Clean Code

Enterprise Java Training
© Copyright Victor Rentea P.F.A. 2015
www.VictorRentea.ro
The Art of
Clean Code
Victor Rentea
March 23, 2016
@
Bucharest Java User Group
Voxxed Days
Bucharest 2016

Clean Code | Enterprise Java Training © Copyright Victor Rentea 2015
About Me
 Victor Rentea, PhD(CS)
- Technical Team Lead,
Senior Java Engineer
at IBM Romania
- Independent Trainer PFA:
- victorrentea@gmail.com
- www.VictorRentea.ro
-
- @victorrentea
2
Who am I ?

Introduction
Names
Functions
Classes
Formatting & Comments
Contents
Agenda
3

Clean Code …
…does one thing well
…reads like well written prose
...looks like it was written by someone who cares
...is when each method you read turns out to be pretty
much what you expected
[The Principle of Least Astonishment]
anyone can write code that a computer understands,
but few programmers know how to write code
that a human can understand
Introduction
What is Clean Code ?
4
Bjarne Stroustrup
inventor of C++
Grady Booch
inventor of UML
Michael Feathers
author of Working Effectively with Legacy Code
Martin Fowler
author of Refactoring
Ward Cunningham
inventor of Wiki, coinventor of eXtreme Programming

The true cost of software = its maintenance
- 80% of the total cost+effort of software is in maintenance. Why ?
- Because we get slower and slower,
as the code gets dirtier
and begins to ROT
We READ 10x more time than we WRITE
Do your best to increase readability, even if it makes writing harder
 Always check in cleaner code than you found (Boy scout rule)
Introduction
Why Clean Code ?
5

Introduction
Names
Functions
Classes
Contents
Agenda
6

Packages, namespaces
Classes, interfaces
Functions
Variables, parameters
Class fields
Constants, enums, ...
 We do a lot of naming, so we must be good at it
Names
We name everything
7

Packages – 1 word
Classes, interfaces
Functions
Variables, parameters
Class fields
Constants
enum values ≈constants
customer.service
Customer, OrderService
placeOrder()
customerName
age, phoneNumber
CUSTOMER_PAGE_SIZE
{SINGLE, MULTIPLE}
Names
The JavaCaseConventions
8

Functions are verbs
getName(), sendEmail(), execute(), placeOrder()
- Boolean functions should answer yes/no
isGoldClient(), hostsAreValid()
Classes are nouns
Customer, OrderLine, SpecialSalesOffer, OrderFacade
- Iron out as much domain concepts ASAP
- Avoid meaningless prefixes or suffixes:
ICustomerService, OrderInfo, OrderData, OrderServiceImpl
Names
“Well Written Prose”
9

Why it exists? What it does? How it's used?
You should NOT need:
1. an /* explanatory comment */
2. to descifer its implementation, or
3. to hunt down the code using it
Make the name speak for itself !!
Just go back and rename it whenever you feel so
- Your team will be grateful! And even you will, in 3 months!
- With an IDE, it's a matter of seconds with almost no risk
Names should…
Names Explain Intent
10

Pronounceable
int getinvcdtlmt() vs. int getInvoiceableCreditLimit()
- Avoid abbreviations ! (except a fundamental business concept)
Consistent
DAO.findXyz() .fetchXyz() or .getXyz() – pick one but stick to it
- When you add code, choose names that preserve consistency
(read some code to understand the de facto naming conventions)
Unique: Synonyms are bad, they confuse
When referring to a customer, don't also use buyer or client in code
- Use ubiquitous language: customer if that’s the prevalent business term
Names: Several Guidelines
Names Should be …
11

Provide Meaningful Context
Order.customerFirstName
- But don't repeat yourself: Order.orderCreationDate
Sort nicely at CTRL-SPACE
- Related classes should begin similarly
CustomerService, CustomerDAO, Customer, CustomerExporter
Think positive about booleans
isActive() instead of isInactive() or isNotActive()
- Read this: !isInactive() :-S
Names: Several Guidelines
Names Should…
12

Variables: big scope  long descriptive name
- For loop variables can be 1 letter
- Method variables: 1+ word
- Method parameters: 1++ word
 Documentation: are shown by the IDE
- Fields: a bit longer
- Constants: self-explanatory names
Functions & Classes: big scope  short name
- public names should be convenient: short and easy to remember
- private functions should have long descriptive names
Names
Name Length
13
public class AClass {
public final static int CONST_WITH_EXPLANATORY_NAME;
private String instanceField;
public void myMethod(String parameter) {
String variable;
for (int i = 0; i < 100; i++) { ... }
}
}

Introduction
Names
Functions
Classes
Contents
Agenda
14

"A function should do one thing, it should do it well
and it should do it ONLY"[UBob]
They should be
Small
in order not to surprise us
- Principle of Least Astonishment
Functions
Functions
15

How small ?
- 4 lines is best; 5 is ok; 6 is already too much
- By any means, keep your functions smaller than a page of your IDE !
Why so small ?!
To be sure that they do only 1 THING
In a few lines, you can’t do much
Its name can then tell you everything
Functions
Functions Should be Small !!
16

The layout of a long function
is like a landscape
- Humans, are good at memorizing landscape
This landscape is familiar to the author
But for the rest of the team,
this function is completely unknown,
it's like wilderness
Functions
Why do small functions scare us?
Function Landscape
17
if
for
if
if
if
for
else
if
for if
if
for

Instead of a familiar landscape
- in which only you (the author) orient very well
You're now swimming in a sea of small functions
- You're not so comfortable anymore,
- But it's to the end good of the entire team !
Choose long descriptive names
- They will act as signposts guiding you and your team
- The reader will now be able to quickly understand your code
Functions
Why do small functions scare us?
A Sea of Small Functions
18
(the author)
else
for if

This is why we want to leave landmarks behind;
we split long functions
into many small functions
with carefully chosen long names
that guide the reader
Functions
Act Professional
19
sendActivation
EmailToCustomer()
removeAllPast
CancelledOrders
OfConsumer()

Same Goal: the code should speak for itself
- The reader shouldn't have to look at the function definition nor to its usages
Max 2 parameters
- For ≥ 3 parameters, it's hard to read: their order becomes a problem
 group them in a Parameter Object/DTO
Avoid output parameters
- The caller may be surprised that an argument changed
No flag parameters (boolean, enum) = laziness/fear
- It shouts that the function does more than one thing !
 Split it into smaller functions
customer.setLoggedIn(true);
Functions
Function Signature (1)
20
myBadMethod("John","Michael",
"Paris","St. Albergue");
checkCustomer(customer, order);
removeOrders(customer, false, true);
params.setStreet("Paris");…
myBadMethod(params);
?!

No nullable parameters
- It's like a boolean  split that function in 2: one for null, one for not-null
- Thoroughly check parameters at boundaries (=defensive programming)
Don’t return null
- Null is the worst mistake in IT [dzone]
 Consider Optional<> / throw exception / Null Object Pattern
Unchecked Exceptions won !
- No more try/throws/empty catch(); no return status codes
- Runtime Exceptions don’t annoy the caller
 Can be handled uniformly in a transparent layer at the entry points
 Can hold enum error codes for (i18n) user messages
 Define custom exception sub-types for recoverable errors (selective catch)
Functions
Function Signature (2)
21

By continuous refactoring to clean up the code
You are not done when the code starts working!
It is then when you should clean up your code !!
- IDEs help you a lot
- But only rigorous unit tests can guarantee that you don't break anything
 Test-Driven Development
Functions - Techniques
How to write such Functions ?
22

You Extract Till You Drop: [UBob]
Try to extract as many functions as possible
Into methods that tell what they do
- by their long nice name
- (you document your code this way making it easier to understand)
… until you simply cannot extract any more [opt]
- (when extracting again would basically duplicate a method)
= Extreme
Split Your Functions !
23

Reduces the number of indentations
- Max number of TABs can be restricted: Readability ++
Function body : Early returns
24
public List<Integer> stringsToInts(
List<String> strings) {
if (strings != null) {
List<Integer> integers = new ArrayList<>();
for (String s : strings) {
integers.add(Integer.parseInt(s));
}
return integers;
} else {
return null;
}
}
public List<Integer> stringsToInts2(
List<String> strings) {
if (strings == null) {
return null;
}
List<Integer> integers = new ArrayList<>();
for (String s : strings) {
integers.add(Integer.parseInt(s));
}
return integers;
}

= how to refactor God Methods into shiny new classes ?
Extract Method Object refactor
25
public String aLongMethod(String text) {
Set<String> words = new HashSet<String>();
int totalWords = 0;
int totalPhrases = 0;
StringTokenizer tokenizer = …
while (tokenizer.hasMoreTokens()) {
String nextToken = tokenizer.nextToken(); …
if (…) {
for (…) {…}
…
} else {
…
}
}
}
public class ExtractedMethodCommand {
private final String text;
private Set<String> words = new HashSet<>();
private int totalWords = 0;
private int totalPhrases = 0;
private StringTokenizer tokenizer;
public ExtractedMethodCommand(String text) {
this.text = text;
tokenizer = new StringTokenizer(text, " ");
}
public String execute() {…}
}
Extracting this block as a method
require lots of parameters, output parameters
or may even be impossible
These methods use instance fields,
and take no/fewer arguments
String result = aLongMethod(”bla”); String result = new ExtractedMethodCommand(”bla”).execute();

= when a function changes the state of the system
- e.g. loginUser(), openConnection(), persistCustomer(), checkUser()
A necessary evil: they can surprise us
- and break the Least Astonishment Principle
Explain them
- checkUserAndInitializeHisSession(User)
Functions – Advanced Topics
Side Effects
26

1.A Query doesn't have side effects
and returns a result
getState():State, isLoggedIn():boolean, findCustomer():Customer
2.A Command changes the system state
and returns void
processOrder(Order), consumeMessage(Message), setName(String)
Avoid doing both in the same method
processOrder(Order):boolean (that returns false if it failed)
(It forces the caller to check the result right away. Prefer exceptions instead!)
Command / Query Separation
27
?
!

A consequence of side effects
Open the DB connection  do stuff  close DB connection
Leads to mysterious code
- “Hack!! No one told me that I should have called .initialize() first !!"
Can be partially avoided by passing a block
- Execute on connection ( set_of_instructions )
Spring’s JdbcTemplate, RestTemplate, …
- Java8 lambdas make it a lot easier ()->{stuff;}
Temporal Coupling
28

Introduction
Names
Functions
Classes
Contents
Agenda
29

The member order: [Standard Java Conventions]
1. Fields
2. Constructors
3. Methods
- Start with the public methods
- That call private methods
 That call private methods down... etc
The reader should search down
Classes
Basic Shape
30
public class ExtractedMethodCommand {
private static final String MY_CONST=“val”;
private Set<String> words = new HashSet<String>();
private int totalWords = 0;
private int totalPhrases = 0;
private StringTokenizer tokenizer;
public ExtractedMethodCommand(String text) {…
}
public String execute() {…}
private void doInternalVeryUsefulStuff() {…}
}

Define a constant in the class that uses it the most
- Even in a domain entity class
- DO NOT create a dreadful 4000-lines Constants class !
The same for enums
- Next to that class or even nested inside
Classes
Constants
32

=classes that expose all their internal state
- to external procedures that manipulate them
- These classes don't usually hold much functionality
Objects vs. Data Structures
Data Structures (≈ C struct)
33
Getters/setters
exposing all private fields
is NOT encapsulation !
public class PhoneBookEntryDTO {
private String firstName;
private String lastName;
private String phoneNumber;
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() { … }
public void setLastName(…) { … }
public String getPhoneNumber() { … }
public void setPhoneNumber(…) { … }
}
public class SimpleImmutableStruct {
private final String firstName;
private final String lastName;
public SimpleImmutableStruct(
String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
public String getFirstName() {
return firstName;
}
public String getLastName() {
return lastName;
}
}
Why we Immutable Objects:
- Preserve their consistency after creation
- Thread safe
- Safe for sort-/hash-based collections
public class PhoneBookEntryDTO {
public String firstName;
public String lastName;
public String phoneNumber;
}
≈

=hide implementation details from your client
Expose operations rather than data
car.startEngine() instead of car.setEngineStarted(true)
or car.engineStarted=true
- Data is concrete, it's an implementation detail
Try to minimize the object's observable state
Expose data as abstract as possible
car.getPercentageFuelLeft() instead of getGasolineInLiters()
True Encapsulation
34
car.getEstimatedRemainingKm()

=classes that hide their state behind abstractions
- and expose functionality as public methods, so that
- their caller works with them without knowing their implementation
True Objects (as in OOP)
35
public class Car {
private double remainingGasoline;
private double reservoirCapacity;
public double getRemainingGasolineInLiters() {
return remainingGasoline;
}
public double getReservoirCapacityInLiters() {
return reservoirCapacity;
}
public double getRemainingFuelRatio() {
return remainingGasoline/reservoirCapacity;
}
} structs OOP

void m(P p) {
memberF();
field.fieldF();
p.parameterF();
new Obj().f();
Math.sqrt(4);
Don’t ask the state of other objects, instead,
tell them what to do
1. Encapsulation
- Keep behavior next to state
2. Low coupling
- Less knowledge about other objects
Avoid Train Wrecks:
- getRoot().getManager().getMessageWidget().showMessage("blah")
Classes
Tell, Don’t Ask Principle
36
The Law of Demeter
A function m() of ClassX
should call only methods of:
• ClassX
• a field of ClassX
• m()'s parameters
• objects created within m()
• globals
(in short: "use only one dot")

=code that works with too much data of another class
…probably belongs inside that other class
- This refactoring usually simplifies the code
- Did you thought about the Law of Demeter ? 
- Is the envy gone from our example ?
Classes
Feature Envy
37
public class Rental {
private MovieCopy movieCopy;
…
public String getStatement() {
return "t" + movieCopy.getMovie().getTitle() +
"t" + movieCopy.getMovie().getPrice() +
"n";
}
}
public class MovieCopy {
private Movie movie;
public String getStatement() {
return "t" + movie.getTitle() +
"t" + movie.getPrice() + "n";
}
}
return movieCopy.getStatement();

Conditionals over Polymorphic
Data Structures Objects
 Easy to add functions
+1 type-specific functionality
= a new switch
e.g. getMaxLoanDays()
 Dangerous to add types
+1 type = a new case in all the
existing switches
abstract methods need to be
defined by each concrete type
 Safe to add types
- Java forces us to implement
all the necessary methods
 More artifacts: + 3 classes
38
Objects vs. Data Structures: Replace Conditionals with Polymorphism
VS
switch (movie.getType()) {
case NEW_RELEASE: … break;
case REGULAR: … break;
case CHILDRENS: … break;
}
public class NewReleaseMovie
extends Movie {
@Override
public double func(…) { … }
}
Movie
#func()
RegularMovie
ChildrensMovie

Runtime dependency usually goes the same
way as source code dependency
- Class A imports the class B (source code dependency)
- …so that A can then invoke a method in B (runtime dependency)
With OO, we can inverse the source code dep.
Classes
Best of OOP:[UBob]
The Plugin Model
39
Module 2Module 1
A B
import B;
b.method()

Make A depend on an interface I
- That contains the method(s) that need to be invoked in B
Make B implement I
- It thus must have code source dependency on I
A needs an instance of type I  give it a B instance
- Easily done using an dependency injection framework (EJB, Spring, Guice)
- At runtime, A still invokes B (), but Module 1 does not depend on Module 2 anymore
Classes
The Plugin Model: Dependency Inversion
40
Module 2Module 1
A B
import
I
.method()
import
= abstractions should not depend on details, details should depend on abstractions

Introduction
Names
Functions
Classes
Contents
Agenda
41

It's all about communication
- Communication is the most important activity of a professional developer
Don't ignore the formatting rules,
Nor follow them blindly
- The purpose is to express your intent in code as clearly as possible
Have the IDE help you
Formatting
Code Formatting
42

< 100-120 chars/line
 < 100-200 lines/file, never > 500
 < 5-10 lines/method, never > 1 screen
Indentation: Team
2-3-4 chars or TAB ? Egyptian-style or not ? ...
- Establish a team style and follow it [+stylesheet?]
- Never reformat the entire file using your IDE – only several selected lines
 Manual formatting can be more expressive:
 We just merge conflicts at commit, don’t we 
Don't collapse:
{ return 0; } or if (cond) doStuff();
Formatting
Several Suggestions
43
Customer customer = new Customer.Builder()
.withName("John Doe")
.withAddress(new Address.Builder()
.withStreetName("Viorele")
.build())
.build();
{
}

=they compensate for our inability to express in code
Always before dropping a comment
And see whether you can express it in code
Why should we hate comments ?
Because they usually LIE !
- As they immediately fall out of sync with the code
Comments
Comments are Failures
44

public List<int[]> getFlaggedCells(){
List<int[]> flaggedCells = new A…L…<…>();
for (int[] cell : gameBoard) {
boolean isFlagged = cell[STATUS_VALUE] == FLAGGED;
if (isFlagged)
flaggedCells.add(x);
}
return flaggedCells;
}
public List<Cell> getFlaggedCells(){
List<Cell> flaggedCells = new A…L…<Cell>();
for (Cell cell : gameBoard) {
if (cell.isFlagged())
flaggedCells.add(cell);
}
return flaggedCells;
}
Constants
- Instead of magic numbers
Explanatory variables
- Locals that store intermediary results
Explanatory methods
- To name a given computation
- "Encapsulate Conditionals"
Split the code
- into many small private functions with long descriptive names, remember?
This is how to document your code !
Comments
Code Expressiveness ++
45
public List<int[]> getCells(){
List<int[]> list1 = new ArrayList<int[]>();
for (int[] x : theList) {
if (x[0] == 4)
list1.add(x);
}
return list1;
}

Mumbling
- Unclear
- Rush/lazy
Redundant
Mandated or Noise
/** Default constructor. */
Position Markers
//////// Accessors //////
Commented-out Code
- Don't read it. Just DELETE IT!
- (You SCM rocks !)
HTML JavaDocs
Non-local Information
- Comment code on that line (±1)
Comments
Bad Comments
46
/**
* Returns the day of the month.
*
* @return the day of the month.
*/
public int getDayOfMonth() {
return dayOfMonth;
}
DUH !!
stream.close();
} catch (IOException e) {
// Oh! Give me a break!
}

Intent: WHY?(what?)
- When the code just can't say it
specific algorithms/calculus
bug workarounds
- An URL link can explain it all
Clarifications
To explain calls to an external API
(if its usage is strange)
Warning of consequences
- that might not be obvious
TODOs
// TODO vrentea Fetch the order
- Assume ownership
Public API JavaDocs
- for libraries/frameworks you write
Legal
// Copyright (C) 2013, …
Comments
Good Comments
47
SimpleDateFormat sdf = new SimpleDateFormat(…); // Not thread-safe. Keep it local!

48
Names
 Choose descriptive names
 Unambiguous names, no synonyms
 Use an ubiquitous language if possible
 Use long variable names for long scopes
 Make side-effects clear
 Avoid encodings (e.g. m_...)
Functions
 Too many arguments
 Output arguments
 Flag arguments
 Dead function (no callers)
 Functions should do one thing
 Function names should say what they do
Comments
 Redundant Comment
 Obsolete Comment
 Commented-out code
General
 Duplication: DRY
 Multiple languages in one source file
 Incorrect behavior at the boundaries
 Clutter: default constructor, unused code, …
 Feature Envy
 Avoid transitive navigation(Law of Demeter)
 Hidden temporal couplings
 Follow standard conventions
 Use explanatory variables and functions
 Extract magic numbers as constants
 Encapsulate conditionals
 Avoid negative conditions
Tests
 Insufficient tests: use a test coverage tool
 Don't skip trivial tests
 Test boundary conditions
 Exhaustively test near bugs
 Tests should be fast !
Summary – Code Smells and Heuristics & Part2Part1

49
Special Thanks to
Ionuţ Chereji
Laurenţiu Carată
Victor Bucuţea
Florin Diaconu
Ionuţ Scutaru
Leonard Giura

Clean Code
Robert C. Martin (aka Uncle Bob), 2008
Clean Coders videos
http://www.cleancoders.com/ : Uncle Bob
Refactoring
Martin Fowler, 1999
 NULL – the worst mistake in IT
- https://dzone.com/articles/the-worst-mistake-of-computer-science-1
 The Clean Architecture:
- http://blog.8thlight.com/uncle-bob/2012/08/13/the-clean-architecture.html
 Some  Programming Jargon
- http://blog.codinghorror.com/new-programming-jargon/
Want to dig more ?
References
51

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