C# – Polymorphism Trouble C Sharp Part 2 >.<

The clearest way to express polymorphism is via an abstract base class (or interface)

public abstract class Human{
   ...
   public abstract void goPee();
}

This class is abstract because the goPee() method is not definable for Humans. It is only definable for the subclasses Male and Female. Also, Human is an abstract concept — You cannot create a human that is neither Male nor Female. It’s got to be one or the other.

So we defer the implementation by using the abstract class.

public class Male extends Human{
...
    @Override
    public void goPee(){
        System.out.println("Stand Up");
    }
}

and

public class Female extends Human{
...
    @Override
    public void goPee(){
        System.out.println("Sit Down");
    }
}

Now we can tell an entire room full of Humans to go pee.

public static void main(String[] args){
    ArrayList<Human> group = new ArrayList<Human>();
    group.add(new Male());
    group.add(new Female());
    // ... add more...

    // tell the class to take a pee break
    for (Human person : group) person.goPee();
}

Running this would yield:

Stand Up
Sit Down
...

If you think about the Greek roots of the term, it should become obvious.

• Poly = many: polygon = many-sided, polystyrene = many styrenes ^(a), polyglot = many languages, and so on.
• Morph = change or form: morphology = study of biological form, Morpheus = the Greek god of dreams able to take any form.

So polymorphism is the ability (in programming) to present the same interface for differing underlying forms (data types).

For example, in many languages, integers and floats are implicitly polymorphic since you can add, subtract, multiply and so on, irrespective of the fact that the types are different. They're rarely considered as objects in the usual term.

But, in that same way, a class like BigDecimal or Rational or Imaginary can also provide those operations, even though they operate on different data types.

The classic example is the Shape class and all the classes that can inherit from it (square, circle, dodecahedron, irregular polygon, splat and so on).

With polymorphism, each of these classes will have different underlying data. A point shape needs only two co-ordinates (assuming it's in a two-dimensional space of course). A circle needs a center and radius. A square or rectangle needs two co-ordinates for the top left and bottom right corners and (possibly) a rotation. An irregular polygon needs a series of lines.

By making the class responsible for its code as well as its data, you can achieve polymorphism. In this example, every class would have its own Draw() function and the client code could simply do:

shape.Draw()

to get the correct behavior for any shape.

This is in contrast to the old way of doing things in which the code was separate from the data, and you would have had functions such as drawSquare() and drawCircle().

Object orientation, polymorphism and inheritance are all closely-related concepts and they're vital to know. There have been many "silver bullets" during my long career which basically just fizzled out but the OO paradigm has turned out to be a good one. Learn it, understand it, love it - you'll be glad you did :-)

^(a) I originally wrote that as a joke but it turned out to be correct and, therefore, not that funny. The monomer styrene happens to be made from carbon and hydrogen, C8H8, and polystyrene is made from groups of that, (C8H8)n.

Perhaps I should have stated that a polyp was many occurrences of the letter p although, now that I've had to explain the joke, even that doesn't seem funny either.

Sometimes, you should just quit while you're behind :-)