Posted on Apr 2, 2022 • Edited on Apr 4, 2022 • Originally published at maximilianocontieri.com

Code Smell 126 - Fake Null Object

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

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 numbers' null object.

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

An empty collection is 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 //We 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); //we have our null object //we should NOT assign it to a singleton, static or global //It behaves like a null object. That's enough //No premature optimizations