Local variables are specific to the local scope.
Global variables have global scope.
Object instance variables have scope within the current object.
class Square def initialize(side_length) @side_length = side_length # w w w . ja v a2 s.c om end def area @side_length * @side_length end end a = Square.new(10) b = Square.new(5) puts a.area puts b.area
Object variables are prefixed with an @ symbol.
In the Square class, you assign the side_length provided to the class to @side_length.
@side_length, as an object variable, is then accessible from any other method inside that object.
That's how the area method can then use @side_length to calculate the area of the square represented by the object: