class Examples
This page was last reviewed on Apr 8, 2022.
Dot Net Perls
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.
Detail In this method, assign 2 fields (width and height) to the arguments. Fields use a leading "at" character.
Detail 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.
And If the internals change, we can modify just get and set. When fewer code changes are made, fewer things are likely to break.
Detail A custom syntax exists for getters and setters. In a getter, we have no arguments and a single statement—the return value.
Detail 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
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.
Detail 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.
Detail 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()
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.
Detail This is a hybrid of public and private—it means public only to subclasses, private to everything else.
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()
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.
Detail Modules can be reopened and changed many times. We can modify them. We just specify the module another time to make changes.
Detail 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
A summary. Built around classes, Ruby has robust support for object-oriented programming. Classes encapsulate functionality and data. They reside in memory.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Apr 8, 2022 (edit).
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