Conditionals and Polymorphism

Tell me and I'll forget.
Show me and I might remember.
But involve me and I will understand.

1

Puzzle
Imagine arranging 6 glasses in a row and filling three
of them with water like this….

Every time you pick up a glass it counts as a move.
What is the smallest number of moves that needs to be
made in order to leave the glasses alternating full and
empty? -– Explain your answer.
4

Conditionals and Polymorphism

Presented by

Ashok Guduru
Technical Architect
Copyright © 2013 Coextrix Technologies Pvt. Ltd., Bangalore

5

Inheritance & Polymorphism


6

Premise

Most ifs can be replaced with
polymorphism


7

Introduction
- Not all ifs but most of them cab be
replaced by polymorphism

- It is kind of fun to write code without ifs
(e.g. Smalltalk )


8

• Why?: Why we need to avoid ifs
-

Functions without ifs are easier to read

-

Functions without ifs are easier to test

-

Functions without ifs are easier to
maintain and extend

9

NOTE: One of the grandest sounding words in object jargon is polymorphism.

• Polymorphism: What it is?:
-

-

You dispatch into a method who’s binding
not known at compile time (a good old
polymorphism)
E.g. a Class and a bunch of sub-classes
inheriting from for printing
You say print and its not known it is
printing to printer, screen or to a PDF

10

•

How do we get rid of ifs?
•

Use Polymorphism

-

If an object should behave differently
based on its state
If you have to check the same condition in
multiple places
Usually happens using flags and your code
is sprinkled with a bunch of ifs all over the
codebase

-


11


• But… Use Conditionals
-

-

Mainly to do comparisons of primitive
objects: >, <, >=, <=, ==, != etc.
There are other uses but today we focus
only on avoiding ifs


12


We’re going to focus on business logic that
should behave differently based on
conditionals


13

•

To be if free…
-

Never return a null, instead return NullObject (Typically
an object that doesn’t do anything)
-

e.g. An empty list, A null logger

-

Nulls are your friends inside of a development test but
enemies in production code

-

When you return a null from a method you can’t
dispatch on null. You get a null pointer exception

-

Don’t return error codes, instead throw an exception
(Java RunTime Exception only.)

14


• What is a subclass?
- aka Base Class, Super Class, Parent Class etc.


15


• Polymorphism uses subclassing
WARNING:
- Be careful about runaway subclassing
- Not to go and crazy on using more subclassing
- Deep inheritance hierarchies creates problems on
testability, understandability and couple of other
abilities as well.

16


State Based Behavior


17

Replace Conditionals with Polymorphism


18

•

You have a conditional that chooses different
behavior depending on the type of an object
- Move each leg of the conditional to an overriding
method in a subclass. Make the original method
abstract.
- The common logic stays in base class (super class)


19

public double GetSpeed() {
switch(_type)
{
case EUROPEAN:
return getBaseSpeed();
case AFRICAN:
return getBaseSpeed() – getLoadFactor() * _numberOfCoconuts;
case NORWEGIAN_BLUE:
return isNailed ? 0 : getBaseSpeed(_voltage)
default:
throw new ArgumentException(“Should be unreachable”);
// Java RuntimeException(“Should be unreachable”);
}
}


20



21

public abstract class Bird {
public abstract double GetSpeed();
protected double getBaseSpeed();
}
public class EuropeanBird : Bird {
return getBaseSpeed()
}
}

22

public class AfricanBird : Bird {
return getBaseSpeed() – getLoadFactor() * _numberOfCoconuts;
}
}
public class NorwegianBlueBird : Bird {
return isNailed ? 0 : getBaseSpeed(_voltage)
}

}


23

public class BirdFactory {
public double createInstance(_type) {
switch (_type)
case EUROPEAN
return new EuropeanBird();
case AFRICAN:
return new AfricanBird()
case NORWEGIAN_BLUE:
return new NorwegianBlueBird();
default:
throw new ArgumentException(“Should be unreachable”);
//Java RuntimeException(“Should be unreachable”);
}
}


24

public static main() {
Bird bird = BirdFactory.createInstance(BirdType.EUROPEAN);
SpeedCalculator sc = new SpeedCalculator(bird);
sc.Calculate();
}
public class SpeedCalculator {
private Bird _bird;
void SpeedCalculator(Bird bird) {
_bird = bird;
}
public void Calculate() {
return _bird.GetSpeed();
}
}

25

Summary
•

A Polymorphic solution is often better
because
- New behavior can be added without having the
original source code, and
- Each operation/concern is separated in a separate
file which makes it easy to test/understand and
maintain


26

Summary
•

Prefer polymorphic over conditionals:
- Switch almost always means you should use
polymorphism
- Sometimes if…elseif… means same as switch
- In both of above cases the code is begging for
polymorphism. It is almost a rule… I can say!
- if is more subtle… sometimes an if is just and if

27

Simple things should be simple and complex things
should be possible.
- Alan Kay

28

Puzzle
Imagine arranging 6 glasses in a row and filling three of them
with water like this….

Every time you pick up a glass it counts as a move. What is the
smallest number of moves that needs to be made in order to leave
the glasses alternating full and empty? -– Explain your answer.
29

Puzzle
Imagine arranging 6 glasses in a row and filling three of them
with water like this….

Every time you pick up a glass it counts as a move. What is the
smallest number of moves that needs to be made in order to leave
the glasses alternating full and empty? -– Explain your answer.
30

Puzzle
… and the answer is ONE

31

Books:
1. Agile Software Development
Principles, Patterns, and Practices
by Robert C. Martin (aka Bob Martin or Uncle Bob)

2. Effective Java by Joshua Bloch
3. Code Complete (2nd Edition) by Steve McConnell
Code Complete: A Practical Handbook of Software Construction


32

The amateur software engineer is always in
search of magic.
-- Grady Booch

Thank you!
Ashok Guduru
Blog: http://www.ashokg.com
Twitter: @AshokGudur


34

Conditionals and Polymorphism

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