处理对象的多种状态及其相互转换——状态模式（三）
3 完整解决方案
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】