A class variable (@@) is shared among the class and all of its descendants. A class instance variable (@) is not shared by the class’s descendants. Class variable (@@) Let’s have a class Foo with a class variable @@i, and accessors for reading and writing @@i: class Foo @@i = 1 def self.i @@i end … Read more
Instance variable on a class: class Parent @things = [] def self.things @things end def things self.class.things end end class Child < Parent @things = [] end Parent.things << :car Child.things << :doll mom = Parent.new dad = Parent.new p Parent.things #=> [:car] p Child.things #=> [:doll] p mom.things #=> [:car] p dad.things #=> [:car] … Read more
Upcasting is casting to a supertype, while downcasting is casting to a subtype. Upcasting is always allowed, but downcasting involves a type check and can throw a ClassCastException. In your case, a cast from a Dog to an Animal is an upcast, because a Dog is-a Animal. In general, you can upcast whenever there is … Read more
Variables declared inside the class definition, but not inside a method are class or static variables: >>> class MyClass: … i = 3 … >>> MyClass.i 3 As @millerdev points out, this creates a class-level i variable, but this is distinct from any instance-level i variable, so you could have >>> m = MyClass() >>> … Read more