初始化它：

  NSMutableArray * array = [NSMutableArray array]; 
 pre> 

你得到一个NSMutableArray。Objective-C的一个很大的特点是类方法是由子类继承的。
 

所以，在类方法中，你可以这样做：
  +（id）array {
 return [[[self alloc] init] autorelease]; 
} 
 
和 self 将引用代码正在执行的类对象（ NSArray 或 NSMutableArray ）。
 
If you initialize an NSMutableArray with NSArray's convenience method as above, do you get an NSArray or an NSMutableArray?

Any consequences?

(I know that NSMutableArray has "arrayWithCapacity:, I'm just curious)
If you initialize it using:
NSMutableArray *array = [NSMutableArray array];
you get a NSMutableArray. One great feature of Objective-C is that class methods are inherited by subclasses.

So, in a class method you can do something like this:
+(id)array {
    return [[[self alloc] init] autorelease];
}
and self will be referencing to the class object where the code is executing (NSArray or NSMutableArray).