一起来学设计模式之状态模式

public interface State {
    void handle(Context context);
}
public class Context {
    private State state;
    public Context() {
        state = new ConcreteStateA();
    }
    public void setState(State state) {
        this.state = state;
    }
    public void request() {
        state.handle(this);
    }
}
public class ConcreteStateA implements State {
    @Override
    public void handle(Context context) {
        System.out.println("当前状态是 A.");
        context.setState(new ConcreteStateB());
    }
}
public class ConcreteStateB implements State {
    @Override
    public void handle(Context context) {
        System.out.println("当前状态是 B.");
        context.setState(new ConcreteStateA());
    }
}
public class Client {
    public static void main(String[] args) {
        Context context = new Context();
        context.request();
        context.request();
        context.request();
        context.request();
//        当前状态是 A.
//        当前状态是 B.
//        当前状态是 A.
//        当前状态是 B.
    }
}

public interface OrderStatus {
    void process(Order order);
}
// 下单状态实现类
public class CreatedStatus implements OrderStatus {
     @Override
    public void process(Order order) {
        System.out.println("订单已经下单，等待付款");
        order.setStatus(new PaidStatus());
    }
}
// 付款状态实现类
public class PaidStatus implements OrderStatus {
     @Override
    public void process(Order order) {
        System.out.println("订单已经付款，等待发货");
        order.setStatus(new ShippedStatus());
    }
}
// 发货状态实现类
public class ShippedStatus implements OrderStatus {
     @Override
    public void process(Order order) {
        System.out.println("订单已经发货，等待确认收货");
        order.setStatus(new DeliveredStatus());
    }
}
// 确认收货状态实现类
public class DeliveredStatus implements OrderStatus {
     @Override
    public void process(Order order) {
        System.out.println("订单已经确认收货，交易完成");
        order.setStatus(this);
    }
}
// 订单类
public class Order {
    private OrderStatus status;
     public Order() {
        status = new CreatedStatus();
    }
     public void setStatus(OrderStatus status) {
        this.status = status;
    }
     public void process() {
        status.process(this);
    }
}
// 测试代码
public class Client {
    public static void main(String[] args) {
        Order order = new Order();
        order.process();
        order.process();
        order.process();
        order.process();
    }
}

//        订单已经下单，等待付款
//        订单已经付款，等待发货
//        订单已经发货，等待确认收货
//        订单已经确认收货，交易完成