Null Object Design Pattern

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In short, a design where "nothing will come of nothing"</p> <p>Use the Null Object pattern when</p> <ul> <li>an object requires a collaborator. The Null Object pattern does not introduce this collaboration--it makes use of a collaboration that already exists</li> <li>some collaborator instances should do nothing</li> <li>you want to abstract the handling of null away from the client</li> </ul> <h3>Problem</h3> <p>Given that an object reference may be optionally null, and that the result of a null check is to do nothing or use some default value, how can the absence of an object — the presence of a null reference — be treated transparently?</p> <h3>Discussion</h3> <p>Sometimes a class that requires a collaborator does not need the collaborator to do anything. However, the class wishes to treat a collaborator that does nothing the same way it treats one that actually provides behavior.</p> <p>Consider for example a simple screen saver which displays balls that move about the screen and have special color effects. This is easily achieved by creating a Ball class to represent the balls and using a Strategy pattern to control the ball's motion and another Strategy pattern to control the ball's color.</p> <p>It would then be trivial to write strategies for many different types of motion and color effects and create balls with any combination of those. However, to start with you want to create the simplest strategies possible to make sure everything is working. And these strategies could also be useful later since you want as strategies as possible strategies.</p> <p class="image"> <img src="/files/v2/content/patterns/Null_Object1.png" srcset="/files/v2/content/patterns/Null_Object1-2x.png 2x" alt="Null Object scheme"/> </p> <p>Now, the simplest strategy would be no strategy. That is do nothing, don't move and don't change color. However, the Strategy pattern requires the ball to have objects which implement the strategy interfaces. This is where the Null Object pattern becomes useful.</p> <p>Simply implement a NullMovementStrategy which doesn't move the ball and a NullColorStrategy which doesn't change the ball's color. Both of these can probably be implemented with essentially no code. All the methods in these classes do "nothing". They are perfect examples of the Null Object Pattern.</p> <p>The key to the Null Object pattern is an abstract class that defines the interface for all objects of this type. The Null Object is implemented as a subclass of this abstract class. Because it conforms to the abstract class' interface, it can be used any place this type of object is needed. As compared to using a special "null" value which doesn't actually implement the abstract interface and which must constantly be checked for with special code in any object which uses the abstract interface.</p> <p>It is sometimes thought that Null Objects are over simple and "stupid" but in truth a Null Object always knows exactly what needs to be done without interacting with any other objects. So in truth it is very "smart."</p> <h3>Structure</h3> <p class="image"> <img src="/files/v2/content/patterns/Null_Object2.png" srcset="/files/v2/content/patterns/Null_Object2-2x.png 2x" alt="Null Object diagram"/> </p> <ul> <li><strong><code>Client</code></strong> - <ul><li>requires a collaborator.</li> </ul></li> <li><strong><code>AbstractObject</code></strong> - <ul><li>declares the interface for Client's collaborator</li> <li>implements default behavior for the interface common to all classes, as appropriate</li> </ul></li> <li><code><strong>RealObject</strong></code> - <ul> <li>defines a concrete subclass of AbstractObject whose instances provide useful behavior that Client expects</li> </ul></li> <li><code><strong>NullObject</strong></code> - <ul> <li>provides an interface identical to AbstractObject's so that a null object can be substituted for a real object</li> <li>implements its interface to do nothing. What exactly it means to do nothing depends on what sort of behavior Client is expecting</li> <li>when there is more than one way to do nothing, more than one NullObject class may be required</li> </ul> </li> </ul> <h3>Rules of thumb</h3> <ul> <li>The Null Object class is often implemented as a Singleton. Since a null object usually does not have any state, its state can't change, so multiple instances are identical. Rather than use multiple identical instances, the system can just use a single instance repeatedly.</li> <li>If some clients expect the null object to do nothing one way and some another, multiple NullObject classes will be required. If the do nothing behavior must be customized at run time, the <code>NullObject</code> class will require pluggable variables so that the client can specify how the null object should do nothing (see the discussion of pluggable adaptors in the Adapter pattern). This may generally be a symptom of the <code>AbstractObject</code> not having a well defined (semantic) interface.</li> <li>A Null Object does not transform to become a Real Object. If the object may decide to stop providing do nothing behavior and start providing real behavior, it is not a null object. It may be a real object with a do nothing mode, such as a controller which can switch in and out of read-only mode. If it is a single object which must mutate from a do nothing object to a real one, it should be implemented with the State pattern or perhaps the Proxy pattern. In this case a Null State may be used or the proxy may hold a Null Object.</li> <li>The use of a null object can be similar to that of a Proxy, but the two patterns have different purposes. A proxy provides a level of indirection when accessing a real subject, thus controlling access to the subject. A null collaborator does not hide a real object and control access to it, it replaces the real object. A proxy may eventually mutate to start acting like a real subject. A null object will not mutate to start providing real behavior, it will always provide do nothing behavior.</li> <li>A Null Object can be a special case of the Strategy pattern. Strategy specifies several ConcreteStrategy classes as different approaches for accomplishing a task. If one of those approaches is to consistently do nothing, that <code>ConcreteStrategy</code> is a <code>NullObject</code>. For example, a Controller is a View's Strategy for handling input, and NoController is the Strategy that ignores all input.</li> <li>A Null Object can be a special case of the State pattern. Normally, each ConcreteState has some do nothing methods because they're not appropriate for that state. In fact, a given method is often implemented to do something useful in most states but to do nothing in at least one state. If a particular ConcreteState implements most of its methods to do nothing or at least give null results, it becomes a do nothing state and as such is a null state.</li> <li>A Null Object can be used to allow a Visitor to safely visit a hierarchy and handle the null situation.</li> <li>Null Object is a concrete collaborator class that acts as the collaborator for a client which needs one. The null behavior is not designed to be mixed into an object that needs some do nothing behavior. It is designed for a class which delegates to a collaborator all of the behavior that may or may not be do nothing behavior.</li> </ul> <div class="banner prom" data-id="DP: Part of the ebook 3d" data-creative-id="standard" data-position="content_bottom"> <div class="line-guide"><div class="line-guide-helper"></div></div> <div class="image"> <a href="/design-patterns-ebook"><img width="200" src="/images/content-public/didp-cover-en.png?id=2e1103ea2edb33a8979eebb8e1ba73b6" srcset="/images/content-public/didp-cover-en-2x.png?id=0be443437e7b9e0d53cfbeef0092d658 2x"/></a> </div> <div class="caption"> <h3 class="title">Support our free website and own the eBook!</h3> <ul style="font-size: 15px;"> <li style="margin: 0;">22 design patterns and 8 principles explained in depth</li> <li style="margin: 0;">406 well-structured, easy to read, jargon-free pages</li> <li style="margin: 0;">228 clear and helpful illustrations and diagrams</li> <li style="margin: 0;">An archive with code examples in 4 languages</li> <li style="margin: 0;">All devices supported: EPUB/MOBI/PDF formats</li> </ul> <a class="btn btn-secondary" href="/design-patterns-ebook"><i class="fa fa-book" aria-hidden="true"></i> Learn more...</a> </div> </div> </article> <section class="patterns-examples"> <h3>Code examples</h3> <div class="table-responsive"> <table class="table table-bordered table-no-striping table"><tbody> <tr><td>Java</td> <td><a href="/design_patterns/null_object/java/1" >Null Object in Java</a></td> <td><a href="/design_patterns/null_object/java/2" >Null Object in Java</a></td> </tr> <tr><td>Python</td> <td><a href="/design_patterns/null_object/python/1" >Null Object in Python</a></td> </tr> </tbody> </table> </div> </section> <nav class="prev-next"> <div class="next"> <h4>Read next</h4> <a rel="next" href="/design_patterns/observer" class="btn btn-primary">Observer Design Pattern <span class="fa fa-arrow-right"></span></a> </div> <div class="prev"> <h4>Return</h4> <a rel="prev" href="/design_patterns/memento" class="btn btn-default"><span class="fa fa-arrow-left"></span> Memento Design Pattern </a> </div> </nav> </div> <aside class="feature content-secondary"> <div class="book-button"> <div class="prom text" data-id="DP: Nav button with image3d" data-creative-id="Blue button" data-position="sidebar_bottom"> <div class="image" style="margin-bottom: 1rem;"> <div class="image3d-book-right"> <div class="image3d-cover" style="background: #0b3752;"> <a href="/design-patterns-ebook"><img width="250" src="/images/content-public/didp-cover-en.png?id=2e1103ea2edb33a8979eebb8e1ba73b6" srcset="/images/content-public/didp-cover-en-2x.png?id=0be443437e7b9e0d53cfbeef0092d658 2x"/></a> </div> </div> </div> <div style="text-align: center"> <a class="btn btn-landing-dp btn-hg btn-block btn-secondary" href="/design-patterns-ebook"><i class="fa fa-book"></i> Dive Into Design Patterns <span style="background-color:red; color:white; padding: 1px 3px; display:inline-block; position: absolute;right: -20px;top: -5px;font-size: 16px;">new</span></a> </div> <p style="margin-top: 1rem;">Hey, check out our new <a href="/design-patterns-ebook">ebook on design patterns</a>. 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<a href="/design_patterns/singleton"> Singleton Design Pattern </a> </li> </ul> </li> <li class=""> <a href="/design_patterns/structural_patterns"> Structural patterns </a> <ul > <li class=""> <a href="/design_patterns/adapter"> Adapter Design Pattern </a> </li> <li class=""> <a href="/design_patterns/bridge"> Bridge Design Pattern </a> </li> <li class=""> <a href="/design_patterns/composite"> Composite Design Pattern </a> </li> <li class=""> <a href="/design_patterns/decorator"> Decorator Design Pattern </a> </li> <li class=""> <a href="/design_patterns/facade"> Facade Design Pattern </a> </li> <li class=""> <a href="/design_patterns/flyweight"> Flyweight Design Pattern </a> </li> <li class=""> <a href="/design_patterns/private_class_data"> Private Class Data </a> </li> <li class=""> <a href="/design_patterns/proxy"> Proxy Design Pattern </a> </li> </ul> </li> <li class="trail"> <a href="/design_patterns/behavioral_patterns"> Behavioral patterns </a> <ul > <li class=""> <a href="/design_patterns/chain_of_responsibility"> Chain of Responsibility </a> </li> <li class=""> <a href="/design_patterns/command"> Command Design Pattern </a> </li> <li class=""> <a href="/design_patterns/interpreter"> Interpreter Design Pattern </a> </li> <li class=""> <a href="/design_patterns/iterator"> Iterator Design Pattern </a> </li> <li class=""> <a href="/design_patterns/mediator"> Mediator Design Pattern </a> </li> <li class=""> <a href="/design_patterns/memento"> Memento Design Pattern </a> </li> <li class="trail active"> <a href="/design_patterns/null_object"> Null Object Design Pattern </a> </li> <li class=""> <a href="/design_patterns/observer"> Observer Design Pattern </a> </li> <li class=""> <a href="/design_patterns/state"> State Design Pattern </a> </li> <li class=""> <a href="/design_patterns/strategy"> Strategy Design Pattern </a> </li> <li class=""> <a href="/design_patterns/template_method"> Template Method Design Pattern </a> </li> <li class=""> <a href="/design_patterns/visitor"> Visitor Design Pattern </a> </li> </ul> </li> </ul> </li> <li class=""> <a href="/refactoring"> <i class="fa fa-fw fa-scissors" aria-hidden="true"></i> Refactoring </a> <ul > <li class=""> <a href="/refactoring/smells"> Code Smells </a> <ul > <li class=""> <a href="/refactoring/smells/bloaters"> Bloaters </a> <ul > <li class=""> <a href="/refactoring/smells/long-method"> Long Method </a> </li> <li class=""> <a href="/refactoring/smells/large-class"> Large Class </a> </li> <li class=""> <a href="/refactoring/smells/primitive-obsession"> Primitive Obsession </a> </li> <li class=""> <a href="/refactoring/smells/long-parameter-list"> Long Parameter List </a> </li> <li class=""> <a href="/refactoring/smells/data-clumps"> Data Clumps </a> </li> </ul> </li> <li class=""> <a href="/refactoring/smells/oo-abusers"> Object-Orientation Abusers </a> <ul > <li class=""> <a href="/refactoring/smells/switch-statements"> Switch Statements </a> </li> <li class=""> <a href="/refactoring/smells/temporary-field"> Temporary Field </a> </li> <li class=""> <a href="/refactoring/smells/refused-bequest"> Refused Bequest </a> </li> <li class=""> <a href="/refactoring/smells/alternative-classes-with-different-interfaces"> Alternative Classes with Different Interfaces </a> </li> </ul> </li> <li class=""> <a href="/refactoring/smells/change-preventers"> Change Preventers </a> <ul > <li class=""> <a href="/refactoring/smells/divergent-change"> Divergent Change </a> </li> <li class=""> <a href="/refactoring/smells/shotgun-surgery"> Shotgun Surgery </a> </li> <li class=""> <a href="/refactoring/smells/parallel-inheritance-hierarchies"> Parallel Inheritance Hierarchies </a> </li> </ul> </li> <li class=""> <a href="/refactoring/smells/dispensables"> Dispensables </a> <ul > <li class=""> <a href="/refactoring/smells/comments"> Comments </a> </li> <li class=""> <a href="/refactoring/smells/duplicate-code"> Duplicate Code </a> </li> <li class=""> <a href="/refactoring/smells/lazy-class"> Lazy Class </a> </li> <li class=""> <a href="/refactoring/smells/data-class"> Data Class </a> </li> <li class=""> <a href="/refactoring/smells/dead-code"> Dead Code </a> </li> <li class=""> <a href="/refactoring/smells/speculative-generality"> Speculative Generality </a> </li> </ul> </li> <li class=""> <a href="/refactoring/smells/couplers"> Couplers </a> <ul > <li class=""> <a href="/refactoring/smells/feature-envy"> Feature Envy </a> </li> <li class=""> <a href="/refactoring/smells/inappropriate-intimacy"> Inappropriate Intimacy </a> </li> <li class=""> <a href="/refactoring/smells/message-chains"> Message Chains </a> </li> <li class=""> <a href="/refactoring/smells/middle-man"> Middle Man </a> </li> </ul> </li> <li class=""> <a href="/refactoring/smells/other"> Other Smells </a> <ul > <li class=""> <a href="/refactoring/smells/incomplete-library-class"> Incomplete Library Class </a> </li> </ul> </li> </ul> </li> <li class=""> <a href="/refactoring/refactorings"> Refactoring techniques </a> <ul > <li class=""> <a href="/refactoring/composing-methods"> Composing Methods </a> <ul > <li class=""> <a href="/refactoring/extract-method"> Extract Method </a> </li> <li class=""> <a href="/refactoring/inline-method"> Inline Method </a> </li> <li class=""> <a href="/refactoring/extract-variable"> Extract Variable </a> </li> <li class=""> <a href="/refactoring/inline-temp"> Inline Temp </a> </li> <li class=""> <a href="/refactoring/replace-temp-with-query"> Replace Temp with Query </a> </li> <li class=""> <a href="/refactoring/split-temporary-variable"> Split Temporary Variable </a> </li> <li class=""> <a href="/refactoring/remove-assignments-to-parameters"> Remove Assignments to Parameters </a> </li> <li class=""> <a href="/refactoring/replace-method-with-method-object"> Replace Method with Method Object </a> </li> <li class=""> <a href="/refactoring/substitute-algorithm"> Substitute Algorithm </a> </li> </ul> </li> <li class=""> <a href="/refactoring/moving-features-between-objects"> Moving Features between Objects </a> <ul > <li class=""> <a href="/refactoring/move-method"> Move Method </a> </li> <li class=""> <a href="/refactoring/move-field"> Move Field </a> </li> <li class=""> <a href="/refactoring/extract-class"> Extract Class </a> </li> <li class=""> <a href="/refactoring/inline-class"> Inline Class </a> </li> <li class=""> <a href="/refactoring/hide-delegate"> Hide Delegate </a> </li> <li class=""> <a href="/refactoring/remove-middle-man"> Remove Middle Man </a> </li> <li class=""> <a href="/refactoring/introduce-foreign-method"> Introduce Foreign Method </a> </li> <li class=""> <a href="/refactoring/introduce-local-extension"> Introduce Local Extension </a> </li> </ul> </li> <li class=""> <a href="/refactoring/organizing-data"> Organizing Data </a> <ul > <li class=""> <a href="/refactoring/self-encapsulate-field"> Self Encapsulate Field </a> </li> <li class=""> <a href="/refactoring/replace-data-value-with-object"> Replace Data Value with Object </a> </li> <li class=""> <a href="/refactoring/change-value-to-reference"> Change Value to Reference </a> </li> <li class=""> <a href="/refactoring/change-reference-to-value"> Change Reference to Value </a> </li> <li class=""> <a href="/refactoring/replace-array-with-object"> Replace Array with Object </a> </li> <li class=""> <a href="/refactoring/duplicate-observed-data"> Duplicate Observed Data </a> </li> <li class=""> <a href="/refactoring/change-unidirectional-association-to-bidirectional"> Change Unidirectional Association to Bidirectional </a> </li> <li class=""> <a href="/refactoring/change-bidirectional-association-to-unidirectional"> Change Bidirectional Association to Unidirectional </a> </li> <li class=""> <a href="/refactoring/replace-magic-number-with-symbolic-constant"> Replace Magic Number with Symbolic Constant </a> </li> <li class=""> <a href="/refactoring/encapsulate-field"> Encapsulate Field </a> </li> <li class=""> <a href="/refactoring/encapsulate-collection"> Encapsulate Collection </a> </li> <li class=""> <a href="/refactoring/replace-type-code-with-class"> Replace Type Code with Class </a> </li> <li class=""> <a href="/refactoring/replace-type-code-with-subclasses"> Replace Type Code with Subclasses </a> </li> <li class=""> <a href="/refactoring/replace-type-code-with-state-strategy"> Replace Type Code with State/Strategy </a> </li> <li class=""> <a href="/refactoring/replace-subclass-with-fields"> Replace Subclass with Fields </a> </li> </ul> </li> <li class=""> <a href="/refactoring/simplifying-conditional-expressions"> Simplifying Conditional Expressions </a> <ul > <li class=""> <a href="/refactoring/decompose-conditional"> Decompose Conditional </a> </li> <li class=""> <a href="/refactoring/consolidate-conditional-expression"> Consolidate Conditional Expression </a> </li> <li class=""> <a href="/refactoring/consolidate-duplicate-conditional-fragments"> Consolidate Duplicate Conditional Fragments </a> </li> <li class=""> <a href="/refactoring/remove-control-flag"> Remove Control Flag </a> </li> <li class=""> <a href="/refactoring/replace-nested-conditional-with-guard-clauses"> Replace Nested Conditional with Guard Clauses </a> </li> <li class=""> <a href="/refactoring/replace-conditional-with-polymorphism"> Replace Conditional with Polymorphism </a> </li> <li class=""> <a href="/refactoring/introduce-null-object"> Introduce Null Object </a> </li> <li class=""> <a href="/refactoring/introduce-assertion"> Introduce Assertion </a> </li> </ul> </li> <li class=""> <a href="/refactoring/simplifying-method-calls"> Simplifying Method Calls </a> <ul > <li class=""> <a href="/refactoring/rename-method"> Rename Method </a> </li> <li class=""> <a href="/refactoring/add-parameter"> Add Parameter </a> </li> <li class=""> <a href="/refactoring/remove-parameter"> Remove Parameter </a> </li> <li class=""> <a href="/refactoring/separate-query-from-modifier"> Separate Query from Modifier </a> </li> <li class=""> <a href="/refactoring/parameterize-method"> Parameterize Method </a> </li> <li class=""> <a href="/refactoring/replace-parameter-with-explicit-methods"> Replace Parameter with Explicit Methods </a> </li> <li class=""> <a href="/refactoring/preserve-whole-object"> Preserve Whole Object </a> </li> <li class=""> <a href="/refactoring/replace-parameter-with-method-call"> Replace Parameter with Method Call </a> </li> <li class=""> <a href="/refactoring/introduce-parameter-object"> Introduce Parameter Object </a> </li> <li class=""> <a href="/refactoring/remove-setting-method"> Remove Setting Method </a> </li> <li class=""> <a href="/refactoring/hide-method"> Hide Method </a> </li> <li class=""> <a href="/refactoring/replace-constructor-with-factory-method"> Replace Constructor with Factory Method </a> </li> <li class=""> <a href="/refactoring/replace-error-code-with-exception"> Replace Error Code with Exception </a> </li> <li class=""> <a href="/refactoring/replace-exception-with-test"> Replace Exception with Test </a> </li> </ul> </li> <li class=""> <a href="/refactoring/dealing-with-generalisation"> Dealing with Generalisation </a> <ul > <li class=""> <a href="/refactoring/pull-up-field"> Pull Up Field </a> </li> <li class=""> <a href="/refactoring/pull-up-method"> Pull Up Method </a> </li> <li class=""> <a href="/refactoring/pull-up-constructor-body"> Pull Up Constructor Body </a> </li> <li class=""> <a href="/refactoring/push-down-method"> Push Down Method </a> </li> <li class=""> <a href="/refactoring/push-down-field"> Push Down Field </a> </li> <li class=""> <a href="/refactoring/extract-subclass"> Extract Subclass </a> </li> <li class=""> <a href="/refactoring/extract-superclass"> Extract Superclass </a> </li> <li class=""> <a href="/refactoring/extract-interface"> Extract Interface </a> </li> <li class=""> <a href="/refactoring/collapse-hierarchy"> Collapse Hierarchy </a> </li> <li class=""> <a href="/refactoring/form-template-method"> Form Template Method </a> </li> <li class=""> <a href="/refactoring/replace-inheritance-with-delegation"> Replace Inheritance with Delegation </a> </li> <li class=""> <a href="/refactoring/replace-delegation-with-inheritance"> Replace Delegation with Inheritance </a> </li> </ul> </li> </ul> </li> </ul> </li> <li class=""> <a href="/uml"> <i class="fa fa-fw fa-sitemap" aria-hidden="true"></i> UML </a> <ul > <li class=""> <a href="/uml/introduction"> Introduction </a> </li> <li class=""> <a href="/uml/basic-principles"> Basic Principles and Background </a> <ul > <li class=""> <a href="/uml/basic-principles-and-background/introduction-to-the-case-study"> Introduction to the Case Study </a> </li> <li class=""> <a href="/uml/basic-principles-and-background/models-views-and-diagrams"> Models, Views, and Diagrams </a> </li> <li class=""> <a href="/uml/basic-principles-and-background/information-systems-and-it-systems"> Information Systems and IT Systems </a> </li> <li class=""> <a href="/uml/basic-principles-and-background/the-models-of-our-case-study"> The Models of our Case Study </a> </li> <li class=""> <a href="/uml/basic-principles-and-background/history-of-uml-methods-and-notations"> History of UML: Methods and Notations </a> </li> <li class=""> <a href="/uml/basic-principles-and-background/requirement-specification"> Requirement Specification </a> </li> <li class=""> <a href="/uml/basic-principles-and-background/uml2"> UML 2.0 </a> </li> </ul> </li> <li class=""> <a href="/uml/modeling-business-systems"> Modeling Business Systems </a> <ul > <li class=""> <a href="/uml/modeling-business-systems/business-processes-and-business-systems"> Business Processes and Business Systems </a> </li> <li class=""> <a href="/uml/modeling-business-systems/one-model-two-views"> One Model—Two Views </a> </li> <li class=""> <a href="/uml/modeling-business-systems/external-view"> External View </a> </li> <li class=""> <a href="/uml/modeling-business-systems/the-elements-of-view"> The Elements of a View </a> </li> <li class=""> <a href="/uml/modeling-business-systems/external-view/use-case-diagrams"> Use Case Diagrams </a> </li> <li class=""> <a href="/uml/modeling-business-systems/external-view/constructing-use-case-diagrams"> Constructing Use Case Diagrams </a> </li> <li class=""> <a href="/uml/modeling-business-systems/external-view/activity-diagrams"> Activity Diagrams </a> </li> <li class=""> <a href="/uml/modeling-business-systems/external-view/constructing-activity-diagrams"> Constructing Activity Diagrams </a> </li> <li class=""> <a href="/uml/modeling-business-systems/external-view/sequence-diagrams"> Sequence Diagrams </a> </li> <li class=""> <a href="/uml/modeling-business-systems/external-view/constructing-sequence-diagrams"> Constructing Sequence Diagrams </a> </li> <li class=""> <a href="/uml/modeling-business-systems/external-view/high-level-sequence-diagrams"> High-Level Sequence Diagrams </a> </li> <li class=""> <a href="/uml/modeling-business-systems/external-view/sequence-diagrams-for-scenarios-of-business-use-cases"> Sequence Diagrams for Scenarios of Business Use Cases </a> </li> <li class=""> <a href="/uml/modeling-business-systems/the-internal-view"> Internal View </a> </li> <li class=""> <a href="/uml/modeling-business-systems/internal-view/package-diagram"> Package Diagram </a> </li> <li class=""> <a href="/uml/modeling-business-systems/internal-view/constructing-package-diagrams"> Constructing Package Diagrams </a> </li> <li class=""> <a href="/uml/modeling-business-systems/internal-view/class-diagram"> Class Diagram </a> </li> <li class=""> <a href="/uml/modeling-business-systems/internal-view/constructing-class-diagrams"> Constructing Class Diagrams </a> </li> <li class=""> <a href="/uml/modeling-business-systems/internal-view/activity-diagram"> Activity Diagram </a> </li> </ul> </li> <li class=""> <a href="/uml/modeling-it-systems"> Modeling IT Systems </a> <ul > <li class=""> <a href="/uml/modeling-it-systems/external-view"> External View </a> </li> <li class=""> <a href="/uml/modeling-it-systems/external-view/the-user-view-or-i-dont-care-how-it-works-as-long-as-it-works"> The User View or "I don't care how it works, as long as it works." </a> </li> <li class=""> <a href="/uml/modeling-it-systems/external-view/the-elements-of-view"> The Elements of a View </a> </li> <li class=""> <a href="/uml/modeling-it-systems/external-view/the-elements-of-view/use-case-diagram"> Use Case Diagram </a> </li> <li class=""> <a href="/uml/modeling-it-systems/external-view/query-events-and-mutation-events"> Query Events and Mutation Events </a> </li> <li class=""> <a href="/uml/modeling-it-systems/external-view/use-case-sequence-diagram"> Use Case Sequence Diagram </a> </li> <li class=""> <a href="/uml/modeling-it-systems/external-view/constructing-the-external-view"> Constructing the External View </a> </li> <li class=""> <a href="/uml/modeling-it-systems/structural-view"> Structural View </a> </li> <li class=""> <a href="/uml/modeling-it-systems/structural-view/objects-and-classes"> Objects and Classes </a> </li> <li class=""> <a href="/uml/modeling-it-systems/structural-view/generalization-specialization-and-inheritance"> Generalization, Specialization, and Inheritance </a> </li> <li class=""> <a href="/uml/modeling-it-systems/structural-view/static-and-dynamic-business-rules"> Static and Dynamic Business Rules </a> </li> <li class=""> <a href="/uml/modeling-it-systems/structural-view/elements-of-the-view"> Elements of the View </a> </li> <li class=""> <a href="/uml/modeling-it-systems/structural-view/class-diagram"> Class Diagram </a> </li> <li class=""> <a href="/uml/modeling-it-systems/structural-view/constructing-class-diagrams"> Constructing Class Diagrams </a> </li> <li class=""> <a href="/uml/modeling-it-systems/the-behavioral-view"> The Behavioral View </a> </li> <li class=""> <a href="/uml/modeling-it-systems/the-behavioral-view/the-life-of-an-object"> The Life of an Object </a> </li> <li class=""> <a href="/uml/modeling-it-systems/the-behavioral-view/the-elements-of-the-view"> The Elements of the View </a> </li> <li class=""> <a href="/uml/modeling-it-systems/the-behavioral-view/statechart-diagram"> Statechart Diagram </a> </li> <li class=""> <a href="/uml/modeling-it-systems/the-behavioral-view/constructing-statechart-diagrams"> Constructing Statechart Diagrams </a> </li> <li class=""> <a href="/uml/modeling-it-systems/interaction-view"> Interaction View </a> </li> <li class=""> <a href="/uml/modeling-it-systems/interaction-view/seeing-what-happens-inside-the-it-system"> Seeing What Happens Inside the IT System </a> </li> <li class=""> <a href="/uml/modeling-it-systems/interaction-view/elements-of-the-view"> Elements of the View </a> </li> <li class=""> <a href="/uml/modeling-it-systems/interaction-view/communication-diagram"> Communication Diagram </a> </li> <li class=""> <a href="/uml/modeling-it-systems/interaction-view/sequence-diagram"> Sequence Diagram </a> </li> <li class=""> <a href="/uml/modeling-it-systems/interaction-view/constructing-communication-diagrams"> Constructing Communication Diagrams </a> </li> <li class=""> <a href="/uml/modeling-it-systems/interaction-view/constructing-sequence-diagrams"> Constructing Sequence Diagrams </a> </li> </ul> </li> <li class=""> <a href="/uml/modeling-for-system-integration"> Modeling for System Integration </a> <ul > <li class=""> <a href="/uml/modeling-for-system-integration/terminology-of-system-integration"> Terminology of System Integration </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/messages-in-uml"> Messages in UML </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/one-model-two-views"> One Model—Two Views </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/process-view"> Process View </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/process-view/the-business-system-model-as-foundation"> The Business System Model as Foundation </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/process-view/elements-of-the-view"> Elements of the View </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/process-view/activity-diagrams"> Activity Diagrams </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/process-view/sequence-diagram"> Sequence Diagram </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/process-view/constructing-diagrams-in-the-process-view"> Constructing Diagrams in the Process View </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/the-static-view"> The Static View </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/the-static-view/elements-of-the-view"> Elements of the View </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/the-static-view/class-diagram"> Class Diagram </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/the-static-view/constructing-class-diagrams"> Constructing Class Diagrams </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/the-static-view/transforming-data-from-the-it-system-to-the-mess"> Transforming Data from the IT System to the Message "passenger list" </a> </li> <li class=""> <a href="/uml/modeling-for-system-integration/the-static-view/transformation-of-uml-messages-into-various-stan"> Transformation of UML Messages into Various Standard Formats </a> </li> </ul> </li> </ul> </li> </ul> </div> <div class="main-menu-aux-controls"> <a href="https://sourcemaking.com/login" title="Sign in"><svg style="width: 16px; height: 16px; filter:drop-shadow(0 1px 1px #0d5e4e)" 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Null Object Design Pattern