Sunny软件公司开发人员使用状态模式来解决账户状态的转换问题，客户端只需要执行简单的存款和取款操作，系统根据余额将自动转换到相应的状态，其基本结构如图4所示：
图4 银行账户结构图
在图4中，Account充当环境类角色，AccountState充当抽象状态角色，NormalState、OverdraftState和RestrictedState充当具体状态角色。完整代码如下所示：
温馨提示：代码有点长，需要有耐心！
//银行账户：环境类 class Account { private AccountState state; //维持一个对抽象状态对象的引用 private String owner; //开户名 private double balance = 0; //账户余额 public Account(String owner,double init) { this.owner = owner; this.balance = balance; this.state = new NormalState(this); //设置初始状态 System.out.println(this.owner + "开户，初始金额为" + init); System.out.println("---------------------------------------------"); } public double getBalance() { return this.balance; } public void setBalance(double balance) { this.balance = balance; } public void setState(AccountState state) { this.state = state; } public void deposit(double amount) { System.out.println(this.owner + "存款" + amount); state.deposit(amount); //调用状态对象的deposit()方法 System.out.println("现在余额为"+ this.balance); System.out.println("现在帐户状态为"+ this.state.getClass().getName()); System.out.println("---------------------------------------------"); } public void withdraw(double amount) { System.out.println(this.owner + "取款" + amount); state.withdraw(amount); //调用状态对象的withdraw()方法 System.out.println("现在余额为"+ this.balance); System.out.println("现在帐户状态为"+ this. state.getClass().getName()); System.out.println("---------------------------------------------"); } public void computeInterest() { state.computeInterest(); //调用状态对象的computeInterest()方法 } } //抽象状态类 abstract class AccountState { protected Account acc; public abstract void deposit(double amount); public abstract void withdraw(double amount); public abstract void computeInterest(); public abstract void stateCheck(); } //正常状态：具体状态类 class NormalState extends AccountState { public NormalState(Account acc) { this.acc = acc; } public NormalState(AccountState state) { this.acc = state.acc; } public void deposit(double amount) { acc.setBalance(acc.getBalance() + amount); stateCheck(); } public void withdraw(double amount) { acc.setBalance(acc.getBalance() - amount); stateCheck(); } public void computeInterest() { System.out.println("正常状态，无须支付利息！"); } //状态转换 public void stateCheck() { if (acc.getBalance() > -2000 && acc.getBalance() <= 0) { acc.setState(new OverdraftState(this)); } else if (acc.getBalance() == -2000) { acc.setState(new RestrictedState(this)); } else if (acc.getBalance() < -2000) { System.out.println("操作受限！"); } } } //透支状态：具体状态类 class OverdraftState extends AccountState { public OverdraftState(AccountState state) { this.acc = state.acc; } public void deposit(double amount) { acc.setBalance(acc.getBalance() + amount); stateCheck(); } public void withdraw(double amount) { acc.setBalance(acc.getBalance() - amount); stateCheck(); } public void computeInterest() { System.out.println("计算利息！"); } //状态转换 public void stateCheck() { if (acc.getBalance() > 0) { acc.setState(new NormalState(this)); } else if (acc.getBalance() == -2000) { acc.setState(new RestrictedState(this)); } else if (acc.getBalance() < -2000) { System.out.println("操作受限！"); } } } //受限状态：具体状态类 class RestrictedState extends AccountState { public RestrictedState(AccountState state) { this.acc = state.acc; } public void deposit(double amount) { acc.setBalance(acc.getBalance() + amount); stateCheck(); } public void withdraw(double amount) { System.out.println("帐号受限，取款失败"); } public void computeInterest() { System.out.println("计算利息！"); } //状态转换 public void stateCheck() { if(acc.getBalance() > 0) { acc.setState(new NormalState(this)); } else if(acc.getBalance() > -2000) { acc.setState(new OverdraftState(this)); } } }编写如下客户端测试代码：
class Client { public static void main(String args[]) { Account acc = new Account("段誉",0.0); acc.deposit(1000); acc.withdraw(2000); acc.deposit(3000); acc.withdraw(4000); acc.withdraw(1000); acc.computeInterest(); } }编译并运行程序，输出结果如下：
段誉开户，初始金额为0.0
---------------------------------------------
段誉存款1000.0
现在余额为1000.0
现在帐户状态为NormalState
---------------------------------------------
段誉取款2000.0
现在余额为-1000.0
现在帐户状态为OverdraftState
---------------------------------------------
段誉存款3000.0
现在余额为2000.0
现在帐户状态为NormalState
---------------------------------------------
段誉取款4000.0
现在余额为-2000.0
现在帐户状态为RestrictedState
---------------------------------------------
段誉取款1000.0
帐号受限，取款失败
现在余额为-2000.0
现在帐户状态为RestrictedState
---------------------------------------------
计算利息！
【作者：刘伟 http://blog.csdn.net/lovelion】
相关资源：Java 面经手册·小傅哥(公众号：bugstack虫洞栈).pdf