CSC/ECE 517 Fall 2007/wiki2 8 c9

Topic 8 : Overloading Vs Overriding

Overloading vs. overriding. Skrien delineates the difference between overloading and overriding in Section 2.8. He does this in the context of Java. How do other languages make this distinction, especially those like CLOS that define multi-methods?

Polymorphism

Polymorphism is usually expressed by the phrase “One interface, multiple methods”. This allows a super class to specify methods that will be common to all of its subclasses, while allowing the subclasses to define their own specific methods.It means design a generic interface and use it for group of related activities. This helps reduce complexity by allowing the same interface to be used to specify a general class of action. It is the compiler’s job to select the specific action (that is, method ) as it applies to each situation.

Overloading

In Java it is possible to define 2 or more methods within the same class that share the same name, as long as their parameter declarations are different. The methods are said to be overloaded. This process is referred as method overloading. It is one of the way by which java implements polymorphism.Java uses the type and /or number of arguments to determine which of the overloaded methods to be overloaded. The overloaded methods may differ in return type or the number of parameters in the method call.

Example

class Overload   
{
  void show() {

   System.out.println(" There are no parameters in this method");
             }

  void show(int a) {
  System.out.println("this has one parameter : a" +a);
                   }

  void show(int a,int b){
  System.out.println("this has 2 parameters a"+a+ "b:"+b);
                         }

  double show(double a){
  System.out.println("this has a double parameter  a"+a);
  return a*a
                        }
}



class overloadtest {
public static void main(String args[]){

Overload obj=new Overload();
double result;

obj.show();
obj.show(10);
obj.show(10,20);
result=obj.show(100.5);
System.out.println("result of obj.show(100.5) is" + result);
                                     }
                     }

Output

There are no parameters in this method
this has one parameter : a 10
this has 2 parameters a 10 b 20
this has a double parameter a 100.5
the result of obj.show(100.5) is 10100.25

Overriding

Overriding occurs only with respect to inheritance and occurs when a subclass has a method with the same signature as a parent's method. The method in the subclass is said to override the method in the superclass. Java always invokes the most derived viewable version of a method based on the scope of the present reference. However , if we wish to access superclass version of overridden method, then we can use the keyword “super” and then access it.

Example

class Figure {
double length;
double breadth;

  Figure(double a, double b)
  {
    length =a;
    breadth=b;
  }

   double area(){
   System.out.println(“area for figure is undefined”);
   return 0;
                }
   }


class Rectangle extends Figure {
 
 Rectangle (Double a, double b) {
 super(a,b);
                                }

  // override the area for rectangle
  double area()  
 {
  System.out.println(“inside area for rectangle”);
  return length* breadth
  }
                                }


class Triangle extends Figure {
 Triangle (double a,double b)
 {
  super(a,b);
 }
 // override area for triangle
 double area() {
 System.out.println(“inside area of triangle”);
 return length*breadth /2;
                }
                                }          


class areastest {
public static void main(String args[]){
Figure obf=new figure(10,10);
Rectangle obr=new rectangle(25,50);
Triangle obt=new triangle(20,40);

Figure ref;
ref=obr;
System.out.println(“area is”+ ref.area());

ref=obt;
System.out.println(“area is”+ ref.area());

ref=obf;
System.out.println(“area is”+ ref.area());
}
}

Output

Inside area for rectangle
Area is 1250
Inside area of triangle
Area is 400
Area for figure is undefined
Area is 0

Differences between Overloading and Overriding

Overloaded	Overridden
supplements each other	replaces the method it overrides
can exist in any number within same class	Each method in a parent class can be overridden at most once in any one class
must have different arguments	must have argument list of identical type and order
return type may be freely chosen	return type must be identical
determined at compile time	determined at run time

Note: Overloaded methods should be used sparingly, as they can make code much less readable.

CLOS (Common Lisp Object System)

Multi Methods

References

1) Complete Reference in Java Text book
2) Object Oriented Programming in Common Lisp Text book
3) A Brief guide to CLOS http://www.aiai.ed.ac.uk/~jeff/clos-guide.html
4) Overloading and Overriding in Java http://ww2.cs.fsu.edu/~steele/FALL_03_CIS3931/REFERENCES/OVER/over.html
5) Overloading and overriding in Java http://smccd.net/accounts/greenm/Overloading.pdf