如何从子类目标C中的NSMutableArray读取数字

Question

我要在子类内部进行计算。但是我完全陷入困境，因为我无法在子类内部的方法中读取属性self.productNumber。我需要创建方法CalculatePayment。我该如何解决这个问题？

Products.h

-1

#import <Foundation/Foundation.h>

@interface Product : NSObject

//Property
@property int ProductID;
@property (strong, nonatomic)NSString* productName;
@property float productPrice;
@property (strong, nonatomic)NSString* productMadeInCountry;

//Contructor
-(id) initWithProductID:(int)productID productName:(NSString*)productName productPrice:(float)productPrice productMadeInCountry:(NSString*)productMadeInCountry;
-(id) initWithProductName:(NSString*)productName;

@end

Products.m

    #import "Product.h"

    @implementation Product

    - (id) initWithProductID:(int)productID productName:(NSString*)productName productPrice:(float)productPrice productMadeInCountry:(NSString*)productMadeInCountry
    {
        self = [super init];
        if(self)
        {
            self.ProductID = productID;
            self.productName = productName;
            self.productPrice = productPrice;
            self.productMadeInCountry = productMadeInCountry;
        }
        return self;
    }

    -(id) initWithProductName:(NSString*)productName
    {
        self = [super init];
        if(self)
        {
            self.productName = productName;
        }
        return self;
    }
    @end

 Shopping_Cart.h

        #import <Foundation/Foundation.h>
#import "Product.h"

@interface Shopping_Cart : NSObject
//property
@property NSMutableArray<Product*>* items;
//Constructor

-(id)initWithItems:(NSMutableArray<Product*>*)items productPrice:(float)productPrice;

//Methods(APIs)
-(void)addItem:(Product*) product;
-(float)calculatePurchase:(Product*) productPrice;
-(void)printProductName:(Product*)productName;


@end