`Class`

With classes in Ruby, we construct complex models. Classes have constructors: this is the initialize method. We invoke "initialize" with new() calls.

By hiding information, classes help with complexity. Understanding objects is important in program design. It helps to know when classes are not needed.

First example

Here we use a custom class called Box. We add a constructor to this class—in Ruby, the constructor is called initialize(). The one here receives two arguments.

Info In initialize(), we assign 2 fields (width and height) to the arguments. Fields use a leading "at" character.

Next When we call new, the initialize method is run. The arguments are stored in the memory of the class fields.

Info We call the constructor by using the class name (Box) and the new method. Box.new returns a new instance of the Box class.

class Box
    # The constructor.
    def initialize(width, height)
        # Assign fields from arguments.
        @width = width
        @height = height
    end

    # A custom method.
    def display()
        puts @width
        puts @height
    end
end

# Create a new Box.
x = Box.new(10, 5)

# Call the display method.
puts x.display()10
5

Get, set

Classes use getters and setters to control access to their fields. This improves program design. It makes classes more separated from external code that uses them.

Important If the internals change, we can modify just get and set. When fewer code changes are made, fewer things are likely to break.

Info A custom syntax exists for getters and setters. In a getter, we have no arguments and a single statement—the return value.

And In a setter, we use the equals sign at the end of the name. Then in the body, we assign the targeted field.

class Box
    # Getter.
    def size
        @size
    end

    # Setter.
    def size=(number)
        @size = number
    end
end

# Create a new Box.
x = Box.new()

# Use setter.
x.size = 10

# Use getter and display.
puts x.size
10

To_s

Each class can have a to_s method defined. This method should return a string that contains the class data. We can specify a string, and this is automatically returned as the result.

Here We specify the to_s method on the Box class. The width and height fields are inserted into the string.

Info When a class is passed to a method like puts, the to_s method is automatically invoked. This all happens implicitly.

Tip We have no need to call to_s in many methods. But to_s can be directly called.

class Box
    def initialize(width, height)
        # Initialize the class.
        @width = width
        @height = height
    end

    def to_s
        # Return this string.
        "Box, width: #@width, height: #@height"
    end
end

# Create new Box.
b = Box.new(10, 20)

# Display it (calls to_s).
puts b
Box, width: 10, height: 20

Inheritance

One class can inherit from another. The derived class, a subclass, gains the abilities (like methods) from the parent class. The "less than" operator indicates a subclass.

Here With a call to super() we invoke a same-named method from the class parent. So from Square's super() we invoke Shape display().

class Shape
    def display()
        puts("Shape display")
    end
end

# This class inherits from Shape.
class Square < Shape
    def display()
        puts("Square display")
        # Call superclass method of the same name.
        super()
    end
end

# Create the class.
item = Square.new()
item.display()Square display
Shape display

Self

The self object is available in all classes. It refers to the immediate class instance. We can access methods with self, or pass self to an external method.

Here We call test() and pass it the self instance. The self variable evaluates to an object.

Note The self instance is the same thing as a "this" instance in other languages like C# or Java.

def test(s)
    # See if the Shape's color is red.
    puts(s.color == "red")
end

class Shape
    def initialize(color)
        @color = color
    end

    def color
        @color
    end

    def analyze()
        # Pass this object to another method.
        test(self)
    end
end

# Create Shape and call the analyze method.
s = Shape.new("red")
s.analyze()
true

Public, private

Methods can be modified with visibilities. Public things can be called from external locations, outside of the class. Something private is only used inside the class.

class Navigator
    def initialize(location)
        @location = location
    end

    def location()
        update()
        @location
        puts @location
    end

    def update()
        @location = @location.upcase()
    end

    public :location
    private :update
end

# Create new class instance and call location public method.
n = Navigator.new("japan")
n.location()
JAPAN

Private method error

Let us get into some mischief. Here we try to call a private method, update() from the same class as above. We get a NoMethodError. And the program terminates.

n = Navigator.new("japan")
n.update()/Users/sam/Documents/test.rb:24:in '<main>':
private method 'update' called for #<Navigator:0x007fa3cb082200
@location="japan"> (NoMethodError)

`Module`

This is an organization unit, much like a namespace in other languages. We use modules to organize classes and methods. To use a module in our file, we use the "include" keyword.

Tip Modules can be reopened and changed many times. We can modify them. We just specify the module another time to make changes.

Tip 2 We can access a method or type (like a class) within a module with two ":" characters. We access "Cat::meow" as an example.

module Cat
    def pet()
        puts "Cat petted"
    end
end

# Reopen and change the module (use mix-in).
module Cat
    def meow()
        puts "Cat meows"
    end
end

# Include the module and call its methods.
include Cat

Cat::pet()
Cat::meow()Cat petted
Cat meows

Built around classes, Ruby has robust support for object-oriented programming. Classes encapsulate functionality and data, and they reside in memory.