Explanation of the UML arrows

Question

I have recently been studying UML and drawing simple diagrams with ordinary plain arrows between classes  but I know it s not enough  There are plenty of other arrows  generalization  realisation and etc  which have meaning to the diagram reader    Is there a nice resource which could explain each arrow  ordinary  plain  dotted  diamond-filled  diamond     It would be the best if it will have some code examples for them

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The accepted answer being said  It is missing some explanations  For example  what is the difference between a uni-directional and a bi-directional association  In the provided example  both do exist    Both  5 s in the arrows   If looking for a more complete answer and have more time  here is a thorough explanation

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If you are more of a MOOC person  one free course that I d recommend that teaches you all the in and outs of most UML diagrams is this one from Udacity  https   www udacity com course software-architecture-design--ud821

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Here is simplified tutorial    Practical UML  For more I recommend to get some literature

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A very easy to understand description is the documentation of yuml  with examples for class diagrams  use cases  and activities

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My favourite UML  cheat sheet  is UML Distilled  by Martin Fowler  It s the only one of his books that I ve read that I do recommend

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Here s some explanations from the Visual Studio 2015 docs  UML Class Diagrams  Reference  https   msdn microsoft com library dd409437 28VS 140 29 aspx   5  Association  A relationship between the members of two classifiers  5a  Aggregation  An association representing a shared ownership relationship  The Aggregation property of the owner role is set to Shared  5b  Composition  An association representing a whole-part relationship  The Aggregation property of the owner role is set to Composite  9  Generalization  The specific classifier inherits part of its definition from the general classifier  The general classifier is at the arrow end of the connector  Attributes  associations  and operations are inherited by the specific classifier  Use the Inheritance tool to create a generalization between two classifiers     13  Import  A relationship between packages  indicating that one package includes all the definitions of another  14  Dependency  The definition or implementation of the dependent classifier might change if the classifier at the arrowhead end is changed     15  Realization  The class implements the operations and attributes defined by the interface  Use the Inheritance tool to create a realization between a class and an interface  16  Realization  An alternative presentation of the same relationship  The label on the lollipop symbol identifies the interface   UML Class Diagrams  Guidelines  http   msdn microsoft com library dd409416 28VS 140 29 aspx  Properties of an Association Aggregation  This appears as a diamond shape at one end of the connector  You can use it to indicate that instances at the aggregating role own or contain instances of the other  Is Navigable  If true for only one role  an arrow appears in the navigable direction  You can use this to indicate navigability of links and database relations in the software     Generalization  Generalization means that the specializing or derived type inherits attributes  operations  and associations of the general or base type  The general type appears at the arrowhead end of the relationship  Realization  Realization means that a class implements the attributes and operations specified by the interface  The interface is at the arrow end of the connector   Let me know if you have more questions

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Aggregations and compositions are a little bit confusing  However  think like compositions are a stronger version of aggregation  What does that mean  Let s take an example   Aggregation  1  Take a classroom and students  In this case  we try to analyze the relationship between them  A classroom has a relationship with students  That means classroom comprises of one or many students  Even if we remove the Classroom class  the Students class does not need to destroy  which means we can use Student class independently    Composition  2  Take a look at pages and Book Class  In this case  pages is a book  which means collections of pages makes the book  If we remove the book class  the whole Page class will be destroyed  That means we cannot use the class of the page independently   If you are still unclear about this topic  watch out this short wonderful video  which has explained the aggregation more clearly   https   www youtube com watch v d5ecYmyFZW0

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I think these pictures are understandable

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For quick reference along with clear concise examples  Allen Holub s UML Quick Reference is excellent   http   www holub com goodies uml    There are quite a few specific examples of arrows and pointers in the first column of a table  with descriptions in the second column

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A nice cheat sheet  http   loufranco com wp-content uploads 2012 11 cheatsheet pdf    It covers    Class Diagram  Sequence Diagram  Package Diagram  Object Diagram  Use Case Diagram   And provides a few samples

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