clean-code-java

Table of Contents

1. Introduction
2. Variables
3. Functions
4. Objects and Data Structures
5. Classes
6. SOLID
7. Testing
8. Concurrency
9. Error Handling
10. Formatting
11. Comments
12. Translation

Introduction

Software engineering principles, from Robert C. Martin's book Clean Code, adapted for Java. This is not a style guide. It's a guide to producing readable, reusable, and refactorable software in Java.

Not every principle herein has to be strictly followed, and even fewer will be universally agreed upon. These are guidelines and nothing more, but they are ones codified over many years of collective experience by the authors of Clean Code.

Our craft of software engineering is just a bit over 50 years old, and we are still learning a lot. When software architecture is as old as architecture itself, maybe then we will have harder rules to follow. For now, let these guidelines serve as a touchstone by which to assess the quality of the Java code that you and your team produce.

One more thing: knowing these won't immediately make you a better software developer, and working with them for many years doesn't mean you won't make mistakes. Every piece of code starts as a first draft, like wet clay getting shaped into its final form. Finally, we chisel away the imperfections when we review it with our peers. Don't beat yourself up for first drafts that need improvement. Beat up the code instead!

Variables

Use meaningful and pronounceable variable names

Bad:

String yyyymmdstr = new SimpleDateFormat("YYYY/MM/DD").format(new Date());

Good:

String currentDate = new SimpleDateFormat("YYYY/MM/DD").format(new Date());

⬆ back to top

Use the same vocabulary for the same type of variable

Bad:

getUserInfo();
getClientData();
getCustomerRecord();

Good:

getUser();

⬆ back to top

Use searchable names

We will read more code than we will ever write. It's important that the code we do write is readable and searchable. By not naming variables that end up being meaningful for understanding our program, we hurt our readers. Make your names searchable.

Bad:

// What the heck is 86400000 for?
setTimeout(blastOff, 86400000);

Good:

// Declare them as capitalized `const` globals.
public static final int MILLISECONDS_IN_A_DAY = 86400000;

setTimeout(blastOff, MILLISECONDS_IN_A_DAY);

⬆ back to top

Use explanatory variables

Bad:

String address = "One Infinite Loop, Cupertino 95014";
String cityZipCodeRegex = "/^[^,\\\\]+[,\\\\\\s]+(.+?)\\s*(\\d{5})?$/";

saveCityZipCode(address.split(cityZipCodeRegex)[0],
                address.split(cityZipCodeRegex)[1]);

Good:

  String address = "One Infinite Loop, Cupertino 95014";
  String cityZipCodeRegex = "/^[^,\\\\]+[,\\\\\\s]+(.+?)\\s*(\\d{5})?$/";

  String city = address.split(cityZipCodeRegex)[0];
  String zipCode = address.split(cityZipCodeRegex)[1];

  saveCityZipCode(city, zipCode);

⬆ back to top

Avoid Mental Mapping

Don’t force the reader of your code to translate what the variable means. Explicit is better than implicit. Bad:

String [] l = {"Austin", "New York", "San Francisco"};

for (int i = 0; i < l.length; i++) {
    String li = l[i];
    doStuff();
    doSomeOtherStuff();
    // ...
    // ...
    // ...
    // Wait, what is `$li` for again?
    dispatch(li);
 }

Good:

String[] locations = {"Austin", "New York", "San Francisco"};

for (String location : locations) {
    doStuff();
    doSomeOtherStuff();
    // ...
    // ...
    // ...
    dispatch(location);
 }

⬆ back to top

Don't add unneeded context

If your class/object name tells you something, don't repeat that in your variable name.

Bad:

class Car {
  public String carMake = "Honda";
  public String carModel = "Accord";
  public String carColor = "Blue";
}

void paintCar(Car car) {
  car.carColor = "Red";
}

Good:

class Car {
  public String make = "Honda";
  public String model = "Accord";
  public String color = "Blue";
}

void paintCar(Car car) {
  car.color = "Red";
}

⬆ back to top

Functions

Function arguments (2 or fewer ideally)

Limiting the amount of function parameters is incredibly important because it makes testing your function easier. Having more than three leads to a combinatorial explosion where you have to test tons of different cases with each separate argument.

One or two arguments is the ideal case, and three should be avoided if possible. Anything more than that should be consolidated. Usually, if you have more than two arguments then your function is trying to do too much. In cases where it's not, most of the time a higher-level object will suffice as an argument.

Bad:

Good:

⬆ back to top

Functions should do one thing

This is by far the most important rule in software engineering. When functions do more than one thing, they are harder to compose, test, and reason about. When you can isolate a function to just one action, they can be refactored easily and your code will read much cleaner. If you take nothing else away from this guide other than this, you'll be ahead of many developers.

Bad:

public void emailClients(List<Client> clients) {
    for (Client client : clients) {
        Client clientRecord = repository.findOne(client.getId());
        if (clientRecord.isActive()){
            email(client);
        }
    }
}

Good:

public void emailClients(List<Client> clients) {
    for (Client client : clients) {
        if (isActiveClient(client)) {
            email(client);
        }
    }
}

private boolean isActiveClient(Client client) {
    Client clientRecord = repository.findOne(client.getId());
    return clientRecord.isActive();
}

⬆ back to top

Function names should say what they do

Bad:

private void addToDate(Date date, int month){
    //..
}

Date date = new Date();

// It's hard to to tell from the method name what is added
addToDate(date, 1);

Good:

private void addMonthToDate(Date date, int month){
    //..
}

Date date = new Date();
addMonthToDate(1, date);

⬆ back to top

Functions should only be one level of abstraction

When you have more than one level of abstraction your function is usually doing too much. Splitting up functions leads to reusability and easier testing.

Bad:

Good:

⬆ back to top

Remove duplicate code

Do your absolute best to avoid duplicate code. Duplicate code is bad because it means that there's more than one place to alter something if you need to change some logic.

Imagine if you run a restaurant and you keep track of your inventory: all your tomatoes, onions, garlic, spices, etc. If you have multiple lists that you keep this on, then all have to be updated when you serve a dish with tomatoes in them. If you only have one list, there's only one place to update!

Oftentimes you have duplicate code because you have two or more slightly different things, that share a lot in common, but their differences force you to have two or more separate functions that do much of the same things. Removing duplicate code means creating an abstraction that can handle this set of different things with just one function/module/class.

Getting the abstraction right is critical, that's why you should follow the SOLID principles laid out in the Classes section. Bad abstractions can be worse than duplicate code, so be careful! Having said this, if you can make a good abstraction, do it! Don't repeat yourself, otherwise you'll find yourself updating multiple places anytime you want to change one thing.

Bad:

Good:

⬆ back to top

Set default objects with Object.assign

Bad:

Good:

⬆ back to top

Don't use flags as function parameters

Flags tell your user that this function does more than one thing. Functions should do one thing. Split out your functions if they are following different code paths based on a boolean.

Bad:

Good:

⬆ back to top

Avoid Side Effects (part 1)

A function produces a side effect if it does anything other than take a value in and return another value or values. A side effect could be writing to a file, modifying some global variable, or accidentally wiring all your money to a stranger.

Now, you do need to have side effects in a program on occasion. Like the previous example, you might need to write to a file. What you want to do is to centralize where you are doing this. Don't have several functions and classes that write to a particular file. Have one service that does it. One and only one.

The main point is to avoid common pitfalls like sharing state between objects without any structure, using mutable data types that can be written to by anything, and not centralizing where your side effects occur. If you can do this, you will be happier than the vast majority of other programmers.

Bad:

Good:

⬆ back to top

Avoid Side Effects (part 2)

In JavaScript, primitives are passed by value and objects/arrays are passed by reference. In the case of objects and arrays, if your function makes a change in a shopping cart array, for example, by adding an item to purchase, then any other function that uses that cart array will be affected by this addition. That may be great, however it can be bad too. Let's imagine a bad situation:

The user clicks the "Purchase", button which calls a purchase function that spawns a network request and sends the cart array to the server. Because of a bad network connection, the purchase function has to keep retrying the request. Now, what if in the meantime the user accidentally clicks "Add to Cart" button on an item they don't actually want before the network request begins? If that happens and the network request begins, then that purchase function will send the accidentally added item because it has a reference to a shopping cart array that the addItemToCart function modified by adding an unwanted item.

A great solution would be for the addItemToCart to always clone the cart, edit it, and return the clone. This ensures that no other functions that are holding onto a reference of the shopping cart will be affected by any changes.

Two caveats to mention to this approach:

1. There might be cases where you actually want to modify the input object, but when you adopt this programming practice you will find that those cases are pretty rare. Most things can be refactored to have no side effects!

Bad:

Good:

⬆ back to top

Don't write to global functions

Polluting globals is a bad practice in JavaScript because you could clash with another library and the user of your API would be none-the-wiser until they get an exception in production. Let's think about an example: what if you wanted to extend JavaScript's native Array method to have a diff method that could show the difference between two arrays? You could write your new function to the Array.prototype, but it could clash with another library that tried to do the same thing. What if that other library was just using diff to find the difference between the first and last elements of an array? This is why it would be much better to just use ES2015/ES6 classes and simply extend the Array global.

Bad:

Good:

⬆ back to top

Favor functional programming over imperative programming

JavaScript isn't a functional language in the way that Haskell is, but it has a functional flavor to it. Functional languages are cleaner and easier to test. Favor this style of programming when you can.

Bad:

Good:

⬆ back to top

Encapsulate conditionals

Bad:

Good:

⬆ back to top

Avoid negative conditionals

Bad:

Good:

⬆ back to top

Avoid conditionals

This seems like an impossible task. Upon first hearing this, most people say, "how am I supposed to do anything without an if statement?" The answer is that you can use polymorphism to achieve the same task in many cases. The second question is usually, "well that's great but why would I want to do that?" The answer is a previous clean code concept we learned: a function should only do one thing. When you have classes and functions that have if statements, you are telling your user that your function does more than one thing. Remember, just do one thing.

Bad:

Good:

⬆ back to top

Don't over-optimize

Modern browsers do a lot of optimization under-the-hood at runtime. A lot of times, if you are optimizing then you are just wasting your time. There are good resources for seeing where optimization is lacking. Target those in the meantime, until they are fixed if they can be.

Bad:

Good:

⬆ back to top

Remove dead code

Dead code is just as bad as duplicate code. There's no reason to keep it in your codebase. If it's not being called, get rid of it! It will still be safe in your version history if you still need it.

Bad:

Good:

⬆ back to top

Objects and Data Structures

Use getters and setters

Using getters and setters to access data on objects could be better than simply looking for a property on an object. "Why?" you might ask. Well, here's an unorganized list of reasons why:

• When you want to do more beyond getting an object property, you don't have to look up and change every accessor in your codebase.
• Makes adding validation simple when doing a set.
• Encapsulates the internal representation.
• Easy to add logging and error handling when getting and setting.
• You can lazy load your object's properties, let's say getting it from a server.

Bad:

Good:

⬆ back to top

Make objects have private members

This can be accomplished through closures (for ES5 and below).

Bad:

Good:

⬆ back to top

Prefer composition over inheritance

As stated famously in Design Patterns by the Gang of Four, you should prefer composition over inheritance where you can. There are lots of good reasons to use inheritance and lots of good reasons to use composition. The main point for this maxim is that if your mind instinctively goes for inheritance, try to think if composition could model your problem better. In some cases it can.

You might be wondering then, "when should I use inheritance?" It depends on your problem at hand, but this is a decent list of when inheritance makes more sense than composition:

1. Your inheritance represents an "is-a" relationship and not a "has-a" relationship (Human->Animal vs. User->UserDetails).
2. You can reuse code from the base classes (Humans can move like all animals).
3. You want to make global changes to derived classes by changing a base class. (Change the caloric expenditure of all animals when they move).

Bad:

Good:

⬆ back to top

SOLID

Single Responsibility Principle (SRP)

As stated in Clean Code, "There should never be more than one reason for a class to change". It's tempting to jam-pack a class with a lot of functionality, like when you can only take one suitcase on your flight. The issue with this is that your class won't be conceptually cohesive and it will give it many reasons to change. Minimizing the amount of times you need to change a class is important. It's important because if too much functionality is in one class and you modify a piece of it, it can be difficult to understand how that will affect other dependent modules in your codebase.

Bad:

Good:

⬆ back to top

Open/Closed Principle (OCP)

As stated by Bertrand Meyer, "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification." What does that mean though? This principle basically states that you should allow users to add new functionalities without changing existing code.

Bad:

Good:

⬆ back to top

Liskov Substitution Principle (LSP)

This is a scary term for a very simple concept. It's formally defined as "If S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., objects of type S may substitute objects of type T) without altering any of the desirable properties of that program (correctness, task performed, etc.)." That's an even scarier definition.

The best explanation for this is if you have a parent class and a child class, then the base class and child class can be used interchangeably without getting incorrect results. This might still be confusing, so let's take a look at the classic Square-Rectangle example. Mathematically, a square is a rectangle, but if you model it using the "is-a" relationship via inheritance, you quickly get into trouble.

Bad:

Good:

⬆ back to top

Interface Segregation Principle (ISP)

JavaScript doesn't have interfaces so this principle doesn't apply as strictly as others. However, it's important and relevant even with JavaScript's lack of type system.

ISP states that "Clients should not be forced to depend upon interfaces that they do not use." Interfaces are implicit contracts in JavaScript because of duck typing.

A good example to look at that demonstrates this principle in JavaScript is for classes that require large settings objects. Not requiring clients to setup huge amounts of options is beneficial, because most of the time they won't need all of the settings. Making them optional helps prevent having a "fat interface".

Bad:

Good:

⬆ back to top

Dependency Inversion Principle (DIP)

This principle states two essential things:

1. High-level modules should not depend on low-level modules. Both should depend on abstractions.
2. Abstractions should not depend upon details. Details should depend on abstractions.

This can be hard to understand at first, but if you've worked with AngularJS, you've seen an implementation of this principle in the form of Dependency Injection (DI). While they are not identical concepts, DIP keeps high-level modules from knowing the details of its low-level modules and setting them up. It can accomplish this through DI. A huge benefit of this is that it reduces the coupling between modules. Coupling is a very bad development pattern because it makes your code hard to refactor.

As stated previously, JavaScript doesn't have interfaces so the abstractions that are depended upon are implicit contracts. That is to say, the methods and properties that an object/class exposes to another object/class. In the example below, the implicit contract is that any Request module for an InventoryTracker will have a requestItems method.

Bad:

Good:

⬆ back to top

Testing

Testing is more important than shipping. If you have no tests or an inadequate amount, then every time you ship code you won't be sure that you didn't break anything. Deciding on what constitutes an adequate amount is up to your team, but having 100% coverage (all statements and branches) is how you achieve very high confidence and developer peace of mind. This means that in addition to having a great testing framework, you also need to use a good coverage tool.

There's no excuse to not write tests. There's plenty of good test frameworks, so find one that your team prefers. When you find one that works for your team, then aim to always write tests for every new feature/module you introduce. If your preferred method is Test Driven Development (TDD), that is great, but the main point is to just make sure you are reaching your coverage goals before launching any feature, or refactoring an existing one.

Single concept per test

Bad:

Good:

⬆ back to top

Error Handling

Thrown errors are a good thing! They mean the runtime has successfully identified when something in your program has gone wrong and it's letting you know by stopping function execution on the current stack, killing the process (in Node), and notifying you in the console with a stack trace.

Don't ignore caught errors

Doing nothing with a caught error doesn't give you the ability to ever fix or react to said error. Logging the error to the console (console.log) isn't much better as often times it can get lost in a sea of things printed to the console. If you wrap any bit of code in a try/catch it means you think an error may occur there and therefore you should have a plan, or create a code path, for when it occurs.

Bad:

Good:

⬆ back to top

Formatting

Formatting is subjective. Like many rules herein, there is no hard and fast rule that you must follow. The main point is DO NOT ARGUE over formatting. There are tons of tools to automate this. Use one! It's a waste of time and money for engineers to argue over formatting.

For things that don't fall under the purview of automatic formatting (indentation, tabs vs. spaces, double vs. single quotes, etc.) look here for some guidance.

Use consistent capitalization

JavaScript is untyped, so capitalization tells you a lot about your variables, functions, etc. These rules are subjective, so your team can choose whatever they want. The point is, no matter what you all choose, just be consistent.

Bad:

Good:

⬆ back to top

Function callers and callees should be close

If a function calls another, keep those functions vertically close in the source file. Ideally, keep the caller right above the callee. We tend to read code from top-to-bottom, like a newspaper. Because of this, make your code read that way.

Bad:

Good:

⬆ back to top

Comments

Only comment things that have business logic complexity.

Comments are an apology, not a requirement. Good code mostly documents itself.

Bad:

// Creating a List of customer names 
List<String> customerNames = Arrays.asList('Bob', 'Linda', 'Steve', 'Mary'); 

// Using Stream findFirst() 
Optional<String> firstCustomer = customerNames.stream().findFirst(); 

// if the stream is empty, an empty 
// Optional is returned. 
if (firstCustomer.isPresent()) { 
    System.out.println(firstCustomer.get()); 
} 
else { 
    System.out.println("no value"); 
} 

Good:

List<String> customerNames = Arrays.asList('Bob', 'Linda', 'Steve', 'Mary'); 

Optional<String> firstCustomer = customerNames.stream().findFirst(); 

if (firstCustomer.isPresent()) { 
    System.out.println(firstCustomer.get()); 
} 
else { 
    System.out.println("no value"); 
} 

⬆ back to top

Don't Use a Comment When You Can Use a Function or a Variable

The best comment is no comment

Bad:

//Check to see if order is eligible to ship
if((order.isPaid & order.isLabeled) && CUSTOMER_FLAG) {
  // ...
}

Good:

if(order.isEligibleToShip()) {
  // ...
}

⬆ back to top

Don't leave commented out code in your codebase

Version control exists for a reason. Leave old code in your history.

Bad:

doStuff();
// doOtherStuff();
// doSomeMoreStuff();
// doSoMuchStuff();

Good:

doStuff();

⬆ back to top

Don't have journal comments

Remember, use version control! There's no need for dead code, commented code, and especially journal comments. Use git log to get history!

Bad:

/**
 * 2021-03-06: Renamed clean to cleanCode (DL)
 * 2020-01-03: Changed return value (LB)
 * 2019-05-12: Added clean method (DL)
 */
 cleanCode(String code) {
   return null;
 }

Good:

 cleanCode(String code) {
   return null;
 }

⬆ back to top

Avoid positional markers

They usually just add noise. Let the functions and variable names along with the proper indentation and formatting give the visual structure to your code.

Bad:

////////////////////////////////////////////////////////////////////////////////
// Instantiate Order List
////////////////////////////////////////////////////////////////////////////////
List<Order> orders = new ArrayList();

////////////////////////////////////////////////////////////////////////////////
// Ship Orders that are eligible
////////////////////////////////////////////////////////////////////////////////
 
orders.filter(Order::isEligibleToShip).forEach(x -> ship(x));

Good:

List<Order> orders = new ArrayList();

orders.filter(Order::isEligibleToShip).forEach(x -> ship(x));

⬆ back to top

Translation

Open for translations.

⬆ back to top