Explanation of the UML arrows

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.

Here's some explanations from the Visual Studio 2015 docs:

UML Class Diagrams: Reference: https://docs.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2015/modeling/uml-class-diagrams-reference

> 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.

I think these pictures are understandable.

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.

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.

Here is simplified tutorial:

Practical UML

For more I recommend to get some literature.

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.)

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

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.

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

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