hfooad chapter 3 change: the constant in software development

HFOOAD Chapter 3

Change: the constant in software development

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And you’re responsible for changing your programs

Great dog door, but…

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Can you add some hardware to recognize Fido’s bark when he wants go to out and come back in and automatically open the door? That way we don’t need to hear him or find that darn remote that keeps getting lost.

What might change?

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Hardware (new and modified)

The use case

The code (implementation and tests)

…

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3

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The old scenario

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The dog door opens (again).6.5

Fido barks to be let out.1

Todd or Gina hears Fido barking.2

Todd or Gina presses the button on the remote control.

3The dog door opens.4

Fido goes outside.5

Fido does his business.6

The door shuts automatically.6.1

Fido barks to be let back inside.6.2

Todd or Gina hears Fido barking (again).

6.3

Todd or Gina presses the button on the remote control.Fido goes back inside.7The door shuts automatically.8

Woof! Woof!

Gina, open the dog door…Fido won’t quit barking!

I feel much better now!

Woof! Woof!

Again with the barking! Someone let Fido back inside

6.4

The new scenario

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The dog door opens (again).6.5

Fido barks to be let out.1

Todd or Gina hears Fido barking.

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Todd or Gina presses the button on the remote control.

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The dog door opens.4

Fido goes outside.5

Fido does his business.6

The door shuts automatically.6.1

Fido barks to be let back inside.6.2

Todd or Gina hears Fido barking (again).

6.3

Todd or Gina presses the button on the remote control.

Fido goes back inside.7The door shuts automatically.8

6.4

The bark recognizer “hears” a bark (again).

6.3.1

The bark recognizer sends a request to the door to open.

6.4.1

The bark recognizer “hears” a bark.

2.1

The bark recognizer sends a request to the door to open.

3.1

We now have an alternate path

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Does this make sense to you?How would you improve it?

An improved document

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A chain of development artifacts

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Requirements

Design documents and

classes

Code

Tests

Documentation

Review the old design

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Association

Multiplicity

Our new design

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The new BarkRecognizer class

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public class BarkRecognizer {

private DogDoor door;

public BarkRecognizer(DogDoor door) {

this.door = door; }

public void recognize(String bark) {

System.out.println(" BarkRecognizer: Heard a '" + bark + "'"); door.open(); }}

This is really simple, trivial actually.

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It’s really great that we applied the Information Expert.

Why is it so great?

Is there a principle here?

If we hadn’t refactored …

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Just copy the code for closing the door automatically from the Remote to the BarkRecognizer.

This is Doug. He’s not a programmer.

What’s wrong with Doug’s idea?

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You might copy the code incorrectly

If you add another type of device you have to copy the code again

It’s just plain sloppy and ugly

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The more copies you have the more you have to change

when requirements change

Speaking of ugly

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public class BarkRecognizer {

private DogDoor door;

public BarkRecognizer(DogDoor door) {

this.door = door; }

public void recognize(String bark) {

System.out.println(" BarkRecognizer: Heard a '" + bark + "'"); door.open(); }}

This is ugly code

This is much better

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/** * This class represents the bark recognizer that opens the device it * controls if presented with a known bark. * * @author gpollice */public class BarkRecognizer {

private DogDoor door;/** * Constructor initializes this recognizer by storing the device it * controls and the bark it recognizes. * * @param door- door the recognizer controls */public BarkRecognizer(DogDoor door){

this.door = door;}

…

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Woof! (Which means: What’s the purpose of the Bark parameter in the BarkRecognizer?)

• What is the Bark for?• Do we need it?• What should we do?

Our second release is ready to go

‣ We’ve gone through another iteration

‣ Requirements

‣ Design

‣ Code

‣ Test

‣ We took on a manageable chunk of work

‣ We delivered a working system

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Things to think about‣ Are the Remote and the

BarkRecognizer similar?‣ Can you encapsulate the

similarities?

‣ How would this change the system?

‣ We haven’t implemented a Bark class. How would you do that?‣ What do we gain from it?

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Time to refactor

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Modify the code.How is it better?