conditionals and polymorphism

1

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

2

A misconception

3

A misconception

4

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.

Conditionals and Polymorphism

Copyright © 2013 Coextrix Technologies Pvt. Ltd., Bangalore 5

Presented by

Ashok GuduruTechnical Architect

Inheritance & Polymorphism


Premise


Most ifs can be replaced with polymorphism

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 )



• 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



• 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

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



• 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



• But… Use Conditionals

- Mainly to do comparisons of primitive objects: >, <, >=, <=, ==, != etc.

- There are other uses but today we focus only on avoiding ifs



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

conditionals



• 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.)



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



• Polymorphism uses subclassingWARNING:

- 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.



State Based Behavior


Replace Conditionals with Polymorphism



• 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)



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”);

}

}



public abstract class Bird {

public abstract double GetSpeed();

protected double getBaseSpeed();

}

public class EuropeanBird : Bird {


return getBaseSpeed()

}

}



public class AfricanBird : Bird {


return getBaseSpeed() – getLoadFactor() * _numberOfCoconuts;

}

}

public class NorwegianBlueBird : Bird {


return isNailed ? 0 : getBaseSpeed(_voltage)

}

}



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”);

}

}



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();

}

}

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

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

28

Simple things should be simple and complex things should be possible.

- Alan Kay

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

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.

31

Puzzle

… and the answer is ONE

Books:


1. Agile Software Development Principles, Patterns, and Practicesby Robert C. Martin (aka Bob Martin or Uncle Bob)

2. Effective Java by Joshua Bloch

3. Code Complete (2nd Edition) by Steve McConnellCode Complete: A Practical Handbook of Software Construction

Thank you!

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


Ashok GuduruBlog: http://www.ashokg.com

Twitter: @AshokGudur

conditionals and polymorphism

Technology

coextrix technologies

ifs functions

bunch of ifs

bangalore7most ifs

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compile time

object jargon

grandest sounding words