Why Overridden Methods?

Why Overridden Methods?

 

As stated earlier, overridden methods allow Java to support run-time polymorphism.

 

Polymorphism is essential to object-oriented programming for one reason: it allows a general class to specify methods that will be common to all of its derivatives, while allowing subclasses to define the specific implementation of some or all of those methods. Overridden methods are another way that Java implements the "one interface, multiple methods" aspect of polymorphism.

 

Part of the key to successfully applying polymorphism is understanding that the superclasses and subclasses form a hierarchy which moves from lesser to greater specialization. Used correctly, the superclass provides all elements that a subclass can use directly. It also defines those methods that the derived class must implement on its own.

 

This allows the subclass the flexibility to define its own methods, yet still enforces a consistent interface. Thus, by combining inheritance with overridden methods, a superclass can define the general form of the methods that will be used by all of its subclasses.

 

Dynamic, run-time polymorphism is one of the most powerful mechanisms that object-oriented design brings to bear on code reuse and robustness. The ability of existing code libraries to call methods on instances of new classes without recompiling while maintaining a clean abstract interface is a profoundly powerful tool.

 

Applying Method Overriding

Let's look at a more practical example that uses method overriding. The following program creates a superclass called Figure that stores the dimensions of various two-dimensional objects. It also defines a method called area( ) that computes the area of an object.

 

The program derives two subclasses from Figure. The first is Rectangle and the second is Triangle. Each of these subclasses overrides area( ) so that it returns the area of a rectangle and a triangle, respectively.

 

// Using run-time polymorphism.

class Figure {

double dim1;

double dim2;

Figure(double a, double b) {

dim1 = a;

dim2 = 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 area for rectangle

double area() {

System.out.println("Inside Area for Rectangle.");

return dim1 * dim2;

}

}

class Triangle extends Figure {

Triangle(double a, double b) {

super(a, b);

}

// override area for right triangle

double area() {

System.out.println("Inside Area for Triangle.");

return dim1 * dim2 / 2;

}

}

class FindAreas {

public static void main(String args[]) {

Figure f = new Figure(10, 10);

Rectangle r = new Rectangle(9, 5);

Triangle t = new Triangle(10, 8);

Figure figref;

figref = r;

System.out.println("Area is " + figref.area());

figref = t;

System.out.println("Area is " + figref.area());

figref = f;

System.out.println("Area is " + figref.area());

}

}

 

The output from the program is shown here:

 

Inside Area for Rectangle.

Area is 45

Inside Area for Triangle.

Area is 40

Area for Figure is undefined.

Area is 0

 

Through the dual mechanisms of inheritance and run-time polymorphism, it is possible to define one consistent interface that is used by several different, yet related, types of objects. In this case, if an object is derived from Figure, then its area can be obtained by calling area( ). The interface to this operation is the same no matter what type of figure is being used.