Defining all abstract method in one concrete class

Question

I have major issues in understanding the major topic of OOP. Though I been through Dietel book. I have an abstract class shape.

public abstract class Shape
{
public abstract area();
public absract volume;
}
public class Circle extends Shape
{
//definition of area method
}
public class Sphere extends Shape
{
//define volume method
// define area method
}

Now as I have Shape class as Parent class and Circle class as child class, at same time Shape class is an abstract class, Now I want to define area method in Circle class but not volume method, I want to define volume method in Sphere class. But when i do this, it shows me error but if i define both method in Circle class then it works fine. But then Circle class have extra code of volume which is not use of Circle class.

Answer 1

An abstract member is not an optional member. It is a member that MUST be implemented for implementing a concrete class. If one of the abstract member is not defined in a derived class, this class is also abstract and cannot be instantiated.

This is an excellent example for the Interface Segregation Principle : your interface for Shape combines the interfaces of different potentially unrelated concepts: area() which is only relevant for 2D shapes, and volume() is only relevant for 3D shapes.

If you want a clean SOLID design, you'd need to consider a Shape2D with area() and perimeter() and a Shape3D with a surface() and volume() .

If you think that the area() is in fact the same thing than a surface() , you may consider an abstract parent Shape that provides an abstract surface() , a child Shape2D that provides for the perimeter() and another child Shape3D that provides the volume() .

More pragmatically, you could consider that the volume() of of a Circle or any other 2D shape is simply 0 and stick to your original design.