Ruby Object Oriented Programming (OOP)

Ruby is a pure object-oriented language and everything appears to Ruby as an object. Every value in Ruby is an object, even the most primitive things: strings, numbers and even true and false. Even a class itself is an object that is an instance of the Class class. This chapter will take you through all the major functionalities related to Object Oriented Ruby.

Table of Contents

Ruby Class Definition

A class is used to specify the form of an object and it combines data representation and methods for manipulating that data into one neat package. The data and methods within a class are called members of the class.

When you define a class, you define a blueprint for a data type. This doesn't actually define any data, but it does define what the class name means, that is, what an object of the class will consist of and what operations can be performed on such an object.

A class definition starts with the keyword class followed by the class name and is delimited with an end. For example, we defined the Box class using the keyword class as follows −

class Box
        code
    end

The name must begin with a capital letter and by convention names that contain more than one word are run together with each word capitalized and no separating characters (CamelCase).

Define Ruby Objects

A class provides the blueprints for objects, so basically an object is created from a class. We declare objects of a class using new keyword. Following statements declare two objects of class Box −

box1 = Box.new
    box2 = Box.new

The initialize Method

The initialize method is a standard Ruby class method and works almost same way as constructor works in other object oriented programming languages. The initialize method is useful when you want to initialize some class variables at the time of object creation. This method may take a list of parameters and like any other ruby method it would be preceded by def keyword as shown below −

class Box
        def initialize(w,h)
            @width, @height = w, h
        end
    end

The to_s Method

Any class you define should have a to_s instance method to return a string representation of the object. Following is a simple example to represent a Box object in terms of width and height −

#!/usr/bin/ruby -w

    class Box
        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end
        # define to_s method
        def to_s
            # string formatting of the object.
            "(w:#@width ,h:#@height)"  
        end
    end

    # create an object
    box = Box.new(10, 20)

    # to_s method will be called in reference of string automatically.
    puts "String representation of box is : #{box}"

When the above code is executed, it produces the following result −

String representation of box is : (w:10 ,h:20)

The instance Variables and instance Methods

The instance Variables

The instance variables are kind of class attributes and they become properties of objects once objects are created using the class. Every object's attributes are assigned individually and share no value with other objects. They are accessed using the @ operator within the class but to access them outside of the class we use public methods, which are called accessor methods. If we take the above defined class Box then @width and @height are instance variables for the class Box.

class Box
        def initialize(w,h)
            # assign instance variables
            @width, @height = w, h
        end
    end

The instance Methods

The instance methods are also defined in the same way as we define any other method using def keyword and they can be used using a class instance only as shown below. Their functionality is not limited to access the instance variables, but also they can do a lot more as per your requirement.

#!/usr/bin/ruby -w

    # define a class
    class Box
        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end
        # instance method
        def getArea
            @width * @height
        end
    end

    # create an object
    box = Box.new(10, 20)

    # call instance methods
    a = box.getArea()

    puts "Area of the box is : #{a}"

When the above code is executed, it produces the following result −

Area of the box is : 200

The class Methods and Variables

The class variables is a variable, which is shared between all instances of a class. In other words, there is one instance of the variable and it is accessed by object instances. while the class method is a method which used with class without need for creating a new instance.

  • class variables : are prefixed with two @ characters (@@). A class variable must be initialized within the class definition as shown below.

  • class method : is defined using def self.methodname(), which ends with end delimiter and would be called using the class name as classname.methodname as shown below.

#!/usr/bin/ruby -w

    class Box
        # Initialize our class variables
        @@count = 0
        def initialize(w,h)
            # assign instance avriables
            @width, @height = w, h

            @@count += 1
        end

        def self.printCount()
            puts "Box count is : #@@count"
        end
    end

    # create two object
    box1 = Box.new(10, 20)
    box2 = Box.new(30, 100)

    # call class method to print box count
    Box.printCount()

When the above code is executed, it produces the following result −

Box count is : 2

Class Constants

You can define a constant inside a class by assigning a direct numeric or string value to a variable, which is defined without using either @ or @@. By convention, we keep constant names in upper-case.

Once a constant is defined, you cannot change its value but you can access a constant directly inside a class much like a variable but if you want to access a constant outside of the class then you would have to use classname::constant as shown in the below example.

#!/usr/bin/ruby -w

    # define a class
    class Box
        BOX_COMPANY = "TATA Inc"
        BOXWEIGHT = 10
        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end
        # instance method
        def getArea
            @width * @height
        end
    end

    # create an object
    box = Box.new(10, 20)

    # call instance methods
    a = box.getArea()
    puts "Area of the box is : #{a}"
    puts Box::BOX_COMPANY
    puts "Box weight is: #{Box::BOXWEIGHT}"

When the above code is executed, it produces the following result −

Area of the box is : 200
TATA Inc
Box weight is: 10

Class constants are inherited and can be overridden like instance methods.

Encapsulation

The accessor & setter Methods

To make the variables available from outside the class, they must be defined within accessor methods, these accessor methods are also known as a getter methods. Ruby also provides a way to set the values of variables from outside of the class using setter methods.

Following example shows the usage of accessor methods and setter methods −

#!/usr/bin/ruby -w

    # define a class
    class Box
        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end

        # accessor methods
        def getWidth
            @width
        end
        def getHeight
            @height
        end

        # setter methods
        def setWidth=(value)
            @width = value
        end
        def setHeight=(value)
            @height = value
        end
    end

    # create an object
    box = Box.new(10, 20)

    # use accessor methods
    x = box.printWidth()
    y = box.printHeight()

    puts "Width of the box is : #{x}"
    puts "Height of the box is : #{y}"

    # use setter methods
    box.setWidth = 30
    box.setHeight = 50

    # use accessor methods
    x = box.getWidth()
    y = box.getHeight()

    puts "Width of the box is : #{x}"
    puts "Height of the box is : #{y}"

When the above code is executed, it produces the following result −

Width of the box is : 10
Height of the box is : 20

Width of the box is : 30
Height of the box is : 50

The attr_reader, attr_writer, and attr_accessor

Ruby also provide a helping methods for setter methods and getter methods:

  • attr_reader : creates only the reader.
  • attr_writer : creates only the writer.
  • attr_accessor : creates reader and writer. 
#!/usr/bin/ruby -w

    # define a class
    class Box
        ## Setter and Getter for width and height 
        attr_accessor :width, :height

        ## Getter for width and height
        # attr_reader :width, :height

        # Setter for width and height
        # attr_writer :width, :height

        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end
    end

    # create an object
    box = Box.new(10, 20)

    # use get width and height
    x = box.width
    y = box.height

    puts "Width of the box is : #{x}"
    puts "Height of the box is : #{y}"

    # use set width and height
    box.width = 30
    box.height = 50

    # use get width and height
    x = box.width
    y = box.height

    puts "Width of the box is : #{x}"
    puts "Height of the box is : #{y}"

When the above code is executed, it produces the following result −

Width of the box is : 10
Height of the box is : 20

Width of the box is : 30
Height of the box is : 50

Access Control

Ruby gives you three levels of protection at instance methods level, which may be public, private, or protected. Ruby does not apply any access control over instance and class variables.

  • Public Methods − A public methods can be called by anyone. Methods are public by default except for initialize, which is always private.

  • Private Methods − A private methods cannot be accessed, or even viewed from outside the class. Only the class methods can access private members.

  • Protected Methods − A protected method can be invoked only by objects of the defining class and its subclasses. Access is kept within the family.

Following is a simple example to show the syntax of all the three access modifiers −

#!/usr/bin/ruby -w

    # define a class
    class Box
        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end

        # instance method by default it is public
        def getArea
            getWidth() * getHeight
        end

        # define private accessor methods
        def getWidth
            @width
        end
        def getHeight
            @height
        end
        # make them private
        private :getWidth, :getHeight

        # instance method to print area
        def printArea
            @area = getWidth() * getHeight
            puts "Big box area is : #@area"
        end
        # make it protected
        protected :printArea
    end

    # create an object
    box = Box.new(10, 20)

    # call instance methods
    a = box.getArea()
    puts "Area of the box is : #{a}"

    # try to call protected or methods
    box.printArea()

When the above code is executed, it produces the following result. Here, first method is called successfully but second method gave a problem.

Area of the box is : 200
test.rb:42: protected method printArea called for # (NoMethodError)

Self

The reserved word self refers to the calling object, which for us can mean a specific instance of the class, and we use it to call methods or variables within class.
Also self can be refers to the class itself, check The Class Methods.

Inheritance

One of the most important concepts in object-oriented programming is that of inheritance. Inheritance allows us to define a class in terms of another class, which makes it easier to create and maintain an application. It provides an opportunity to reuse the code functionality and fast implementation time.

Unfortunately Ruby does not support multiple levels of inheritances but Ruby supports mixins. A mixin is like a specialized implementation of multiple inheritance in which only the interface portion is inherited

When creating a class, instead of writing completely new data members and member functions, the programmer can designate that the new class should inherit the members of an existing class. This existing class is called the base class or superclass or parent class, and the new class is referred to as the derived class or sub-class or child class.

Ruby also supports the concept of subclassing, i.e., inheritance and following example explains the concept. The syntax for extending a class is simple. Just add a < character and the name of the superclass to your class statement. For example, following define a class BigBox as a subclass of Box −

#!/usr/bin/ruby -w

    # define a class
    class Box
        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end
        # instance method
        def getArea
            @width * @height
        end
    end

    # define a subclass
    class BigBox < Box

        # add a new instance method
        def printArea
            @area = @width * @height
            puts "Big box area is : #@area"
        end
    end

    # create an object
    box = BigBox.new(10, 20)

    # print the area
    box.printArea()

When the above code is executed, it produces the following result −

Big box area is : 200

Super

The reserved word super passes any arguments supplied up the method lookup path to the first method of the same name that Ruby finds, which it then invokes. at the next example super will pass arguments from child class's initializer method to parent class's initializer method.

#!/usr/bin/ruby -w

    # define a class
    class Box
        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end
        # instance method
        def getArea
            @width * @height
        end
    end

    # define a subclass
    class BigBox < Box

        def initialize(w,h,l)
            super(w, h)
            @length = l
        end

        # add a new instance method
        def printArea
            @area = @width * @height
            puts "Big box area is : #@area"
        end
    end

    # create an object
    box = BigBox.new(10, 20, 30)

    # print the area
    box.printArea()

When the above code is executed, it produces the following result −

Big box area is : 200

Polymorphism

Methods Overriding

Dynamic or Runtime polymorphism
Though you can add new functionality in a derived class, but sometimes you would like to change the behavior of already defined method in a parent class. You can do so simply by keeping the method name same and overriding the functionality of the method as shown below in the example −

#!/usr/bin/ruby -w

    # define a class
    class Box
        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end
        # instance method
        def getArea
            @width * @height
        end
    end

    # define a subclass
    class BigBox < Box

        # change existing getArea method as follows
        def getArea
            @area = @width * @height
            puts "Big box area is : #@area"
        end
    end

    # create an object
    box = BigBox.new(10, 20)

    # print the area using overriden method.
    box.getArea()

Operator Overloading

Static or Compile time polymorphism
We'd like the + operator to perform vector addition of two Box objects using +, the * operator to multiply a Box width and height by a scalar, and the unary - operator to do negate the width and height of the Box. Here is a version of the Box class with mathematical operators defined −

class Box
        # Initialize the width and height
        def initialize(w,h)     
            @width,@height = w, h
        end

        # Define + to do vector addition
        def +(other)       
            Box.new(@width + other.width, @height + other.height)
        end

        # Define unary minus to negate width and height
        def -@           
            Box.new(-@width, -@height)
        end

        # To perform scalar multiplication
        def *(scalar)           
            Box.new(@width*scalar, @height*scalar)
        end
    end

Freezing Objects

Sometimes, we want to prevent an object from being changed. The freeze method in Object allows us to do this, effectively turning an object into a constant. Any object can be frozen by invoking Object.freeze. A frozen object may not be modified: you can't change its instance variables.

You can check if a given object is already frozen or not using Object.frozen? method, which returns true in case the object is frozen otherwise a false value is return. Following example clears the concept −

#!/usr/bin/ruby -w

    # define a class
    class Box
        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end

        # accessor methods
        def getWidth
            @width
        end
        def getHeight
            @height
        end

        # setter methods
        def setWidth=(value)
            @width = value
        end
        def setHeight=(value)
            @height = value
        end
    end

    # create an object
    box = Box.new(10, 20)

    # let us freez this object
    box.freeze

    if( box.frozen? )
        puts "Box object is frozen object"
    else
        puts "Box object is normal object"
    end

    # now try using setter methods
    box.setWidth = 30
    box.setHeight = 50

    # use accessor methods
    x = box.getWidth()
    y = box.getHeight()

    puts "Width of the box is : #{x}"
    puts "Height of the box is : #{y}"

When the above code is executed, it produces the following result −

Box object is frozen object
test.rb:20:in 'setWidth=': can't modify frozen object (TypeError) from test.rb:39

Create Object Using Allocate

There may be a situation when you want to create an object without calling its constructor initialize i.e. using new method, in such case you can call allocate, which will create an uninitialized object for you as in the following example −

#!/usr/bin/ruby -w

    # define a class
    class Box
        attr_accessor :width, :height

        # constructor method
        def initialize(w,h)
            @width, @height = w, h
        end

        # instance method
        def getArea
            @width * @height
        end
    end

    # create an object using new
    box1 = Box.new(10, 20)

    # create another object using allocate
    box2 = Box.allocate

    # call instance method using box1
    a = box1.getArea()
    puts "Area of the box is : #{a}"

    # call instance method using box2
    a = box2.getArea()
    puts "Area of the box is : #{a}"

When the above code is executed, it produces the following result −

Area of the box is : 200
test.rb:14: warning: instance variable @width not initialized
test.rb:14: warning: instance variable @height not initialized
test.rb:14:in 'getArea': undefined method for nil:NilClass (NoMethodError) from test.rb:29

Class Information

If class definitions are executable code, this implies that they execute in the context of some object: self must reference something. Let's find out what it is.

#!/usr/bin/ruby -w

    class Box
        # print class information
        puts "Type of self = #{self.type}"
        puts "Name of self = #{self.name}"
    end

When the above code is executed, it produces the following result −

Type of self = Class
Name of self = Box

This means that a class definition is executed with that class as the current object. This means that methods in the metaclass and its superclasses will be available during the execution of the method definition.

Coding Examples

Resources

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