(qián)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() {
      synchronized (account) {
        // 賬戶余額大于取錢(qián)數(shù)目
        if (account.getBalance() >= drawAmount) {
           // 吐出鈔票
           System.out.println(getName() + "取錢(qián)成功！吐出鈔票:" + drawAmount);
           try {
              Thread.sleep(1);
           } catch (InterruptedException ex) {
              ex.printStackTrace();
           }
           // 修改余額
           account.setBalance(account.getBalance() - drawAmount);
           System.out.println("\t余額為: " + account.getBalance());
        } else {
           System.out.println(getName() + "取錢(qián)失??！余額不足！");
        }
      }
   }
}

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(qián)
      new DrawThread("甲" , acct , 800).start();
      new DrawThread("乙" , acct , 800).start();
   }
}