Learn To Recognize Code Smell

“Smells,” you say, “and that is supposed to be better than vague aesthetics?”
Well, yes. We have looked at lots of code, written for projects that span the gamut
from wildly successful to nearly dead. In doing so, we have learned to look for
certain structures in the code that suggest—sometimes, scream for—the possibility
of refactoring.

• Kent Beck and Martin Fowler, Refactoring

Code smell is a term used to describe the tell-tail signs of bad software and therefore an indication that refactoring is overdue. Similar to design patterns that we learn to recognize and apply in our software, naming and identifying different code smells can help us triage the existing code base and proceed to apply fixes in the form of refactoring.

Below is a catalog of code smells identified by Martin Fowler

• Alternative Classes with Different Interfaces
• Comments
• Data Class
• Data Clumps
• Divergent Change
• Duplicated Code
• Feature Envy
• Global Data
• Insider Trading
• Large Class
• Lazy Element
• Long Function
• Long Parameter List
• Loops
• Message Chains
• Middle Man
• Mutable Data
• Mysterious Name
• Primitive Obsession
• Refused Bequest
• Repeated Switches
• Shotgun Surgery
• Speculative Generality
• Temporary Field

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A Practical Example: Comments

Comments are helpful when used to make clarifications and explain the purpose of the code. However, comments are also tricky to write because when done poorly, they can be a source of noise and potentially exposed the weakness in software. On top of the techniques of writing good comments, let's take a look at how comments can become code smells that prompt us to refactor.

Comments that describe what a function does

// add expenditure to record
void processInput(int data) {
    //...
}

Course of action: Rename function and remove comment

void recordExpenditure(int amount) {
    //...
}

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Comments that describe what a block of code does

static Command parseInput(String input) throws InvalidInputException {
    String task = Parser.tokenize(input)[0];
    // check if the user input contains recognized keywords
    boolean hasKeyword = Arrays
            .stream(TaskType.values())
            .map(TaskType::getRep)
            .anyMatch(x -> x.equals(task));
    if (!hasKeyword) {
        return false;
    }
    return Parser.interpret(input);
}

Course of action: Extract function

private static boolean hasTaskKeyword(String input) {
    String task = Parser.tokenize(input)[0];
    return Arrays
            .stream(TaskType.values())
            .map(TaskType::getRep)
            .anyMatch(x -> x.equals(task));
}

static Command parseInput(String input) throws InvalidInputException {
    if (!hasTaskKeyword(input)) {
        throw new InvalidInputException(input);
    }
    return Parser.interpret(input);
}

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Comments that describe assumptions

public ShowCommand(Index index) {
    // note that index must not be null!
    // and index must be larger or equal to zero!
    this.index = index;
}

Course of action: Be defensive & use assertions

public ShowCommand(Index index) {
    requireNonNull(index);
    assert index.getZeroBased() >= 0;
    this.index = index;
}

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Further reading

• An short essay on code smell by Martin Fowler
• A catalog of code smells
• A comprehensive discussion on code smell is also part of the book: Refactoring: Improving the Design of Existing Code