Calling class methods via class name vs self
If inside an instance method:
[self runProgram]in this, self means the object instance itself, and thus it will generate a runtime error. You want to use
[[self class] runProgram]instead.
However, if you call this method from another class method, then
[self runProgram]is correct, since now self refers to the class itself. I'd discourage to use
[Calculator runProgram]because then subclasses of the class would erroneously call the superclass' (Calculator's) method instead of a possibly overridden method.