UML - Building Blocks

UML defines how the system or application can be represented visually in number of diagrams. The System's design is basically made of diagrams and the diagrams contain many related elements. A system's architecture and specification is shown with multiple diagrams. There are various diagrams which UML defines which can be used to show the system at various levels and views. All the diagrams and the elements in it constitute the model of the software system or application. The model is nothing but the blueprint of the system.

To understand UML, one needs to understand basic building blocks, rules guiding them and the common design concepts.

The building blocks (vocabulary of language) of UML are:
1. Things
2. Relationships
3. Diagrams
Lets go one by one to get a better picture...

Things are the elements of UML model which is used to build a model
There are four kinds of things in the UML
i) Structural things (nouns) - Classes, Interfaces, Collaborations, Use Cases, Active Classes, components and nodes
ii) Behavioral things (verbs) - Interaction (messages exchanged), State machine
iii) Grouping things (organisation of structural things) - Package
iv) Annotational things (documentation) - Annotations

Relationships are the elements which indicate how two or more things are related to each other. There are various relationships, which are:
i) Dependency (semantic relation) - indicates that change to one thing may affect the semantics of other thing. One of the commonly seen relation is Use-Dependency, where one thing uses other thing to perform certain action.
ii) Association (structural relation) - indicates whole and its part relationship
iii) Generalization (inheritence relation) - the specialized element (the child) are substitutable for objects of generalized element (the parent). In this way the child shares the structure and behavior of the parent.
iv) Realization - one classifier specifies the contract that another classifier gaurantees to carry out. This relationship can be found in two places: between interfaces and the classes or components that realize them, and between use cases and the collaborations that realize them.

Diagrams are the graphical representation using things and relationships to visualize a system with specific perspective. So a diagram is a projection into a system. In theory, a diagram may contain any combination of things and relationships. In Practice, however, a small number of common combinations arise, which are consistent with five most useful views that comprise the architecture of a software-intensive system. In UML1.0 there are nine such diagrams supporting this.
1. Class Diagram
2. Object Diagram
3. Use Case Diagram
4. Sequence Diagram
5. Collaboration Diagram
6. Statechart Diagram
7. Activity Diagram
8. Component Diagram
9. Deployment Diagram

Out of these Class Diagrams, Object Diagrams, Component Diagrams and Deployment Diagrams indicate the structural specification of the software system and the rest of the diagrams are useful in projecting the behaviour of the system.

UML - Why?

The so called "Programming World" started much before the world's first single chip microprocessor, Intel 4004 was invented in November, 1971. It has been evolving since then, with newer paradigms seeding the birth of new languages. As the computing power of the processors increasing at a rapid pace from 8085, 80x86, etc, the programming languages are able to stretch themselves making the language more flexible, efficient and readable. Today, robots are being programmed to understand not just the syntax and semantics but also the pragmatics of the language. Let us not get that far but try to understand where the current programming languages are leading to. For those who are interested in the chronology of languages they can take a look at this site http://www.scriptol.org/history.php.

Broadly the programming styles can be categorized into two: Imperative and Declarative (See http://en.wikipedia.org/wiki/Programming_paradigm for more paradigms). Every language is basically a set of language grammar which the language compiler translates to machine understandable instructions.

Most of the languages which were used for development were typed and with the increasing complexity of software development, there arose a necessity for a language which a human can easily understand. The advent of Object Oriented paradigm helped in this direction with the concepts of objects and behavior similar to the real world objects. During 1990's UML or Unified Modeling Language took birth gradually with the union of ideas from three people Grady Booch, Jim Rumbaugh, and Ivar Jacobson. This language gave a visual representation of the system from various aspects which even an average programmer could easily understand. By being able to put it into diagrams, UML eased the software development to a great extent. With the various views and diagrams of the system one could easily get both the bigger picture as well as the surgical view of the system. In general UML introduced a standardized visual specification for object modeling.

Now lets talk about the need and how UML helps.
In the current situation,


Interested ones can look at the more links below,
UML History graph: http://en.wikipedia.org/wiki/File:OO-historie.jpg
UML Specifications: http://www.omg.org/technology/documents/modeling_spec_catalog.htm#UML