Ruby's class variables and class methods

Like most object-oriented languages, ruby classes also allow you to define class variables and methods. A class variable allows a single variable to be shared among all instances of a class. In Ruby, two @@ numbers are used to indicate class variables. For example, if you want to make all instances of a BankAccount class share the same interest rate, the class might be defined as follows:

Class BankAccount
@ @interestRate = 6.5
def bankaccount.getinterestrate ()
@ @interestRate
End Attr_accessor:balance
def initialize (bal)
@balance = Bal
End

As you can see, class variables must be initialized before use, and like instance variables, you need to write accessor methods if you want to access class variables. Here, I define a class method to return the interest rate. Note that the class name and the period preceding the getinterestrate represent a class method. A class method, regardless of any instance, works the same way-this is to return the same interest rate to all bankaccount instances. To invoke a class method, you need to use the class name as it would in the class method definition:

IRB (main):045:0> bankaccount.getinterestrate
=> 6.5

In fact, the "new" method for creating instances of a class is a class method. So, when you type "Rectangle.new" into your program, you actually call the new class method-This is what Ruby provides by default.

Inherited

One of the principles of object-oriented programming is to support class hierarchies. Like the sort of things in nature, classes allow inheritance from more general classes. The features of object-oriented programming are mainly embodied in the use of methods and variables. For example, a square class inherits some of the characteristics of the rectangle class, such as methods and variables. A square is a more specific type of rectangle (an rectangle instance of equal height and width), but it still has a height and width and an area (and is the same as the calculation method of the rectangle). In Ruby, the square class can be created using the following definitions:

Class Square < Rectangle end

"<rectangle" means that square is a subclass of Rectangle, or, conversely, that Rectangle is a superclass of square. By default, a square instance automatically owns the same properties and methods that all of the rectangle have, including the Height,width and area methods. To ensure that the side length of the square instance is equal, you can overload the existing square Initialize method:

Class Square < Rectangle
def initialize (size)
@height = size
@width = size
End