The three major building blocks of UML are:

1.      The Elements of the model

2.      The Associations between the elements of the model

3.      The UML Diagrams

The basic elements of an UML model are the classes, the interfaces and the components. A class may be defined as a self contained unit that encapsulates a collection of data and methods that operate on the data resulting on an isolation of the contents of this encapsulated from any external interface. An interface is a contract which can be used to define a protocol of behavior. It can be implemented by any class anywhere in the class hierarchy.

UML Relationships

Let us now understand how we can depict relationships between various elements in the UML model of a system. Basically, there are four types of relationships that we use in a UML model of a system. These are as follows.

·         Association

·         Aggregation

·         Generalization or Inheritance

·         Realization

·         Dependency

UML Diagrams

You have in UML various models that define the notation and semantics for a wide variety of domains. The UML models depict how the classes and objects in a system interact with one another. "UML models are diagrams of three primary types: structural, behavioral, and interaction. Generally, structural diagrams define the underlying software system (the code), behavioral diagrams describe what happens in the system under certain conditions, and interaction diagrams explain control flow." UML diagrams are used to provide a graphical representation of the system being modeled. UML 2.0 defines thirteen diagrams that are broadly classified into three categories with each category containing one or more diagrams that fall under that category. These categories are:

·         The Structural Diagrams

·         The Behavioral Diagrams

·         The Interaction Diagrams

The next section discusses these categories and shows the UML diagrams that comprise each of these categories.

The Structural Diagrams

These relate to the static structure of a system, i.e., they represent elements that are static in nature. These diagrams are fundamental to the UML modeling of a system and portray the static structure of the system as a whole. The Structural Diagrams are comprised of the following.

·         The Class diagram

·         The Component diagram

·         The Composite Structure diagram

·         The Deployment diagram

·         The Object diagram

·         The Package diagram

The Behavioral Diagrams

The Behavioral Diagrams model how the system functions. The following are examples of Behavioral Diagrams.

·         Use Case Diagram

·         Activity Diagram

·         State Machine Diagram

The Interaction Diagrams

These diagrams represent how flow of data and control takes place in the system that is being modeled. They are a subset of the Behavioral Diagrams. The examples in this category are:

·         Communication Diagram

·         Sequence Diagram

·         UML Timing Diagram

·         Interaction Overview Diagram

We will learn what each of these diagrams and model systems are in the forthcoming articles in this series. The next section discusses UML Rules.

UML Rules

The UML also defines a set of rules that can be used to check whether the UML is properly being followed. "A well formed model is one that is semantically self-consistent and in harmony with all its related models."

In UML, you have semantic rules for the following:

·         Names

·         Scope

·         Visibility

·         Integrity

·         Execution