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<title>Null object pattern</title>

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Null object pattern

</h1>

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</style><div role="note" class="hatnote navigation-not-searchable">"Null object" redirects here. For the concept in category theory, see <a href="Initial_object" class="mw-redirect" title="Initial object">Initial object</a>.</div>

In <a href="Object-oriented_programming" title="Object-oriented programming">object-oriented</a> <a href="Computer_programming" title="Computer programming">computer programming</a>, a null object is an <a href="Object_(computer_science)" title="Object (computer science)">object</a> with no referenced value or with defined neutral (null) behavior. The null object <a href="Design_pattern" title="Design pattern">design pattern</a>, which describes the uses of such objects and their behavior (or lack thereof), was first published as "Void Value"<a href="#cite_note-1">[1]</a>

and later in the <a href="Pattern_Languages_of_Programs#Publications" title="Pattern Languages of Programs">Pattern Languages of Program Design book series</a> as "Null Object".<a href="#cite_note-2">[2]</a>

<div class="mw-heading mw-heading2"><h2 id="Motivation">Motivation</h2></div>

In most object-oriented languages, such as <a href="Java_(programming_language)" title="Java (programming language)">Java</a> or <a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a>, <a href="Reference_(computer_science)" title="Reference (computer science)">references</a> may be <a href="Null_pointer" title="Null pointer">null</a>. These references need to be checked to ensure they are not null before invoking any <a href="Method_(computer_science)" class="mw-redirect" title="Method (computer science)">methods</a>, because methods typically cannot be invoked on null references.

The <a href="Objective-C" title="Objective-C">Objective-C language</a> takes another approach to this problem and does nothing when sending a message to <code>nil</code>; if a return value is expected, <code>nil</code> (for objects), 0 (for numeric values), <code>NO</code> (for <code>BOOL</code> values), or a struct (for struct types) with all its members initialised to <code>null</code>/0/<code>NO</code>/zero-initialised struct is returned.<a href="#cite_note-3">[3]</a>

<div class="mw-heading mw-heading2"><h2 id="Description">Description</h2></div>

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Instead of using a <a href="Null_reference" class="mw-redirect" title="Null reference">null reference</a> to convey the absence of an object (for instance, a non-existent customer), one uses an object which implements the expected <a href="Interface_(computing)#In_object-oriented_languages" title="Interface (computing)"> interface</a>, but whose method body is empty. A key purpose of using a null object is to avoid conditionals of different kinds, resulting in code that is more focused, and quicker to read and follow – i.e. improved readability.

One advantage of this approach over a working default implementation is that a null object is very predictable and has no side effects: it does nothing.

For example, a function may retrieve a list of files in a folder and perform some action on each. In the case of an empty folder, one response may be to throw an exception or return a null reference rather than a list. Thus, the code expecting a list must verify that it in fact has one before continuing, which can complicate the design.

By returning a null object (i.e., an empty list) instead, there is no need to verify that the return value is in fact a list. The calling function may simply iterate the list as normal, effectively doing nothing. It is, however, still possible to check whether the return value is a null object (an empty list) and react differently if desired.

The null object pattern can also be used to act as a stub for testing, if a certain feature such as a database is not available for testing.

<div class="mw-heading mw-heading2"><h2 id="Example">Example</h2></div>

Given a <a href="Binary_tree" title="Binary tree">binary tree</a>, with this node structure:

<pre>class node {

node left

node right

}

</pre>

One may implement a tree size procedure recursively:

<pre>function tree_size(node) {

return 1 + tree_size(node.left) + tree_size(node.right)

}

</pre>

Since the child nodes may not exist, one must modify the procedure by adding non-existence or null checks:

<pre>function tree_size(node) {

set sum = 1

if node.left exists {

sum = sum + tree_size(node.left)

}

if node.right exists {

sum = sum + tree_size(node.right)

}

return sum

}

</pre>

This, however, makes the procedure more complicated by mixing boundary checks with normal logic, and it becomes harder to read. Using the null object pattern, one can create a special version of the procedure but only for null nodes:

<pre>function tree_size(node) {

return 1 + tree_size(node.left) + tree_size(node.right)

}

</pre>

<pre>function tree_size(null_node) {

return 0

}

</pre>

This separates normal logic from special case handling and makes the code easier to understand.

<div class="mw-heading mw-heading2"><h2 id="Relation_to_other_patterns">Relation to other patterns</h2></div>

It can be regarded as a special case of the <a href="State_pattern" title="State pattern">State pattern</a> and the <a href="Strategy_pattern" title="Strategy pattern">Strategy pattern</a>.

It is not a pattern from <a href="Design_Patterns" title="Design Patterns">Design Patterns</a>, but is mentioned in <a href="Martin_Fowler_(software_engineer)" title="Martin Fowler (software engineer)">Martin Fowler's</a> Refactoring<a href="#cite_note-4">[4]</a> and Joshua Kerievsky's Refactoring To Patterns<a href="#cite_note-5">[5]</a> as the Insert Null Object <a href="Refactoring" class="mw-redirect" title="Refactoring">refactoring</a>.

Chapter 17 of <a href="Robert_Cecil_Martin" class="mw-redirect" title="Robert Cecil Martin">Robert Cecil Martin's</a> Agile Software Development: Principles, Patterns and Practices<a href="#cite_note-6">[6]</a> is dedicated to the pattern.

<div class="mw-heading mw-heading2"><h2 id="Alternatives">Alternatives</h2></div>

From C# 6.0 it is possible to use the "?." operator (aka <a href="Safe_navigation_operator#C.23" title="Safe navigation operator">null-conditional operator</a>), which will simply evaluate to null if its left operand is null.

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>// compile as Console Application, requires C# 6.0 or higher

using System;

namespace ConsoleApplication2

{

class Program

{

static void Main(string[] args)

{

string str = "test";

Console.WriteLine(str?.Length);

Console.ReadKey();

}

}

// The output will be:

// 4

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Extension_methods_and_Null_coalescing">Extension methods and Null coalescing</h3></div>

In some <a href="Microsoft_.NET" class="mw-redirect" title="Microsoft .NET">Microsoft .NET</a> languages, <a href="Extension_method" title="Extension method">Extension methods</a> can be used to perform what is called 'null coalescing'. This is because extension methods can be called on null values as if it concerns an 'instance method invocation' while in fact extension methods are static. Extension methods can be made to check for null values, thereby freeing code that uses them from ever having to do so. Note that the example below uses the <a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> <a href="Null_coalescing_operator" title="Null coalescing operator">Null coalescing operator</a> to guarantee error free invocation, where it could also have used a more mundane if...then...else. The following example only works when you do not care the existence of null, or you treat null and empty string the same. The assumption may not hold in other applications.

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>// compile as Console Application, requires C# 3.0 or higher

using System;

using System.Linq;

namespace MyExtensionWithExample {

public static class StringExtensions {

public static int SafeGetLength(this string valueOrNull) {

return (valueOrNull ?? string.Empty).Length;

}

public static class Program {

// define some strings

static readonly string[] strings = new [] { "Mr X.", "Katrien Duck", null, "Q" };

// write the total length of all the strings in the array

public static void Main(string[] args) {

var query = from text in strings select text.SafeGetLength(); // no need to do any checks here

Console.WriteLine(query.Sum());

}

}

// The output will be:

// 18

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="In_various_languages">In various languages</h2></div>

A language with statically typed references to objects illustrates how the null object becomes a more complicated pattern:

<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>import std;

class Animal {

public:

virtual ~Animal() = default;

virtual void MakeSound() const = 0;

};

class Dog: public Animal {

public:

virtual void MakeSound() const override { std::println("woof!"); }

};

class NullAnimal: public Animal {

public:

};

</pre></div>

Here, the idea is that there are situations where a pointer or reference to an <code>Animal</code> object is required, but there is no appropriate object available. A null reference is impossible in standard-conforming C++. A null <code>Animal*</code> pointer is possible, and could be useful as a place-holder, but may not be used for direct dispatch: <code>a->MakeSound()</code> is undefined behavior if <code>a</code> is a null pointer.

The null object pattern solves this problem by providing a special <code>NullAnimal</code> class which can be instantiated bound to an <code>Animal</code> pointer or reference.

The special null class must be created for each class hierarchy that is to have a null object, since a <code>NullAnimal</code> is of no use when what is needed is a null object with regard to some <code>Widget</code> base class that is not related to the <code>Animal</code> hierarchy.

Note that NOT having a null class at all is an important feature, in contrast to languages where "anything is a reference" (e.g., Java and C#). In C++, the design of a function or method may explicitly state whether null is allowed or not.

<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>// Function which requires an |Animal| instance, and will not accept null.

void DoSomething(const Animal& animal) {

// |animal| may never be null here.

}

// Function which may accept an |Animal| instance or null.

void DoSomething(const Animal* animal) {

// |animal| may be null.

}

</pre></div>

C# is a language in which the null object pattern can be properly implemented. This example shows animal objects that display sounds and a NullAnimal instance used in place of the C# null keyword. The null object provides consistent behaviour and prevents a runtime null reference exception that would occur if the C# null keyword were used instead.

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>/* Null object pattern implementation:

*/

using System;

// Animal interface is the key to compatibility for Animal implementations below.

interface IAnimal

{

void MakeSound();

}

// Animal is the base case.

abstract class Animal : IAnimal

{

// A shared instance that can be used for comparisons

public static readonly IAnimal Null = new NullAnimal();

// The Null Case: this NullAnimal class should be used in place of C# null keyword.

private class NullAnimal : Animal

{

public override void MakeSound()

{

// Purposefully provides no behaviour.

}

public abstract void MakeSound();

}

// Dog is a real animal.

class Dog : Animal

{

{

Console.WriteLine("Woof!");

}

}

/* =========================

* Simplistic usage example in a Main entry point.

*/

static class Program

{

static void Main()

{

IAnimal dog = new Dog();

dog.MakeSound(); // outputs "Woof!"

/* Instead of using C# null, use the Animal.Null instance.

* This example is simplistic but conveys the idea that if the Animal.Null instance is used then the program

* will never experience a .NET System.NullReferenceException at runtime, unlike if C# null were used.

*/

IAnimal unknown = Animal.Null; //<< replaces: IAnimal unknown = null;

unknown.MakeSound(); // outputs nothing, but does not throw a runtime exception

}

}

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Smalltalk">Smalltalk</h3></div>

Following the Smalltalk principle, everything is an object, the absence of an object is itself modeled by an object, called <code>nil</code>. In the GNU Smalltalk for example, the class of <code>nil</code> is <code>UndefinedObject</code>, a direct descendant of <code>Object</code>.

Any operation that fails to return a sensible object for its purpose may return <code>nil</code> instead, thus avoiding the special case of returning "no object" unsupported by Smalltalk designers. This method has the advantage of simplicity (no need for a special case) over the classical "null" or "no object" or "null reference" approach. Especially useful messages to be used with <code>nil</code> are <code>isNil</code>, <code>ifNil:</code> or <code>ifNotNil:</code>, which make it practical and safe to deal with possible references to <code>nil</code> in Smalltalk programs.

<div class="mw-heading mw-heading3"><h3 id="Common_Lisp">Common Lisp</h3></div>

In Lisp, functions can gracefully accept the special object <code>nil</code>, which reduces the amount of special case testing in application code. For instance, although <code>nil</code> is an atom and does not have any fields, the functions <code>car</code> and <code>cdr</code> accept <code>nil</code> and just return it, which is very useful and results in shorter code.

Since <code>nil</code> is the empty list in Lisp, the situation described in the introduction above doesn't exist. Code which returns <code>nil</code> is returning what is in fact the empty list (and not anything resembling a null reference to a list type), so the caller does not need to test the value to see whether or not it has a list.

The null object pattern is also supported in multiple value processing. If the program attempts to extract a value from an expression which returns no values, the behavior is that the null object <code>nil</code> is substituted.

Thus <code>(list (values))</code> returns <code>(nil)</code> (a one-element list containing nil). The <code>(values)</code> expression returns no values at all, but since the function call to <code>list</code> needs to reduce its argument expression to a value, the null object is automatically substituted.

In Common Lisp, the object <code>nil</code> is the one and only instance of the special class <code>null</code>. What this means is that a method can be specialized to the <code>null</code> class, thereby implementing the null design pattern. Which is to say, it is essentially built into the object system:

<div class="mw-highlight mw-highlight-lang-lisp mw-content-ltr" dir="ltr"><pre>;; empty dog class

(defclass dog () ())

;; a dog object makes a sound by barking: woof! is printed on standard output

;; when (make-sound x) is called, if x is an instance of the dog class.

(defmethod make-sound ((obj dog))

(format t "woof!~%"))

;; allow (make-sound nil) to work via specialization to null class.

;; innocuous empty body: nil makes no sound.

</pre></div>

The class <code>null</code> is a subclass of the <code>symbol</code> class, because <code>nil</code> is a symbol.

Since <code>nil</code> also represents the empty list, <code>null</code> is a subclass of the <code>list</code> class, too. Methods parameters specialized to <code>symbol</code> or <code>list</code> will thus take a <code>nil</code> argument. Of course, a <code>null</code> specialization can still be defined which is a more specific match for <code>nil</code>.

<div class="mw-heading mw-heading3"><h3 id="Scheme">Scheme</h3></div>

Unlike Common Lisp, and many dialects of Lisp, the Scheme dialect does not have a nil value which works this way; the functions <code>car</code> and <code>cdr</code> may not be applied to an empty list; Scheme application code therefore has to use the <code>empty?</code> or <code>pair?</code> predicate functions to sidestep this situation, even in situations where very similar Lisp would not need to distinguish the empty and non-empty cases thanks to the behavior of <code>nil</code>.

In <a href="Duck_typing" title="Duck typing">duck-typed</a> languages like <a href="Ruby_(programming_language)" title="Ruby (programming language)">Ruby</a>, language inheritance is not necessary to provide expected behavior.

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>class Dog

def sound

"bark"

end

end

class NilAnimal

def sound(*); end

end

def get_animal(animal=NilAnimal.new)

animal

end

get_animal(Dog.new).sound

=> "bark"

get_animal.sound

=> nil

</pre></div>

Attempts to directly <a href="Monkey-patch" class="mw-redirect" title="Monkey-patch">monkey-patch</a> NilClass instead of providing explicit implementations give more unexpected side effects than benefits.

<div class="mw-heading mw-heading3"><h3 id="JavaScript">JavaScript</h3></div>

In <a href="Duck_typing" title="Duck typing">duck-typed</a> languages like <a href="JavaScript" title="JavaScript">JavaScript</a>, language inheritance is not necessary to provide expected behavior.

<div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>class Dog {

sound() {

return 'bark';

}

}

class NullAnimal {

sound() {

return null;

}

}

function getAnimal(type) {

return type === 'dog' ? new Dog() : new NullAnimal();

}

['dog', null].map((animal) => getAnimal(animal).sound());

// Returns ["bark", null]

</pre></div>

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>public interface Animal {

void makeSound();

}

public class Dog implements Animal {

public void makeSound() {

System.out.println("woof!");

}

}

public class NullAnimal implements Animal {

// silence...

}

}

</pre></div>

This code illustrates a variation of the C++ example, above, using the Java language. As with C++, a null class can be instantiated in situations where a reference to an <code>Animal</code> object is required, but there is no appropriate object available. A null <code>Animal</code> object is possible (<code>Animal myAnimal = null;</code>) and could be useful as a place-holder, but may not be used for calling a method. In this example, <code>myAnimal.makeSound();</code> will throw a NullPointerException. Therefore, additional code may be necessary to test for null objects.

The null object pattern solves this problem by providing a special <code>NullAnimal</code> class which can be instantiated as an object of type <code>Animal</code>. As with C++ and related languages, that special null class must be created for each class hierarchy that needs a null object, since a <code>NullAnimal</code> is of no use when what is needed is a null object that does not implement the <code>Animal</code> interface.

<div class="mw-highlight mw-highlight-lang-php mw-content-ltr" dir="ltr"><pre>interface Animal

{

public function makeSound();

}

class Dog implements Animal

{

public function makeSound()

{

echo "Woof...\n";

}

class Cat implements Animal

{

public function makeSound()

{

echo "Meowww...\n";

}

class NullAnimal implements Animal

{

public function makeSound()

{

// silence...

}

$animalType = 'elephant';

function makeAnimalFromAnimalType(string $animalType): Animal

{

switch ($animalType) {

case 'dog':

return new Dog();

case 'cat':

return new Cat();

default:

return new NullAnimal();

}

makeAnimalFromAnimalType($animalType)->makeSound(); // ..the null animal makes no sound

function animalMakeSound(Animal $animal): void

{

$animal->makeSound();

}

foreach ([

makeAnimalFromAnimalType('dog'),

makeAnimalFromAnimalType('NullAnimal'),

makeAnimalFromAnimalType('cat'),

] as $animal) {

// That's also reduce null handling code

animalMakeSound($animal);

}

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Visual_Basic_.NET">Visual Basic .NET</h3></div>

The following null object pattern implementation demonstrates the concrete class providing its corresponding null object in a static field <code>Empty</code>. This approach is frequently used in the .NET Framework (<code>String.Empty</code>, <code>EventArgs.Empty</code>, <code>Guid.Empty</code>, etc.).

<div class="mw-highlight mw-highlight-lang-vbnet mw-content-ltr" dir="ltr"><pre>Public Class Animal

Public Shared ReadOnly Empty As Animal = New AnimalEmpty()

Public Overridable Sub MakeSound()

Console.WriteLine("Woof!")

End Sub

End Class

Friend NotInheritable Class AnimalEmpty

Inherits Animal

Public Overrides Sub MakeSound()

'

End Sub

End Class

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Criticism">Criticism</h2></div>

This pattern should be used carefully, as it can make errors/bugs appear as normal program execution.<a href="#cite_note-7">[7]</a>

Care should be taken not to implement this pattern just to avoid null checks and make code more readable, since the harder-to-read code may just move to another place and be less standard—such as when different logic must execute in case the object provided is indeed the null object. The common pattern in most languages with <a href="Reference_type" class="mw-redirect" title="Reference type">reference types</a> is to compare a reference to a single value referred to as null or nil. Also, there is an additional need for testing that no code anywhere ever assigns null instead of the null object, because in most cases and languages with static typing, this is not a compiler error if the null object is of a reference type, although it would certainly lead to errors at run time in parts of the code where the pattern was used to avoid null checks. On top of that, in most languages and assuming there can be many null objects (i.e., the null object is a reference type but doesn't implement the <a href="Singleton_pattern" title="Singleton pattern">singleton pattern</a> in one or another way), checking for the null object instead of for the null or nil value introduces overhead, as does the singleton pattern likely itself upon obtaining the singleton reference.

<ul><li><a href="Nullable_type" title="Nullable type">Nullable type</a></li>

<li><a href="Option_type" title="Option type">Option type</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFKühne1996" class="citation conference cs1">Kühne, Thomas (1996). "Void Value". Proceedings of the First International Conference on Object-Oriented Technology, White Object-Oriented Nights 1996 (WOON'96), St. Petersburg, Russia.</cite>

</li>

<li id="cite_note-2"><a href="#cite_ref-2">^</a> <cite id="CITEREFWoolf1998" class="citation book cs1">Woolf, Bobby (1998). "Null Object". In Martin, Robert; Riehle, Dirk; Buschmann, Frank (eds.). Pattern Languages of Program Design 3. Addison-Wesley.</cite>

</li>

<li id="cite_note-3"><a href="#cite_ref-3">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/WorkingwithObjects/WorkingwithObjects.html#//apple_ref/doc/uid/TP40011210-CH4-SW22">"Working with Objects (Working with nil)"</a>. iOS Developer Library. Apple, Inc. 2012-12-13. Retrieved 2014-05-19.</cite>

</li>

<li id="cite_note-4"><a href="#cite_ref-4">^</a> <cite id="CITEREFFowler1999" class="citation book cs1"><a href="Martin_Fowler_(software_engineer)" title="Martin Fowler (software engineer)">Fowler, Martin</a> (1999). <a rel="nofollow" class="external text" href="https://archive.org/details/isbn_9780201485677">Refactoring. Improving the Design of Existing Code</a>. Addison-Wesley. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-201-48567-2</bdi>.</cite>

</li>

<li id="cite_note-5"><a href="#cite_ref-5">^</a> <cite id="CITEREFKerievsky2004" class="citation book cs1">Kerievsky, Joshua (2004). Refactoring To Patterns. Addison-Wesley. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-321-21335-1</bdi>.</cite>

</li>

<li id="cite_note-6"><a href="#cite_ref-6">^</a> <cite id="CITEREFMartin2002" class="citation book cs1"><a href="Robert_Cecil_Martin" class="mw-redirect" title="Robert Cecil Martin">Martin, Robert</a> (2002). <a rel="nofollow" class="external text" href="https://archive.org/details/agilesoftwaredev00robe">Agile Software Development: Principles, Patterns and Practices</a>. Pearson Education. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-13-597444-5</bdi>.</cite>

</li>

<li id="cite_note-7"><a href="#cite_ref-7">^</a> Fowler, Martin (1999). Refactoring, p. 216.

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="http://www.cs.oberlin.edu/~jwalker/nullObjPattern/">Jeffery Walker's account of the Null Object Pattern</a></li>

<li><a rel="nofollow" class="external text" href="http://martinfowler.com/eaaCatalog/specialCase.html">Martin Fowler's description of Special Case, a slightly more general pattern</a></li>

<li><a rel="nofollow" class="external text" href="http://www.owlnet.rice.edu/~comp212/00-spring/handouts/week06/null_object_revisited.htm">Null Object Pattern Revisited</a></li>

<li><a rel="nofollow" class="external text" href="http://refactoring.com/catalog/introduceNullObject.html">Introduce Null Object refactoring</a></li>

<li><a rel="nofollow" class="external text" href="http://sourcemaking.com/design_patterns/null_object">SourceMaking Tutorial</a></li>

<li><a rel="nofollow" class="external text" href="https://medium.com/@eofster/null-object-pattern-in-swift-1b96e03b2756">Null Object Pattern in Swift</a></li></ul>

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</style><div id="Nulls_in_computing108" style="font-size:114%;margin:0 4em">Nulls in <a href="Computing" title="Computing">computing</a></div></th></tr><tr><td colspan="2" class="navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Null_character" title="Null character">Null character</a></li>

<li><a href="Null_device" title="Null device">Null device</a></li>

<li><a href="Null_function" title="Null function">Null function</a></li>

<li><a href="Null_modem" title="Null modem">Null modem</a></li>

<li><a href="Null_pointer" title="Null pointer">Null pointer</a></li>

<li><a href="Empty_string" title="Empty string">Null string</a></li>

<li><a href="Null_coalescing_operator" title="Null coalescing operator">Null coalescing operator</a></li></ul>

</div></td></tr><tr><td class="navbox-abovebelow hlist" colspan="2"><div>

<ul><li>See also: <a href="Null_coalescing_operator" title="Null coalescing operator">Null coalescing operator</a></li>

<li><a href="Nullable_type" title="Nullable type">Nullable type</a></li>

<li><a href="Undefined_value" title="Undefined value">Undefined value</a></li></ul>

</div></td></tr></tbody></table></div>

<div class="navbox-styles"></div><div role="navigation" class="navbox" aria-labelledby="Software_design_patterns225" style="padding:3px"><table class="nowraplinks mw-collapsible autocollapse navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><div id="Software_design_patterns225" style="font-size:114%;margin:0 4em"><a href="Software_design_pattern" title="Software design pattern">Software design patterns</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Design_Patterns" title="Design Patterns">Gang of Four patterns</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Creational_pattern" title="Creational pattern">Creational</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Abstract_factory_pattern" title="Abstract factory pattern">Abstract factory</a></li>

<li><a href="Builder_pattern" title="Builder pattern">Builder</a></li>

<li><a href="Factory_method_pattern" title="Factory method pattern">Factory method</a></li>

<li><a href="Prototype_pattern" title="Prototype pattern">Prototype</a></li>

<li><a href="Singleton_pattern" title="Singleton pattern">Singleton</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Structural_pattern" title="Structural pattern">Structural</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Adapter_pattern" title="Adapter pattern">Adapter</a></li>

<li><a href="Bridge_pattern" title="Bridge pattern">Bridge</a></li>

<li><a href="Composite_pattern" title="Composite pattern">Composite</a></li>

<li><a href="Decorator_pattern" title="Decorator pattern">Decorator</a></li>

<li><a href="Facade_pattern" title="Facade pattern">Facade</a></li>

<li><a href="Flyweight_pattern" title="Flyweight pattern">Flyweight</a></li>

<li><a href="Proxy_pattern" title="Proxy pattern">Proxy</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Behavioral_pattern" title="Behavioral pattern">Behavioral</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Chain-of-responsibility_pattern" title="Chain-of-responsibility pattern">Chain of responsibility</a></li>

<li><a href="Command_pattern" title="Command pattern">Command</a></li>

<li><a href="Interpreter_pattern" title="Interpreter pattern">Interpreter</a></li>

<li><a href="Iterator_pattern" title="Iterator pattern">Iterator</a></li>

<li><a href="Mediator_pattern" title="Mediator pattern">Mediator</a></li>

<li><a href="Memento_pattern" title="Memento pattern">Memento</a></li>

<li><a href="Observer_pattern" title="Observer pattern">Observer</a></li>

<li><a href="State_pattern" title="State pattern">State</a></li>

<li><a href="Strategy_pattern" title="Strategy pattern">Strategy</a></li>

<li><a href="Template_method_pattern" title="Template method pattern">Template method</a></li>

<li><a href="Visitor_pattern" title="Visitor pattern">Visitor</a></li></ul>

</div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Concurrency_pattern" title="Concurrency pattern">Concurrency patterns</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Active_object" title="Active object">Active object</a></li>

<li><a href="Balking_pattern" title="Balking pattern">Balking</a></li>

<li><a href="Binding_properties_pattern" title="Binding properties pattern">Binding properties</a></li>

<li><a href="Double-checked_locking" title="Double-checked locking">Double-checked locking</a></li>

<li><a href="Asynchronous_method_invocation" title="Asynchronous method invocation">Event-based asynchronous</a></li>

<li><a href="Guarded_suspension" title="Guarded suspension">Guarded suspension</a></li>

<li><a href="Monitor_(synchronization)" title="Monitor (synchronization)">Monitor</a></li>

<li><a href="Proactor_pattern" title="Proactor pattern">Proactor</a></li>

<li><a href="Reactor_pattern" title="Reactor pattern">Reactor</a></li>

<li><a href="Readers%E2%80%93writer_lock" title="Readers–writer lock">Read–write lock</a></li>

<li><a href="Scheduler_pattern" class="mw-redirect" title="Scheduler pattern">Scheduler</a></li>

<li><a href="Scheduled-task_pattern" title="Scheduled-task pattern">Scheduled-task pattern</a></li>

<li><a href="Semaphore_(programming)" title="Semaphore (programming)">Semaphore</a></li>

<li><a href="Thread_pool" title="Thread pool">Thread pool</a></li>

<li><a href="Thread-local_storage" title="Thread-local storage">Thread-local storage</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Architectural_pattern" title="Architectural pattern">Architectural patterns</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Front_controller" title="Front controller">Front controller</a></li>

<li><a href="Interceptor_pattern" title="Interceptor pattern">Interceptor</a></li>

<li><a href="Specification_pattern" title="Specification pattern">Specification</a></li>

<li><a href="Publish%E2%80%93subscribe_pattern" title="Publish–subscribe pattern">Publish–subscribe</a></li>

<li><a href="Naked_objects" title="Naked objects">Naked objects</a></li>

<li><a href="Service_locator_pattern" title="Service locator pattern">Service locator</a></li>

<li><a href="Active_record_pattern" title="Active record pattern">Active record</a></li>

<li><a href="Identity_map_pattern" title="Identity map pattern">Identity map</a></li>

<li><a href="Data_access_object" title="Data access object">Data access object</a></li>

<li><a href="Data_transfer_object" title="Data transfer object">Data transfer object</a></li>

<li><a href="Inversion_of_control" title="Inversion of control">Inversion of control</a></li>

<li><a href="JSP_model_2_architecture" title="JSP model 2 architecture">Model 2</a></li>

<li><a href="Broker_pattern" title="Broker pattern">Broker</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Other patterns</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Blackboard_design_pattern" class="mw-redirect" title="Blackboard design pattern">Blackboard</a></li>

<li><a href="Business_delegate_pattern" title="Business delegate pattern">Business delegate</a></li>

<li><a href="Composite_entity_pattern" title="Composite entity pattern">Composite entity</a></li>

<li><a href="Dependency_injection" title="Dependency injection">Dependency injection</a></li>

<li><a href="Guard_(computer_science)" title="Guard (computer science)">Guard clause</a></li>

<li><a href="Intercepting_filter_pattern" title="Intercepting filter pattern">Intercepting filter</a></li>

<li><a href="Lazy_loading" title="Lazy loading">Lazy loading</a></li>

<li><a href="Mock_object" title="Mock object">Mock object</a></li>

<li><a href="Object_pool_pattern" title="Object pool pattern">Object pool</a></li>

<li><a href="Servant_(design_pattern)" title="Servant (design pattern)">Servant</a></li>

<li><a href="Type_Tunnel_pattern" class="mw-redirect" title="Type Tunnel pattern">Type tunnel</a></li>

<li><a href="Method_chaining" title="Method chaining">Method chaining</a></li>

<li><a href="Delegation_pattern" title="Delegation pattern">Delegation</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Books</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Design_Patterns" title="Design Patterns">Design Patterns</a></li>

<li><a href="Enterprise_Integration_Patterns" title="Enterprise Integration Patterns">Enterprise Integration Patterns</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">People</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Christopher_Alexander" title="Christopher Alexander">Christopher Alexander</a></li>

<li><a href="Erich_Gamma" title="Erich Gamma">Erich Gamma</a></li>

<li><a href="Ralph_Johnson_(computer_scientist)" title="Ralph Johnson (computer scientist)">Ralph Johnson</a></li>

<li><a href="John_Vlissides" title="John Vlissides">John Vlissides</a></li>

<li><a href="Grady_Booch" title="Grady Booch">Grady Booch</a></li>

<li><a href="Ward_Cunningham" title="Ward Cunningham">Ward Cunningham</a></li>

<li><a href="Martin_Fowler_(software_engineer)" title="Martin Fowler (software engineer)">Martin Fowler</a></li>

<li><a href="Robert_C._Martin" title="Robert C. Martin">Robert Martin</a></li>

<li><a href="Jim_Coplien" title="Jim Coplien">Jim Coplien</a></li>

<li><a href="Douglas_C._Schmidt" title="Douglas C. Schmidt">Douglas Schmidt</a></li>

<li><a href="Linda_Rising" title="Linda Rising">Linda Rising</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Communities</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="The_Hillside_Group" title="The Hillside Group">The Hillside Group</a></li>

<li><a href="Portland_Pattern_Repository" title="Portland Pattern Repository">Portland Pattern Repository</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">See also</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Anti-pattern" title="Anti-pattern">Anti-pattern</a></li>

<li><a href="Architectural_pattern" title="Architectural pattern">Architectural pattern</a></li></ul>

</div></td></tr></tbody></table></div></div><div><div class="zim-footer">

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