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】