Ben's Blog A place to share my thoughts

Clean Code

Clean code cover

Last (and final) update: 19/03/19

Full Summary: Clean Code Cheatsheet

Today I started reading Clean code: A Handbook of Agile Software Craftsmanship by Robert C. Martin. As a helpful reminder, I will summarise the core concepts from each of the chapters. Conveniently, the guys at have recorded a podcast series discussing each chapter, which I will use to ensure I have fully digested the content! I will provide their links, along with supporting material that I found helpful.

Chapter 2 - Meaningful Names

  • You shouldn’t need to comment to explain a variable
  • Don’t abbreviate variables, dts should be dateTimeStamp or addr should be address
  • Avoid disinformation: a StudentList must be a list or it’s a lie. Students or StudentGroup is better term of internal representation is not a list
  • Use Solution names: StudentList if internally its a List<Student> or name a class CarFactory if its following the Factory design pattern.
    • when solution names aren’t applicable, fallback to domain rules which can be clarified with product owner
  • Use Meaninful names: Copy(string a1, string a2) is meaningless, use something like Copy(string source, string destination)
  • Always Focus on making our code as easy as possible to understand
  • Class NameNoun. Don’t add Manager, Processor, Data or Info to the name of a class.
  • Method NameVerb
  • Variables with context? Move them in a class.
  • Don’t add types in names
  • Refactor if a better name

Chapter 3 - Functions

  • Functions should be small… smaller than that (repeat 3-4 times)
  • Function should do one thing – until you cannot further reduce
    • We know it does more than one thing when we can extract a name that is not a restatement of its implementation
  • Functions that do one thing cannot be reasonably divided into sections (if you see #sections, that’s a strong indicator it does too much)
  • One level of abstraction per function (this also ensures its doing one thing). Mixing abstractions is confusing!
  • Should read like a novel
  • Switch Statements – use only if:
    • It appears once
    • Its used to create a polymorphic object
    • Hidden behind inheritance
  • Functions should avoid Out, return only via return.
  • Number of arguments: 0 is better than 1 (niladic) 1 is better than 2 (monadic), 2 is better than 3 (dyadic), 3 is …. no, 3 is too many.
    • Two arguments are perfectly acceptable if they are cohesive i.e. Point(x,y)
    • Two arguments are not ok in this case: assert(expected, actual) – have you ever mixed the order? We all have! Use extension method so actual.assert(expected)
    • More args makes testing harder!
  • Don’t pass flags as arguments, they complicate things! Create a function which toggles it to true and one for false
  • Use arguments object if cohesive to reduce args: Draw(x,y,radius) -> Draw(circle)
  • Function = Verb, Class = Noun works well Write(name)
  • Avoid side effects! If you DoX(), it should do X, not do X + Y
    • Ensures we don’t unintentionally add Temporaral Coupling; a kind of coupling where code is dependent on time in some way.
  • Command Query Separation – A method does something XOR answer something
    • a method may change the state of an object
    • XOR a method may return info on that object, NEVER both
  • Exceptions are better than error codes
    • Try/Catch should call a method to keep error processing seperated from normal processing i.e. try { DoAction(); } catch (Exception e) { logError(e) }
      • functions should do one thing. Error handling is one, thus should do nothing else.
    • error codes are probably enums who are dependency magnets! If enum changes, all classes that import will be recompiled.
    • Exceptions abide to the Open Close Principle as we can extend new code with new exceptions without modifying existing code. If we add error code, we modify, violating principle.
  • DRY – don’t repeat them functions

Chapter 4 - Comments

  • A comment is to compensate for our failure to express ourselfs in code
    • We comment because its unclear
  • Why failure? Comments are lies! – too often anyway.
  • Inaccurate is much worse than none – code is the only truth.
  • Always try and explain in code:
// check if employee is eligible for full benefits
if(employee.flags == Hourly && age > 64)
  • The only good comment is a:
    • comment you found a way not to write!
    • comment that provides context i.e. explaining a regex matcher
    • clarifying stuff in a library for code you cannot alter
  • TODO is acceptable, just keep on top of it
  • Avoid useless / pointless comments that just repeat the implementation
    • Aim to get the info from code
  • Don’t use a comment when you can use a function or variable
  • Banners (something to draw attention) can be OK, but use little or it will become noise
  • Don’t comment out code, source control!
  • A comment should describe code that doesn’t describe itself (when refactoring not possible)
  • Do not add a comment that is unclear and needs explaining!

Chapter 5 - Formatting

  • Take pride on how your code looks – poorly formatted code will make others think the project lacks attention to detail
  • Code should be formatted like a newspaper – it’s usable and organised
  • Openness to separate concepts – new line between methods, no lines if variables are related etc
  • Vertical density implies close association
  • variable declarations should be as close as usage as possible
  • Keep related things vertically close
  • If function A calls B, then caller should be above callee
    • Function dependencies should point downward

Chapter 6 - Objects and Data Structures

  • Use abstract terms to express our data
  • Objects and DTO (They’re opposites)
    • Objects – hide data behind abstraction and expose functions operating on data
  • DTO – expose data and have no meaningful functions
  • Procedural and OO (They’re opposites)
    • Procedural
      • easy to add new functions without changing existing data structures
      • hard to add new data structures as all functions must change
    • OO
      • easy to add new classes without changing existing functions
      • hard to add new functions because all classes must change (polymorphism)
  • Law of Demeter:
    • method f of class C can only call methods if:
      • object created by f
      • object passed as arg of f
      • object held as instance variable in C “Talk to friends not strangers”
    • Rules above state that we cannot chain methods i.e. a().b().c() but we can set a variable for each and call each in turn i.e. var first = a(), var second = first.b();
      • if they’re dto’s then it’s fine as a properties are exposing data
      • If lot’s of chaining then there probably a deeper problem. Why do you need c(), you should create a method that does the intended action.

Chapter 7 - Error Handling

  • If error handling obscures logic, you’re doing it wrong!
    • Error handling and algorithm should be different concerns try { DoStuff() } catch { HandleError() }
  • Use exceptions over return codes
    • codes used when language did not support exceptions
  • Each thrown exception must provide context to determine source and foundation of error. Must know the intent.
  • Define exception classes for callers needs:
void DoSomething()
    Custom c = new Custom();
    try { c.DoIt(); }
    catch(MyCustomException e) { LogError(e); }
class Custom
    private LibraryAPI lib = new LibraryAPI();
    void DoIt(){
        try { lib.DoIt(); }
        catch(ExceptionLib1 e){ throw MyCustomException(e) }
        catch(ExceptionLib2 e){ throw MyCustomException(e) }
        catch(ExceptionLib3 e){ throw MyCustomException(e) }
  • Above is much cleaner than directly implementing LibraryAPI and referencing that across app. Wrapping 3rd party API also means you’re not tied to a vendor.
  • Never use try catch for flow control
  • Return empty list over null, callers can then check for nulls
    • special case pattern (return an empty object)
  • Don’t ever return null
  • Returning null is bad, passing null in a method is worse!
  • To handle nulls, you can
    • use some guard clause to verify inputs are not null and throw ArgumentException
    • or anything else that checks inputs – it’s dependent on language

Chapter 8 - Boundaries

  • Expose only what needs to be exposed
    • Expose IEnumerable over List, or even better wrap the collection in an object and expose exactly what you want
  • Create interfaces over directly accessing 3rd party libraries i.e.
    • ILogger is better than exposing Log4Net
  • Learning 3rd party code is hard, integrating 3rd party code is hard, doing both together is a magnitude harder
    • use learning tests – Don’t do it all in production, create a dummy unit test project and learn the library
      • controlled experiments
      • with tests, if library is updated we have tests to verify that there are no change breaking updates
  • Use code that does not exists “I wish for that interface”
    • define your own interface before the code even exists
    • makes mocking trivial Code at boundaries needs clear separation and tests to define expectations
    • our code should not know (too much) about 3rd party
    • better to dependend on something you control, not something that controls you
  • Manage 3rd party boundaries by exposing as little as possible
    • can always use Adapter pattern to convert from our perfect interface to the provided interface
  • Clean boundaries promotes consistency and reduces maintenance issues

Chapter 9 - Unit Tests

  • TDD write tests first, code second
  • 3 laws of TDD – ensures tests and production are written together, with tests few secs ahead of production code
    1. You may not write production code until you have a written a failing unit test
    2. You cannot write more of a unit test than is sufficient to fail, and not compiling is failing
    3. You may not write more production code than is sufficient to pass the currently failing test
  • Test code is as important as production code
    • first class citizen, requires thought, design and care – don’t worry about making it as efficient as production code
    • Poorly written tests are as bad if not worse than having no tests. This is due to the difficulties in maintaining it with production code
    • no unit tests means you cannot safely refactor and clean up code
  • Unit tests keep your code flexible
    • unit tests ensures code it flexible, maintainable and reusable
    • Without tests, you will be reluctant to make changes because you might introduce bugs, fact.
    • With tests, you do not fear introducing bugs enabling you to improve architecture and design without fear
  • Key to clean tests: Readability, readability & readability – clarity, simplicity and density of expression (say a lot with little)
    • Don’t flood tests with details, abstract those details and provide intent at the method level
      • Design a testing API (a domain-specific language) to abstract those lower level details, over time, it helps with writing but more importantly in reading
  • Aim for 1 assert per test, but enforce one concept to test per function
  • Clean test FIRST:
    • Fast – make tests fast, or you won’t run them often enough
    • Independent – tests should not depend on each other. Tests should be runnable in any order
    • Repeatable – All tests should be runnable on any environment (home PC, QA, production etc). Otherwise if they fail, you’ll end up making excuses
    • Self-Validating – True or False. If should be obvious if a test has passed or failed
    • Timely – Write tests in a timely fashion. Write before production code that makes them pass. If you write after, you’ll find production code hard to test (the code won’t be testable)
  • If you let your tests rot, your code will rot too. Keep tests clean!

Chapter 10 - Classes

  • Loosening encapsulation for tests is fine - tests rule!
  • Classes should be small with single responsibilities
    • Name the class describing what it fulfils. The more ambiguous the name, the more likely its doing too much
      • ‘Manager’, ‘Processor’, ‘Super’ are smells
    • Describe the class in <25 words, without:
      • ‘if’, ‘and’, ‘or’, ‘but’
  • Adhere to Single Responsibility Principle - the class or module should have 1 and only 1 reason to change
    • Getting code working vs clean code are 2 different activities. When code works, go back and break overstuffed classes into decoupled units with single responsibilities.
      • Some say many single purpose classes makes it difficult to get bigger picture, but actually there is no additional complexity as same moving parts. The difference is few toolboxes with many tools vs lots of small organised toolboxes with a few tools. Or let the dev wade through unnecessary noise vs provide concise and only relevant units
    • In a clean system you should organise classes to reduce the risk of change
      • Why? Change == risk and it breaks the Open/Close principle (open for extension, closed for modification)
  • Cohesive - small number of instance variables
    • Each method should manipulate 1+ of those variables
    • High cohesion - each variable is used by many methods
      • don’t pass lots of params from function to function, make them instance variables
      • If this reduces cohesion, then split them in a smaller class (instance variables + methods = class)
  • Needs change, code will change - therefore if a client depends upon concrete details, its a risk
    • Much safer to depend on a concept (interfaces, abstract classes)
    • Don’t depend on concrete behaviours
      • More testable code
      • less coupled thus more flexible, promotes reuse and better isolated from change
      • less coupling adheres to Dependency Inversion principle (open for extension, closed for modification) - depend on abstractions, not implementations

Chapter 11 - Systems

  • Separate construction from usage - starting up is a concern and usage is another concern
    public Service getService()
      if (service == null) service = new MyServiceImpl(...); // a reasonable default for most cases?
      return service;
    • Above looks reasonable, faster startup due to lazy loading but there are issues:
      • Hard coded a dependency on MyServiceImpl and all constructor args and testing is tricky.
      • Small violation in Single responsibility principle as we have a null test and the block
      • Worst of all, this method has to know the global context
    • It’s acceptable, but will quickly get abused, and you’ll have initialization logic scattered everywhere.
    • Modularity is much more important than convenient idioms! Enforce a global consistent strategy for resolving dependencies!
  • Factories separates application from the building and resolving of dependency
  • Use Dependency Injection and Inversion of control to separate construction from use
    • IOC moves secondary responsibilities from an object to other objects that are dedicated to a single purposes (supports SRP)
      • An object should not be responsible for instantiating dependencies itself, instead, pass responsibility to another authoritative mechanism
    • Construction or Property DI is resolved at runtime. Can normally use to invoke a factory or proxies
  • Focus on todays stories, but be prepared to go back and refactor code
    • Getting it right first time is a myth, so refactor, expand system to implement new stories tomorrow (iterative and incremental agility!)
      • Clean code, TDD and refactoring will make this work at code level
      • At system level, if you have proper separation of concerns, architecture can grow incrementally
  • Big Design up front is harmful since it inhibits adapting to change due to psychological resistance to discarding prior effort!
    • it’s economically feasible if it separates concerns effectively
  • Keep designs naively simple but ensure architecture is decoupled, enabling story delivery to be quick, and adding more infrastructure as we scale up and maintain ability to respond to new information
  • Modularity and separation of concerns make decentralized management and decision making possible. In a large system, no one person can make all decisions.
    • Postpone decisions until last possible moment → more information = better decisions whereas premature decisions is a decision made with suboptimal knowledge
  • Use standards wisely, when they add value → should make it easier t reuse ideas and components, ensure it primarily meets the needs of the adopters; the real intended users
  • Keep it clean, invasive architectures overwhelms domain logical and impacts agility
    • if domain logic obscured, quality suffers, bugs creep in, stories harder to implement, agility is lost

Chapter 12 - Emergence

  • A design is simple when it:

    1. Runs all the tests
      • provides means to verify system works as intended
      • systems that are not testable are not verifiable - should non-verified system be deployed?
      • Testable system = low coupling, high cohesion
        • naturally try make tests simpler, therefore small SRP classes, loosely couples (Dependency Inversion Principle)
    2. no duplication
    3. Express intent of programmer
    4. Minimizes number of classes and methods
  • Rule 2-4: Refactoring

    • tests give security during refactoring
      • every few lines question if design degraded - if so clean it up. Lookout for:
        • higher cohesion
        • decrease coupling
        • separate concerns
        • modularize system concerns
        • shrink functions and classes
        • better names
        • Summary: Remove duplication + ensure expressiveness, minimise classes & methods
    • No Duplication
      • risks are: additional work, more complexity
      • as you remove duplication, violations to SRP will emerge (Template Pattern helps)
    • Expressive
      • majority of cost is maintenance
      • if it makes sense to you now, make sure the code clearly expresses intent of the author
        • the clearer, the less time, reducing defects and cost of maintenance
      • express yourself by:
        • good naming (don’t surprise when you look at responsibilities)
        • small functions and classes - easy to name, write and understand
        • use standard nomenclature - name code patterns after design patterns
        • well-written unit tests - they are documentation
        • take pride - when it works, you must make it clean, take care
    • Minimise classes and methods
      • don’t go too crazy with eliminating duplication, code expressive, SRP - too many tiny classes or too much abstraction is not good.
      • tests, duplication and expression more important than low counts

Chapter 13 - Concurrency!

  • Objects are abstractions of processing. Threads are abstractions of schedule.
  • Concurrency is complex… given
public class X {
  private int lastIdUsed;
  public int getNextId(){
    return ++lastIdUsed;
  • In sample above, given 2 threads, the three possible outcomes:
    • T1 gets value 43, T2 gets value 44, lastIdUsed 44
    • T1 gets value 44, T2 gets value 43, lastIdUsed 44
    • T1 gets value 43, T2 gets value 43, lastIdUsed 43
  • the Java Just-in-time compiler can produce 12,870 possible execution paths. Most will return correct result, but not all! Swap int for long, increases to over 2m!
  • Concurrency Defense principles
    • SRP - concurrency should be its own right, it should be separated from rest of code
    • Limit scope of data - above example, we can use synchronized to protect shared object. But take data encapsulation to heart… limit shared data access!
      • keep sections small as possible or performance decreases and contention increases
    • Avoid shared data - make read-only copies or clone objects and merge results in single thread
    • Threads should be as independent as possible - each does one thing i.e. one client request, local variables
    • Know your library - thread-safe collections!
      • ConcurrentHashMap enables simultaneous concurrent reads and writes
    • Know various execution models (pg 184: Producer-Consumer, Readers-Writes, Dining Philosophers)
  • Test it, a lot
    • Run more threads than processors - exposes problems
    • Look at threading API and make calls to methods that change orders, automate this, try and break it now


Note - there is more but the chapters remaining are case studies, so I will not summarise those here.

This has been the most influential book that I’ve ever read. I found myself a year or so ago not actually knowing what was good clean code. Since having read this book, I now have the skills to recognise and improve the quality in whichever code base I work in.

Supporting Material: