Code Smell 126 — Fake Null Object

Null Objects are great alternatives to The Billion Dollar Mistake. Sometimes we don’t need them

Maximiliano Contieri

Apr 02, 2022

TL;DR: Don’t abuse patterns. Even NullObject.

Problems 😔

Empty Classes
Namespace Polluting
Duplicated Behavior

Solutions 😃

Create Null Objects instantiating real-object classes.

Context 💬

Null Object pattern is a great alternative to Nulls and IFs (Both are code smells).

The structure of the pattern tells us to create a hierarchy.

This is not really necessary, we need real objects to be polymorphic to null objects.

Inheritance is not a proper way to achieve polymorphism.

A simple solution is to create a real object behaving like a null one.

For example: '0' is the numbers' null object.

'' (or "") is String's null object

An empty collection is the collection's null object.

Sample Code 📖

Wrong 🚫

abstract class Address {
    public abstract String getCity();
    public abstract String getState();
    public abstract String getZipCode();
}

// Using inheritance for null objects is a mistake
// You should use interfaces (when available)
public class NullAddress extends Address {

    public NullAddress() { }

    public String getCity() {
        return Constants.EMPTY_STRING;
    }

    public String getState() {
        return Constants.EMPTY_STRING;
    }

    public String getZipCode() {
        return Constants.EMPTY_STRING;
    }

}

public class RealAddress extends Address {

    private String zipCode;
    private String city;
    private String state;

    public RealAddress(String city, String state, String zipCode) {
        this.city = city;
        this.state = state;
        this.zipCode = zipCode;
    }

    public String getZipCode() {
        return zipCode;
    }

    public String getCity() {
        return city;
    }

    public String getState() {
        return state;
    }
}

Right 👉

// There are just "addresses"
public class Address {

    private String zipCode;
    private String city;
    private String state;

    public Address(String city, String state, String zipCode) {
        // Looks anemic :(
        this.city = city;
        this.state = state;
        this.zipCode = zipCode;
    }

    public String zipCode() {
        return zipCode;
    }

    public String city() {
        return city;
    }

    public String state() {
        return state;
    }

}

Address nullAddress = new Address(
    Constants.EMPTY_STRING, 
    Constants.EMPTY_STRING, 
    Constants.EMPTY_STRING);
// You have your null object
// You should NOT assign it to a singleton, static or global
// It behaves like a null object. That's enough
// No premature optimizations