处理对象的多种状态及其相互转换——状态模式（三）

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，其基本结构如图所示：

在图中，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：

//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！