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Notes > Software Engineering > Unified Modelling Language (UML)

UML is a modelling language which can be used to create abstract diagrams and models that represent a real (or imaginary) system. A model is an overall representation of a system which shows all the relevant details of the system at a high level. Diagrams define smaller parts of the overall model in more detail.

Like many other modelling languages that have developed over time, UML has many advantages when it comes to modelling a system:

  • Simplicity in its representation
  • Consistency across a whole system model which leads to less problems related to misunderstandings and discrepancies
  • Hierarchial abstraction - the system can be represented in more detail at lower levels

UML diagrams (also referred to as graphs) consist of four types of UML element:

  • Icons
  • Paths
  • Symbols
  • Strings

Processes in UML are a type of symbol which are represented on a graph using a rectangle which has rounded ends. Decision points are represented using diamond shapes. Strings are used at decision points to show which path represents which outcome from the decision.

The start state within a UML diagram is represented by a solid black dot. A UML end state is represented by a solid black dot surrounded by a white ring surrounded by a black ring.

Classes and Objects in UML

At a lower level in systems analysis, classes and the relationships between these classes are defined in order to reach a final system design. UML sets out the standards for drawing classes.

It is important to note a difference between objects and classes in UML. Objects will have a name that starts with a lower case character whereas a class name starts with a capital letter. The following image shows how a class is drawn in UML:

UML Class / Classes

Instances of this class (objects) will have a "state" which is determined by the data values encapsulated within the object. An Object Identifier (OID) is always unique and does not change. An OID is assigned by the system to an object when it is created. Reference attributes (also known as "complex" attributes) within objects specify relationships with other objects. These attributes refer to other objects.

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