Diagrams in UML
UML 2 defines many basic diagram types, divided into two general sets:
1. Structural Modeling Diagrams
Structure diagrams define the static architecture of a model. They are used to model the ‘things’ that make up a model – the classes, objects, interfaces and physical components. In addition, they are used to model the relationships and dependencies between elements.
2. Behavioral Modeling Diagrams
Behavior diagrams capture the varieties of interaction and instantaneous states within a model as it ‘executes’ over time; tracking how the system will act in a real-world environment, and observing the effects of an operation or event, including its results.
The underlying premise of UML is that no one diagram can capture the different elements of a system in its entirety. Hence, UML is made up of nine diagrams that can be used to model a system at different points of time in the software life cycle of a system.
The UML diagrams are:
Use case diagram: The use case diagram is used to identify the primary elements and processes that form the system. The primary elements are termed as “actors” and the processes are called “use cases.” The use case diagram shows which actors interact with each use case.
Class diagram: The class diagram is used to refine the use case diagram and define a detailed design of the system. The class diagram classifies the actors defined in the use case diagram into a set of interrelated classes. The relationship or association between the classes can be either an “is-a” or “has-a” relationship. Each class in the class diagram may be capable of providing certain functionalities. These functionalities provided by the class are termed “methods” of the class. Apart from this, each class may have certain “attributes” that uniquely identify the class.
Object diagram: The object diagram is a special kind of class diagram. An object is an instance of a class. This essentially means that an object represents the state of a class at a given point of time while the system is running. The object diagram captures the state of different classes in the system and their relationships or associations at a given point of time.
State diagram: A state diagram, as the name suggests, represents the different states that objects in the system undergo during their life cycle. Objects in the system change states in response to events. In addition to this, a state diagram also captures the transition of the object’s state from an initial state to a final state in response to events affecting the system.
Activity diagram: The process flows in the system are captured in the activity diagram. Similar to a state diagram, an activity diagram also consists of activities, actions, transitions, initial and final states, and guard conditions.
Sequence diagram: A sequence diagram represents the interaction between different objects in the system. The important aspect of a sequence diagram is that it is time-ordered. This means that the exact sequence of the interactions between the objects is represented step by step. Different objects in the sequence diagram interact with each other by passing “messages”.
Collaboration diagram: A collaboration diagram groups together the interactions between different objects. The interactions are listed as numbered interactions that help to trace the sequence of the interactions. The collaboration diagram helps to identify all the possible interactions that each object has with other objects.
Component diagram: The component diagram represents the high-level parts that make up the system. This diagram depicts, at a high level, what components form part of the system and how they are interrelated. A component diagram depicts the components culled after the system has undergone the development or construction phase.
Deployment diagram: The deployment diagram captures the configuration of the runtime elements of the application. This diagram is by far most useful when a system is built and ready to be deployed.
Now that we have an idea of the different UML diagrams, let us see if we can somehow group together these diagrams to enable us to further understand how to use them.