Refactorings (21 Part Series)

1 Refactoring 001 – Remove Setters
2 Refactoring 002 – Extract Method
… 17 more parts…
3 Refactoring 003 – Extract Constant
4 Refactoring 004 – Remove Unhandled Exceptions
5 Refactoring 005 – Replace Comment with Function Name
6 Refactoring 006 – Rename Result Variables
7 Refactoring 007 – Extract Class
8 Refactoring 008 – Convert Variables to Constant
9 Refactoring 009 – Protect Public Attributes
10 Refactoring 010 – Extract Method Object
11 Refactoring 011 – Replace Comments with Tests
12 Refactoring 012 – Reify Associative Arrays
13 Refactoring 013 – Remove Repeated Code
14 Refactoring 014 – Remove IF
15 Refactoring 015 – Remove NULL
16 Refactoring 016 – Build With The Essence
17 Refactoring 017 – Convert Attributes to Sets
18 Refactoring 018 – Replace Singleton
19 Refactoring 019 – Reify Email Addresses
20 Refactoring 020 – Transform Static Functions
21 Refactoring 021 – Remove Dead Code

Eliminating The Billion-Dollar Mistake forever

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

#codenewbie #tutorial #oop

#security #cleancode #hacking #null

#javascript #webdev #beginners #programming

#webdev #javascript #beginners #programming

#webdev #javascript #beginners #programming

#webdev #beginners #programming #kotlin

Steps

These steps are a special case of Remove IF Refactoring

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

2. Replace null checks with the polymorphic Null Object

Sample Code

Before

<span>public</span> <span>class</span> <span>SoccerMatch</span> <span>{</span>
    <span>private</span> <span>Team</span> <span>homeTeam</span><span>;</span>
    <span>private</span> <span>Team</span> <span>awayTeam</span><span>;</span>
    <span>private</span> <span>TimeRecord</span> <span>regularTime</span><span>;</span>
    <span>private</span> <span>TimeRecord</span> <span>extraTime</span><span>;</span>
    <span>public</span> <span>SoccerMatch</span><span>(</span><span>Team</span> <span>homeTeam</span><span>,</span> 
                       <span>Team</span> <span>awayTeam</span><span>,</span>
                       <span>TimeRecord</span> <span>regularTime</span><span>,</span> 
                       <span>TimeRecord</span> <span>extraTime</span><span>)</span> <span>{</span>
        <span>this</span><span>.</span><span>homeTeam</span> <span>=</span> <span>homeTeam</span><span>;</span>
        <span>this</span><span>.</span><span>awayTeam</span> <span>=</span> <span>awayTeam</span><span>;</span>
        <span>this</span><span>.</span><span>regularTime</span> <span>=</span> <span>regularTime</span><span>;</span>
        <span>this</span><span>.</span><span>extraTime</span> <span>=</span> <span>extraTime</span><span>;</span>
    <span>}</span>
    <span>public</span> <span>int</span> <span>totalGoals</span><span>()</span> <span>{</span>
        <span>int</span> <span>goals</span> <span>=</span> <span>regularTime</span><span>.</span><span>goals</span><span>();</span>
        <span>// You might forget this if check </span>
        <span>// resulting in a null error</span>
        <span>if</span> <span>(</span><span>extraTime</span> <span>!=</span> <span>null</span><span>)</span> <span>{</span>
            <span>goals</span> <span>+=</span> <span>extraTime</span><span>.</span><span>goals</span><span>();</span>
        <span>}</span>
        <span>return</span> <span>goals</span><span>;</span>
    <span>}</span>
<span>}</span>
<span>class</span> <span>TimeRecord</span> <span>{</span>
    <span>private</span> <span>int</span> <span>goals</span><span>;</span>
    <span>public</span> <span>TimeRecord</span><span>(</span><span>int</span> <span>goals</span><span>)</span> <span>{</span>
        <span>this</span><span>.</span><span>goals</span> <span>=</span> <span>goals</span><span>;</span>
    <span>}</span>
    <span>public</span> <span>int</span> <span>goals</span><span>()</span> <span>{</span>
        <span>return</span> <span>goals</span><span>;</span>
    <span>}</span>
<span>}</span>
<span>public</span> <span>class</span> <span>SoccerMatch</span> <span>{</span>
    <span>private</span> <span>Team</span> <span>homeTeam</span><span>;</span>
    <span>private</span> <span>Team</span> <span>awayTeam</span><span>;</span>
    <span>private</span> <span>TimeRecord</span> <span>regularTime</span><span>;</span>
    <span>private</span> <span>TimeRecord</span> <span>extraTime</span><span>;</span>

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

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

<span>class</span> <span>TimeRecord</span> <span>{</span>
    <span>private</span> <span>int</span> <span>goals</span><span>;</span>

    <span>public</span> <span>TimeRecord</span><span>(</span><span>int</span> <span>goals</span><span>)</span> <span>{</span>
        <span>this</span><span>.</span><span>goals</span> <span>=</span> <span>goals</span><span>;</span>
    <span>}</span>

    <span>public</span> <span>int</span> <span>goals</span><span>()</span> <span>{</span>
        <span>return</span> <span>goals</span><span>;</span>
    <span>}</span>
<span>}</span>
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;
    }
}

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After

<span>// 1. Create a Null Object class that implements the same interface</span>
<span>interface</span> <span>TimeRecord</span> <span>{</span>
    <span>// The common protocol between the real object </span>
    <span>// and the Null Object</span>
    <span>int</span> <span>goals</span><span>();</span>
<span>}</span>
<span>class</span> <span>PlayedTimeRecord</span> <span>implements</span> <span>TimeRecord</span> <span>{</span>
    <span>// This class is suitable both to be</span>
    <span>// a Regular Time or an Extra Time</span>
    <span>private</span> <span>int</span> <span>goals</span><span>;</span>
    <span>public</span> <span>PlayedTimeRecord</span> <span>(</span><span>int</span> <span>goals</span><span>)</span> <span>{</span>
        <span>this</span><span>.</span><span>goals</span> <span>=</span> <span>goals</span><span>;</span>
    <span>}</span>
    <span>public</span> <span>int</span> <span>goals</span><span>()</span> <span>{</span>
        <span>return</span> <span>goals</span><span>;</span>
    <span>}</span>
<span>}</span>
<span>class</span> <span>NoExtraTime</span> <span>implements</span> <span>TimeRecord</span> <span>{</span>
    <span>public</span> <span>int</span> <span>goals</span><span>()</span> <span>{</span>
        <span>// They are polymorphic</span>
        <span>// They don't need IF checks</span>
        <span>return</span> <span>0</span><span>;</span>
    <span>}</span>
<span>}</span>
<span>public</span> <span>class</span> <span>SoccerMatch</span> <span>{</span>
    <span>private</span> <span>Team</span> <span>homeTeam</span><span>;</span>
    <span>private</span> <span>Team</span> <span>awayTeam</span><span>;</span>
    <span>private</span> <span>PlayedTimeRecord</span> <span>regularTime</span><span>;</span>
    <span>private</span> <span>TimeRecord</span> <span>extraTime</span><span>;</span>
    <span>public</span> <span>SoccerMatch</span><span>(</span><span>Team</span> <span>homeTeam</span><span>,</span> 
                       <span>Team</span> <span>awayTeam</span><span>,</span>
                       <span>PlayedTimeRecord</span> <span>regularTime</span><span>,</span>
                       <span>TimeRecord</span> <span>extraTime</span><span>)</span> <span>{</span>
        <span>this</span><span>.</span><span>homeTeam</span> <span>=</span> <span>homeTeam</span><span>;</span>
        <span>this</span><span>.</span><span>awayTeam</span> <span>=</span> <span>awayTeam</span><span>;</span>        
        <span>// There's a business rule telling</span>
        <span>// regular time is not optional</span>
        <span>// Therefore is an instance of PlayedTimeRecord</span>
        <span>this</span><span>.</span><span>regularTime</span> <span>=</span> <span>regularTime</span><span>;</span>
        <span>this</span><span>.</span><span>extraTime</span> <span>=</span> <span>extraTime</span><span>;</span>
    <span>}</span>
    <span>public</span> <span>int</span> <span>totalGoals</span><span>()</span> <span>{</span>
        <span>// 2. Replace null checks with the polymorphic Null Object</span>
        <span>// No Ifs </span>
        <span>// No null checks</span>
        <span>return</span> <span>regularTime</span><span>.</span><span>goals</span><span>()</span> <span>+</span> <span>extraTime</span><span>.</span><span>goals</span><span>();</span>
    <span>}</span>
<span>}</span>
<span>// 1. Create a Null Object class that implements the same interface</span>

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

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

    <span>public</span> <span>PlayedTimeRecord</span> <span>(</span><span>int</span> <span>goals</span><span>)</span> <span>{</span>
        <span>this</span><span>.</span><span>goals</span> <span>=</span> <span>goals</span><span>;</span>
    <span>}</span>

    <span>public</span> <span>int</span> <span>goals</span><span>()</span> <span>{</span>
        <span>return</span> <span>goals</span><span>;</span>
    <span>}</span>
<span>}</span>

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

<span>public</span> <span>class</span> <span>SoccerMatch</span> <span>{</span>
    <span>private</span> <span>Team</span> <span>homeTeam</span><span>;</span>
    <span>private</span> <span>Team</span> <span>awayTeam</span><span>;</span>
    <span>private</span> <span>PlayedTimeRecord</span> <span>regularTime</span><span>;</span>
    <span>private</span> <span>TimeRecord</span> <span>extraTime</span><span>;</span>

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

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

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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

Tags

• Null

Related Refactorings

#webdev #beginners #programming #java

See also

#codenewbie #tutorial #programming #webdev

Wikipedia

Refactoring Guru

This article is part of the Refactoring Series.

#webdev #beginners #programming #tutorial

Refactorings (21 Part Series)

1 Refactoring 001 – Remove Setters
2 Refactoring 002 – Extract Method
… 17 more parts…
3 Refactoring 003 – Extract Constant
4 Refactoring 004 – Remove Unhandled Exceptions
5 Refactoring 005 – Replace Comment with Function Name
6 Refactoring 006 – Rename Result Variables
7 Refactoring 007 – Extract Class
8 Refactoring 008 – Convert Variables to Constant
9 Refactoring 009 – Protect Public Attributes
10 Refactoring 010 – Extract Method Object
11 Refactoring 011 – Replace Comments with Tests
12 Refactoring 012 – Reify Associative Arrays
13 Refactoring 013 – Remove Repeated Code
14 Refactoring 014 – Remove IF
15 Refactoring 015 – Remove NULL
16 Refactoring 016 – Build With The Essence
17 Refactoring 017 – Convert Attributes to Sets
18 Refactoring 018 – Replace Singleton
19 Refactoring 019 – Reify Email Addresses
20 Refactoring 020 – Transform Static Functions
21 Refactoring 021 – Remove Dead Code

原文链接：Refactoring 015 – Remove NULL