求救`：如何让 一个类的两个对象的 属性值 同步改变？

Question

class Account
{
   private double balance;
   public Account(double balance)
   {
       this.balance=balance;
   }   public double getBalance()
   {
      return this.balance;
   }    public void setBlance(double s_balance)
    {
       this.balance=s_balance;
    }
    
}class Customer
{
    private String firstName;
    private String lastName;    private Account account;
  
    public Customer(String f,String l)
    {
        this.firstName=f;
        this.lastName=l;
    }
  
    public Account getAccount(Account acc)
    {
        this.account=acc;
        return this.account;
    }}
如上面两个类。当我定义2个对象，分别为 
Customer c1=new Customer("zhang","san");
Customer c2=new Customer("zhang","si");如何让这两个 对象 共用一个 Account 帐户，即一个Customer 的 Account 的 balance改变，另一个也随之改变
困扰了我好几天了，请高手帮忙，多谢！多谢！

Answer 1

class Account
{
private double balance;
public Account(double balance)
{
this.balance=balance;
}public double getBalance()
{
return this.balance;
}public void setBalance(double s_balance)
{
this.balance=s_balance;
}}class Customer
{
private String firstName;
private String lastName;private Account account;public Customer(String f,String l)
{
this.firstName=f;
this.lastName=l;
}public Account getAccount()
{
return this.account;
}public void setAccount(Account acc)
{
this.account = acc ;
}}public class Test
{
public static void main(String[] arg)
{
Customer c1=new Customer("zhang","san");
Customer c2=new Customer("zhang","si");

Account acc = new Account(2.2);

c1.setAccount(acc);
c2.setAccount(acc);

c1.getAccount().setBalance(3.3);

System.out.println(c2.getAccount().getBalance());
}
}

Answer 2

private static Account account;

Answer 3

class Account
{
private double balance;
public Account(double balance)
{
this.balance=balance;
}public double getBalance()
{
return this.balance;
}public void setBalance(double s_balance)
{
this.balance=s_balance;
}}class Customer
{
private String firstName;
private String lastName;private static Account account;public Customer(String f,String l)
{
this.firstName=f;
this.lastName=l;
}public Account getAccount()
{
return this.account;
}public void setAccount(Account acc)
{
this.account = acc ;
}}public class Test
{
public static void main(String[] arg)
{
Customer c1=new Customer("zhang","san");
Customer c2=new Customer("zhang","si");

Account acc = new Account(2.2);
c1.setAccount(acc);
System.out.println(c2.getAccount().getBalance());
c2.getAccount().setBalance(3.3);
System.out.println(c1.getAccount().getBalance());
}
}

Answer 4

private static Account account;
好像这里的 static 作用不大吧...