Abstract Classes and Methods || what are Abstract Classes and Methods in java? || Abstract Classes and Methods in java with example

Abstract Classes and Methods

That is, sometimes you will want to create a superclass that only defines a generalized form that will be shared by all of its subclasses, leaving it to each subclass to fill in the details. Such a class determines the nature of the methods that the subclasses must implement. You can require that certain methods be overridden by subclasses by specifying the abstract type modifier. These methods are sometimes referred to as subclasser responsibility because they have no implementation specified in the superclass. Thus, a subclass must override them—it cannot simply use the version defined in the superclass. Any class that contains one or more abstract methods must also be declared abstract. To declare a class abstract, you simply use the abstract keyword in front of the class keyword at the beginning of the class declaration. There can be no objects of an abstract class. That is, an abstract class cannot be directly instantiated with the new operator. you cannot declare abstract constructors, or abstract static methods. Any subclass of an abstract class must either implement all of the abstract methods in the superclass, or be itself declared abstract. Although abstract classes cannot be used to instantiate objects, they can be used to create object references, because Java’s approach to run-time polymorphism is implemented through the use of superclass references. Thus, it must be possible to create a reference to an abstract class so that it can be used to point to a subclass object.

Example

// Using abstract methods and classes.
abstract class Figure
{
double dim1;
double dim2;
Figure(double a, double b)
{ dim1 = a;
dim2 = b;
}
// area is now an abstract method
abstract double area();
}
class Rectangle extends Figure
{
Rectangle(double a, double b)
{ super(a, b); }
// override area for rectangle
double area()
{
System.out.println("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("Area for Triangle.");

return dim1 * dim2 / 2;

}

}

class AbstractAreas

{

public static void main(String args[])

{

// Figure f = new Figure(10, 10); // illegal now

Rectangle r = new Rectangle(9, 5);

Triangle t = new Triangle(10, 8);

Figure figref; // this is OK, no object created

figref = r;

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

figref = t;

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

}

}