子类用一个名为subitems的数组属性保存父类对象，为什么不能获取父类对象的信息呢？？？在main函数中subitems为空，不知道为什么。

main.m如下

#import <Foundation/Foundation.h>
#import "BNRContainer.h"
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        //创建一个NSMutableArray对象，并用items变量保存该对象的地址
        BNRContainer *items = [[BNRContainer alloc] init];
        for (int i = 0; i < 10; i++) {
            BNRItem *item = [BNRItem randomItem];
            [items.subitems addObject:item];
        }
        // item数组一直为空，是为什么？？？？？？？？
        for (BNRItem *item in [items subitems]) {
           NSLog(@"%@",item);
       }
        NSLog(@"%@",items);
        //释放items所指向的NSMutableArray对象
        items = nil;
       
    }
    return 0;
}

BNRContainer.h

#import "BNRItem.h"

@interface BNRContainer : BNRItem
@property (nonatomic)NSMutableArray *subitems;
- (NSString *)description;
@end

BNRContainer.m

#import "BNRContainer.h"

@implementation BNRContainer
- (NSString *)description
{
    NSString *string = [[NSString alloc] init];
    int sumtalto = 0;
    NSLog(@"%@",self.subitems);
        NSMutableArray *array = [self subitems];// 这个方法不对吗？？ 不能获取subitems 中的BNRItem对象？？
        for (int i = 0;i < [array count];i++) {
            
            sumtalto += [array[i] valueInDollars] + [self valueInDollars];
           
        }
        string = [NSString stringWithFormat:@"BNRItem and BNRContainer is total %d ",sumtalto];
        return string ;
    

}
@end

BNRItem.h

#import <Foundation/Foundation.h>

@interface BNRItem : NSObject
{
    NSString *_itemName;
    NSString *_serialNumber;
    int _valueInDollars;
    NSDate *_dateCreated;

}

+ (instancetype)randomItem;
//BNRItem类的指定初始化方法
- (instancetype)initWithItemName:(NSString *)name
                  valueInDollars:(int)value
                    serialNumber:(NSString *)sNumber;

    


- (instancetype)initWithItemName:(NSString *)name;

- (instancetype)initWithItemName:(NSString *)name
                    serialNumber:(NSString *)sNumber;
- (void)setItemName:(NSString *)str;
- (NSString *)itemName;

- (void)setSerialNumber:(NSString *)str;
- (NSString *)serialNumber;

- (void)setValueInDollars:(int)v;
- (int)valueInDollars;

- (NSDate *)dateCreated;
@end

BNRItem.m

#import "BNRItem.h"

@implementation BNRItem

+ (instancetype)randomItem
{
   //创建不可变数组对象，包含三个形容词
    NSArray *randomAdjectiveList = @[@"Fluffy",@"Rusty",@"Shiny"];
    
    //创建不可变数组对象，包含三个名词
    
    NSArray *randomNounList = @[@"Bear",@"Spork",@"Mac"];
    
    
    //根据数组对象所含对象的个数，得到随机索引
    //注意：运算符％是模运算符，运算后得到的是余数
    //因此adjectiveIndex 是一个0到2（包括2）的随机数
    
    NSInteger adjectiveIndex = arc4random() % [randomAdjectiveList count];
    NSInteger nounIndex = arc4random() % [randomNounList count];
    
    //注意，类型为NSInteger的变量不是对象
    //NSInteger是一种针对unsigned long(无符号长整数)的类型定义
    
    NSString *randomName = [NSString stringWithFormat:@"%@  %@",
                                  randomAdjectiveList[adjectiveIndex],
                                       randomNounList[nounIndex]];

    int randomValue = arc4random() % 100;
    
    NSString *randomserialNumber = [NSString stringWithFormat:@"%c%c%c%c%c",
                                    '0' + arc4random() % 10,
                                    'A' + arc4random() % 26,
                                    '0' + arc4random() % 10,
                                    'A' + arc4random() % 26,
                                    '0' + arc4random() % 10];
    
    BNRItem *newItem = [[self alloc] initWithItemName:randomName
                                       valueInDollars:randomValue
                                         serialNumber:randomserialNumber];
    
    return newItem;



}
- (instancetype)initWithItemName:(NSString *)name
                  valueInDollars:(int)value
                    serialNumber:(NSString *)sNumber
{
    //调用父类的指定初始化方法
    self = [super init];
    
    //父类的初始化方法是否成功创建了父类对象
    if (self) {
        //为实例变量设定初始值
        
        _itemName = name;
        _valueInDollars = value;
        _serialNumber = sNumber;
        
        //设置_dateCreated 的值为系统当前时间
        _dateCreated = [[NSDate alloc] init];
    }
    
    //返回初始化后对象的新地址
    
    return self;

}
- (instancetype)initWithItemName:(NSString *)name
{
    return [self initWithItemName:name
                   valueInDollars:0
                     serialNumber:@""];

}
- (instancetype)init
{
    return [self initWithItemName:@"Item"];

}
- (instancetype)initWithItemName:(NSString *)name serialNumber:(NSString *)sNumber
{
    return [self initWithItemName:name
                   valueInDollars:0
                     serialNumber:sNumber];

}
- (void)setItemName:(NSString *)str
{
    _itemName = str ;//与self.itemName = str ;有什么区别？
    
}
- (NSString *)itemName
{
    return _itemName;

}
- (void)setSerialNumber:(NSString *)str
{

    _serialNumber = str;
    
}
- (NSString *)serialNumber
{
    return _serialNumber;
}
- (void)setValueInDollars:(int)v
{
    _valueInDollars = v;
}
- (int)valueInDollars
{
    return _valueInDollars;
}
- (NSDate *)dateCreated
{
    return _dateCreated;
}
- (NSString *)description
{
  NSString *descriptionString =
    [[NSString alloc] initWithFormat:@"%@ (%@): Worth $%d,recorded on %@",
                      self.itemName,
                      self.serialNumber,
                      self.valueInDollars,
                      self.dateCreated ];
    
    return descriptionString;
}
@end