Calling class methods via class name vs self

Question

Suppose we have a class named Calculator. There's a class method in it, called runProgram. If I wanted to call this class method, inside the class's implementation, what would the difference between these two be:

[Calculator runProgram]

OR

[self runProgram]

Are these both the same?

Answer 1

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.