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Ring/Lessons/Object Oriented Programming (OOP)

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Object Oriented Programming (OOP)

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In this chapter we are going to learn how to use the Object-Oriented programming paradigm inside the Ring programming language.

We will learn about

  • Classes and Objects
  • Access Objects Using Braces
  • Composition
  • Setter and Getter
  • Private Attributes and Methods
  • Operator Overloading
  • Inheritance
  • Dynamic Attributes
  • Packages



Classes and Objects

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We can define new classes using the next syntax

Syntax: 

	Class <Class Name> [From <Parent Class Name>]
		[Attributes]
		[Methods]
		[Private
                  [Attributes]
		  [Methods]
                ]

And we can create objects using the next syntax

Syntax: 

	New <Object Name> [ (init method parameters) ] |
			  [ { access object data and methods } ]   ---> Object

Example: 

	New point { x=10  y=20  z=30  print() }
	Class Point x y z func print see x + nl + y + nl + z + nl

.. note:: We can use { } to access object data and methods.

.. tip:: we can declare the class attributes directly after the class name.

Output: 

	10
	20
	30

We can rewrite the same program in another style 

	New point 			# create new object using the point class
	{ 				# access the new object attributes and methods
		x = 10 			# set the x attribute to 10
		y = 20 			# set the y attribute to 20
		z = 30 			# set the z attribute to 30
		print() 		# call the print method
	}				# end of object access


Class Point # define the Point class x y z # the class contains three attributes x, y & z func print # define the print method see x + nl + # print the x attribute y + nl + # print the y attribute z + nl # print the z attribute


Also we can write the same program in another way 

	P1 = New Point
	P1.x = 10
	P1.y = 20
	P1.z = 30
	P1.Print()
	Class Point x y z func print see x + nl + y + nl + z + nl

.. note:: we can use the dot operator after the object name to access object members.

Also we can write the same program in another way 

	new point { print() } 	
	Class Point
		x = 10  y = 20  z = 30
		func print see x + nl + y + nl + z + nl

.. note:: we can set the default values for the class attributes when we declare them.

Also we can write the same program in another way 

	new point(10,20,30)
	Class Point
		x y z 
		func init p1,p2,p3 x=p1 y=p2 z=p3 print()
		func print see x + nl + y + nl + z + nl

.. note:: we can call the init method directly using () when we create new objects

Also we can write the same program in another way 

	new point( [ :x = 10 , :y = 20 , :z = 30 ] )
	Class Point x y z
	      func init aPara x = aPara[:x] y = aPara[:y] z = aPara[:z] print()


.. tip:: using Hash for passing method parameters enable us to create optional 

        parameters and change the order of parameters when adding them to the Hash.

Access Objects Using Braces

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We can access the object at any time using braces { }

Inside the braces we can use the object attributes and methods directly

This can be done when we create the object using the New keyword or at any time using the next syntax 

	ObjectName { access object data and methods }

Example: 

	See "Creating the Object" + nl
	o1 = new Point
	See "Using the Object" + nl
	o1 {
		x=5 	
		y=15
		z=25	
		print()
	}
	Class Point x y z func print see x + nl + y + nl + z

We can use braces to access objects when we call functions or methods

Example: 

	o1 = new Point

	print( o1 { x=10 y=20 z=30 } )
	
	func print object
		see object.x + nl + 
		    object.y + nl + 
		    object.z 

	Class Point x y z

We can mix between using braces and the dot operator to access the object in the same expression.


Example: 

	o1 = new Point

	O1 { x=10 y=20 z=30 }.print()
	
	Class Point x y z
		func print see x + nl + y + nl + z

Composition

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The object may contains other objects as attributes.

Using braces to access objects can be nested.

Example: 

	R1 = New Rectangle 
	{

		Name = "Rectangle 1"

		P1 
		{
			X = 10
			Y = 20
		}

		P2 
		{
			X = 200
			Y = 300
		}	

	        Color = "Blue"

	}
	
	see "Name : " + R1.Name + nl +
	    "Color: " + R1.Color + nl +
	    "P1   : (" + R1.P1.X + "," + R1.P1.Y + ")" + nl + 
	    "P2   : (" + R1.P2.X + "," + R1.P2.Y + ")"  

	Class Rectangle
		name  color
		p1 = new Point
		p2 = new Point

	Class Point x y

Output: 

	Name : Rectangle 1
	Color: Blue
	P1   : (10,20)
	P2   : (200,300)

Setter and Getter

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We can define methods to be used when we set and get object attributes.

Syntax: 

	Class ClassName

		AttributeName
		...

		Func SetAttributeName
			...

		Func GetAttributeName
			...

Example: 

	o1 = new person

	o1.name = "Mahmoud"  see o1.name + nl

	o1 { name = "Ahmed"  see name }

	Class Person

		name family = "Fayed"

		func setname value
			see "Message from SetName() Function!" + nl
			name = value + " " + family

		func getname
			see "Message from GetName() Function!" + nl
			return "Mr. " + name

Output: 

	Message from SetName() Function!
	Message from GetName() Function!
	Mr. Mahmoud Fayed
	Message from SetName() Function!
	Message from GetName() Function!
	Mr. Ahmed Fayed

Private Attributes and Methods

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We can define private attributes and methods after the keyword private inside the class body

Example: 

	o1 = new person {
		name = "Test"
		age = 20
		print()
		o1.printsalary()
	}

	try
		see o1.salary
	catch
		see cCatchError + nl
	done

	try
		o1.increasesalary(1000)
	catch
		see cCatchError + nl
	done

	Class Person

		name age 

		func print
			see "Name   : " + name + nl + 
			    "Age    : " + age + nl 

		func printsalary
		  	see "Salary : " + salary + nl 

		private

		salary = 15000

		func increasesalary x
			salary += x

Output: 

	Name   : Test
	Age    : 20
	Salary : 15000
	Error (R27) : Using private attribute from outside the class : salary
	Error (R26) : Calling private method from outside the class : increasesalary

Operator Overloading

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We can add the operator method to our class to enable using operators with the class objects.

Syntax: 

	Class ClassName

		...

		Func operator cOperator,Para

			...

The function operator takes two paramters, the first represent the operator and the second represent the second parameter after the operator.

Example: 

	o1 = new point { x = 10 y = 10 print("P1    : ") }
	o2 = new point { x = 20 y = 40 print("P2    : ") }

	o3 = o1 + o2
	o3.print("P1+P2 : ")

	class point x y

		func operator cOperator,Para
			result = new point	
			switch cOperator
			on "+"
				result.x = x + Para.x
				result.y = y + Para.y
			on "-"
				result.x = x - Para.x
				result.y = y - Para.y
			off
			return result

		func print cPoint
			see cPoint + "X : " + x + " Y : " + y + nl

Output: 

	P1    : X : 10 Y : 10
	P2    : X : 20 Y : 40
	P1+P2 : X : 30 Y : 50

Inheritance

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We can create class from another class in the class definition using the keyword from.

Syntax: 

	Class <Class Name> [From <Parent Class Name>]

We can call a method in the parent class from the child class using the super object.

Syntax: 

	func methodname
		...
		super.methodname()
		...

Example: 

	Func main
		e1 = new Employee {
			Name = "test"
			age = 20
			job = "programmer"
			salary = 20000000
			print()
		}
	
	Class Human 
		Name Age
		func print
			see "Name : " + name + nl + "Age  : " + age + nl

	Class Employee from Human
		Job Salary
		func print
			super.print()
			see "Job  : " + job + nl + "Salary : " + salary + nl

Output: 

	Name : test
	Age  : 20
	Job  : programmer
	Salary : 20000000

Dynamic Attributes

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We can write instructions after the class name to be executed when we create new objects

Example: 

	o1 = new dynamicClass
	see o1.var5 + nl	# output 5

	Class DynamicClass
		for x = 1 to 10
			cStr = "var" + x + " = " + x
			eval(cStr)
		next

.. tip:: in the previous example var1, var2, ..., var10 will be defined as attributes.

.. tip:: The problem with the previous example is that x and cStr will be defined as attributes too!

.. note:: we can write class definitions inside a string then using eval() we can execute the string to define the classes

Packages

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We can create a package (a group of classes under a common name) using the next syntax 

	package PackageName
		Class Class1
			...
		Class Class2
			...
		Class Class3
			...
		...

Example 

	o1 = new System.output.console
	o1.print("Hello World")
	
	Package System.Output
		Class Console
			Func Print cText
				see cText + nl

.. note:: we can use the dot operator as part of the package name

Instead of typing the long name PackageName.ClassName we can use the import command

When we import a package, we can use any class inside this package directly.

Example 

	import system.output
	o1 = new console {
		print("Hello World")
	}
	Package System.Output
		Class Console
			Func Print cText
				see cText + nl

Printing Objects

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We can print the object state (attributes and values) using the see command.

Example: 

	see new point { x=10 y=20 z=30 }
	class point x y z

Output: 

	x: 10.000000
	y: 20.000000
	z: 30.000000


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