处理对象的多种状态及其相互转换——状态模式（三）.md

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！