Refactoring 015 - Remove NULL

Eliminating The Billion-Dollar Mistake forever

Maximiliano Contieri

Jul 28, 2024

TL;DR: Use the Null Object Pattern to eliminate null checks and simplify your code.

Problems Addressed

Frequent null checks and IFs cluttering your code
Null pointer exceptions
Hard-to-read and maintain code
Lack of extensibility
Unnecessary complexity with excessive null checks.
More detail about NULL creation and regrets

Related Code Smells

Code Smell 12 - Null

Code Smell 260 - Crowdstrike NULL

Code Smell 149 - Optional Chaining

Code Smell 212 - Elvis Operator

Code Smell 192 - Optional Attributes

Code Smell 126 - Fake Null Object

Code Smell 208 - Null Island

Steps

These steps are a special case of Remove IF Refactoring

Create a Null Object class that implements the same interface
Replace null checks with the polymorphic Null Object

Sample Code

Before

public class SoccerMatch {
    private Team homeTeam;
    private Team awayTeam;
    private TimeRecord regularTime;
    private TimeRecord extraTime;

    public SoccerMatch(Team homeTeam, 
                       Team awayTeam,
                       TimeRecord regularTime, 
                       TimeRecord extraTime) {
        this.homeTeam = homeTeam;
        this.awayTeam = awayTeam;
        this.regularTime = regularTime;
        this.extraTime = extraTime;
    }

    public int totalGoals() {
        int goals = regularTime.goals();
        // You might forget this if check 
        // resulting in a null error
        if (extraTime != null) {
            goals += extraTime.goals();
        }
        return goals;
    }
}

class TimeRecord {
    private int goals;

    public TimeRecord(int goals) {
        this.goals = goals;
    }

    public int goals() {
        return goals;
    }
}

After

// 1. Create a Null Object class that implements the same interface

interface TimeRecord {
    // The common protocol between the real object 
    // and the Null Object
    int goals();
}

class PlayedTimeRecord implements TimeRecord {
    // This class is suitable both to be
    // a Regular Time or an Extra Time
    private int goals;

    public PlayedTimeRecord (int goals) {
        this.goals = goals;
    }

    public int goals() {
        return goals;
    }
}

class NoExtraTime implements TimeRecord {
    public int goals() {
        // They are polymorphic
        // They don't need IF checks
        return 0;
    }
}

public class SoccerMatch {
    private Team homeTeam;
    private Team awayTeam;
    private PlayedTimeRecord regularTime;
    private TimeRecord extraTime;

    public SoccerMatch(Team homeTeam, 
                       Team awayTeam,
                       PlayedTimeRecord regularTime,
                       TimeRecord extraTime) {
        this.homeTeam = homeTeam;
        this.awayTeam = awayTeam;        
        // There's a business rule telling
        // regular time is not optional
        // Therefore is an instance of PlayedTimeRecord
        this.regularTime = regularTime;
        this.extraTime = extraTime;
    }

    public int totalGoals() {
        // 2. Replace null checks with the polymorphic Null Object
        // No Ifs 
        // No null checks
        return regularTime.goals() + extraTime.goals();
    }
}

As a side note when you prompt Gemini-AI with the After version it says:

While the code is well-structured, there's a minor optimization opportunity. The NoExtraTime class can be eliminated without compromising the code's functionality.

.. and suggests using the first version with the billion-dollar mistake problem!*

Type

[X] Semi-Automatic

The refactoring has a semantic part that needs to find an existing NullObejct in the domain.

Safety

This refactoring is generally safe.

You must ensure that the Null Object's behavior matches the expected behavior for real cases in your code.

The null object needs to be partially polymorphic with real ones.

In strongly typed languages, they must adhere to the same interface or belong to the same hierarchy.

Why is the code better?

You eliminate null checks with accidental IF conditions, making the code cleaner and easier to understand.

The Null Object Pattern ensures you always work with objects, avoiding null pointer exceptions - The Billion Dollar Mistake.

Limitations

Related Refactorings

Refactoring 014 - Remove IF

Clean Code Cookbook

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