Objective-C method_exchangeImplementations result is not as expected - objective-c

Question

I'm learning Objective-C runtime, and try to use method_exchangeImplementations to exchange addObject: method and removeObject: method of NSMutableArray.
My code like this:
int main(int argc, const char * argv[]) {
#autoreleasepool {
Method removeMethod = class_getInstanceMethod(NSMutableArray.class, #selector(removeObject:));
Method addMethod = class_getInstanceMethod(NSMutableArray.class, #selector(addObject:));
method_exchangeImplementations(addMethod, removeMethod);
NSMutableArray *array = [[NSMutableArray alloc] init];
NSObject *obj = [[NSObject alloc] init];
[array removeObject:obj];
NSLog(#"%lu", (unsigned long)array.count); // expect print 1, actual print 1
[array addObject:obj];
NSLog(#"%lu", (unsigned long)array.count); // expect print 0, actual print 2
}
return 0;
}
I expect exchange add/remove function, but seems like only removeObject: has been exchange to addObject: , addObject: still is addObject to array, now I have two addObject method of NSMutableArray
I'm not sure the reason. I try to exchange other method like uppercaseString/lowercaseString of NSString, that work correct.

Answer 1

OK, I solved problem. Thanks #Willeke for the tip.
The real class of my array is not NSMutableArray, so use NSMutableArray.class in class_getInstanceMethod can't get correct method. Use [array class] is the answer.
int main(int argc, const char * argv[]) {
#autoreleasepool {
NSMutableArray *array = [[NSMutableArray alloc] init];
Method removeMethod = class_getInstanceMethod([array class], #selector(removeObject:));
Method addMethod = class_getInstanceMethod([array class], #selector(addObject:));
method_exchangeImplementations(addMethod, removeMethod);
NSObject *obj = [[NSObject alloc] init];
[array removeObject:obj];
NSLog(#"%lu", (unsigned long)array.count); // expect print 1, actual print 1
[array addObject:obj];
NSLog(#"%lu", (unsigned long)array.count); // expect print 0, actual print 0
}
return 0;
}