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设计模式-状态模式

状态模式

角色

上下文（State）

定义了客户端需要的接口，内部维护一个当前状态，并负责具体状态的切换。

抽象状态（State）

定义一个接口，用以封装环境对象中的特定状态所对应的行为，可以有一个或多个行为。

具体状态（Concrete State）

实现抽象状态所对应的行为，并且在需要的情况下进行状态切换。

类图:

案例

银行账号状态

Account: 环境类

public 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
    }
}

AcountState: 抽象状态

public abstract class AccountState {
    protected Account acc;
    public abstract void deposit(double amount); //取款
    public abstract void withdraw(double amount); //透支
    public abstract void computeInterest();
    public abstract void stateCheck();

}

NormalState: 正常状态

public class NormalState extends AccountState {
    public NormalState(Account acc) {
        this.acc = acc;
    }

    public NormalState(AccountState state) {
        this.acc = state.acc;
    }

    @Override
    public void deposit(double amount) {
        acc.setBalance(acc.getBalance() + amount);
        stateCheck();
    }

    @Override
    public void withdraw(double amount) {
        acc.setBalance(acc.getBalance() - amount);
        stateCheck();
    }

    @Override
    public void computeInterest() {
        System.out.println("正常状态，无须支付利息！");
    }

    @Override
    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！");
        }
    }
}

OverdraftState: 透支状态

public 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！");
        }
    }
}

RestrictedState: 受限状态

public 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));
        }
    }
}

Main:

public class Main {
    public static void main(String[] args) {
        Account acc = new Account("evanp",0.0);
        acc.deposit(1000);
        acc.withdraw(2000);
        acc.deposit(3000);
        acc.withdraw(4000);
        acc.withdraw(1000);
        acc.computeInterest();
    }
}

Output:

evanp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！