CSC/ECE 517 Fall 2011/ch4 4a aj
Extending objects. : Consider one or more practical examples of Ruby's extend method--that is, examples that serve a real need and are not contrived. What facilities do other languages have for extending objects? Are they more or less powerful than Ruby's? Also consider prototype-based languages such as Self, which don't even provide classes.
Introduction
One of the most fundamental advantages of using Object Orientation is to reuse code. Rather than coding what was already coded, we can simply extend it to suit the needs of the new class. This helps to develop applications rapidly. Many languages provide built in libraries which can be included in classes. The O-O languages also provide a way to add functionality to existing classes through extension.
Implementation of Code Reuse:
“Inheritance” is the word used in O-O for the process of inheriting the members of a class and extending the functionality. A class can inherit from a single class or “Multiple Classes”, the latter is called as “Multiple Inheritance”. While a child class inherits from the parent class, it tends to inherit all the members of the class.
Comparison of Extend methods in Java, C# and Ruby.
In java:
A class declaration can use the extend keyword on another class as follows:
Class Course extends College
{
..............
}
When a class Child extends class Parent, Child automatically has all variables and methods defined in class Parent. If class Child defines a variable or method that has the same name in class Parent, class Child's definition overrides that of Parent's.
class Parent {
int x;
int y;
int get(int a, int b){
x=a; y=b; return(0);
}
void Show(){
System.out.println(x);
}
}
class Child extends Parent {
public static void main(String args[]){
Parent p = new Parent();
p.get(5,6);
p.Show();
}
void display(){
System.out.println("Child");
}
}
In C#:
Extension methods enable the programmer to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type.
class ExtensionMethod
{
static void Main()
{
int[] s = { 1, 55, 75, 93, 81, 96 };
var r = s.OrderBy(g => g);
foreach (var a in result)
{
System.Console.Write(a + " ");
}
}
}
Output: 1 55 15 81 93 96 The above example shows how to call the standard query operator OrderBy method on an array of integers.
In C++
Similar to Java, C++ also extends functionality of a base class to a derived class. The derived class can use the functions and variables of a base class as well as add their own functionality, thus making them a more specialized version of the base class. Similar to Java, overriding is also supported in C++. Here is an example of how a class is extended in C++.
class Base{
public:
int x;
int y;
int get(int a, int b){
x=a;
y=b;
return(0);
}
void display()
cout<<x<<y;
}
}
class derived:public base{
void Main(){
Base b;
b.get(5,6);
b.display();
void display(){
cout<<”Derived”;
}
}
In Python:
To extend a class in Python, we simply call the parent class in the parenthesis while defining the child class. This will pass all the methods and variables defined in the parent class to the child class. An illustration of how classes are extended in python.
class Parent: a=5 b=4 def add(): print(a+b) class Child(Parent): def difference(): print(a-b) p = Parent() c = Child() c.add() c.difference()
In Ruby:
The include method effectively adds a module as a super class of self. It is used inside a class definition to make the instance methods in the module available to instances of the class. It is sometimes useful to add the instance methods to a particular object. This is done using Object#extend.
module Write
def pen
"#{self} says use a pen not a pencil for your exam!"
end
end
obj = " Proctor"
obj.extend Write
puts obj.pen
Output: Proctor says use a pen not pencil for your exam
If the extend key word is used within a class definition, the modules methods become class methods. This is because extend is equal to self.extend, so the methods are added to self, which in a class definition is the class itself.
module Write
def pen
"#{self} says use a pen not pencil for your exam"
end
end
class Proctor
extend Write
end
puts Proctor.pen
Output: Proctor says use a pen not pencil for your exam
How Extending Objects in Ruby is more powerful than other languages:
The one main disadvantage of inheriting from a class is that, we tend to inherit all of its members, most of which we may not use in our programs. Ruby however gives us the option of extending only what we need. Several modules can be created and these modules can be extended at runtime. The extend keyword makes the module available to all instances of the class. A class can simply extend the modules and uses it the way it wants on the move. This, however, does not change the objects original code. This is really a unique and powerful feature of Ruby.