没有可见的接口错误

我的模型的实现文件中有一个错误，我已将其注释掉。我可以做什么来解决这个问题？

提前致谢。

#import "CalculatorBrain.h"

@interface CalculatorBrain()
@property (nonatomic, strong) NSMutableSet *operandStack;
@end

@implementation CalculatorBrain

@synthesize operandStack = _operandStack;

- (NSMutableArray *)operandStack
{
    if (!_operandStack) {
        _operandStack = [[NSMutableArray alloc] init];
    }
    return _operandStack;
}

-(void)pushOperand:(double)operand
{
    NSNumber *operandObject = [NSNumber numberWithDouble:operand];
    [self.operandStack addObject:operandObject];
}

- (double)popOperand
{
    NSNumber *operandObject = [self.operandStack lastObject]; // No visible interface for 'NSMutableSet' declares the selector 'lastObject'
    if(operandObject)   [self.operandStack removeLastObject]; // No visible interface for 'NSMutableSet' declares the selector 'removeLastObject'
    return [operandObject doubleValue];
}

- (double)performOperation:(NSString *)operation
{
    double result = 0;

    if([operation isEqualToString:@"+"]) {
        result = [self popOperand] + [self popOperand];
    } else if ([@"*" isEqualToString:operation]) {
        result = [self popOperand] * [self popOperand];
    } else if ([operation isEqualToString:@"-"]) {
        double subtrahend = [self popOperand];
        result = [self popOperand] - subtrahend;
    } else if ([operation isEqualToString:@"/"]) {
        double divisor = [self popOperand];
        if (divisor)result = [self popOperand] / divisor;
    }

    [self pushOperand:result];

    return result;

}


@end

您已宣布您的operandStack财产作为NSMutableSet，但你应该将其声明为NSMutableArray:

@property (nonatomic, strong) NSMutableArray *operandStack;