Modeling

This is a printer-friendly version. It omits exercises, optional topics (i.e., four-star topics), and other extra content such as learning outcomes.

Introduction

What

A model is a representation of something else.

A class diagram is a model that represents a software design.

A class diagram is a diagram drawn using the UML modelling notation.
An example class diagram:

A model provides a simpler view of a complex entity because a model captures only a selected aspect. This omission of some aspects implies models are abstractions.

 

Design → Design Fundamentals → Abstraction →

What

Abstraction is a technique for dealing with complexity. It works by establishing a level of complexity we are interested in, and suppressing the more complex details below that level.

The guiding principle of abstraction is that only details that are relevant to the current perspective or the task at hand needs to be considered. As most programs are written to solve complex problems involving large amounts of intricate details, it is impossible to deal with all these details at the same time. That is where abstraction can help.

Ignoring lower level data items and thinking in terms of bigger entities is called data abstraction.

Within a certain software component, we might deal with a user data type, while ignoring the details contained in the user data item such as name, and date of birth. These details have been ‘abstracted away’ as they do not affect the task of that software component.

Control abstraction abstracts away details of the actual control flow to focus on tasks at a simplified level.

print(“Hello”) is an abstraction of the actual output mechanism within the computer.

Abstraction can be applied repeatedly to obtain progressively higher levels of abstractions.

An example of different levels of data abstraction: a File is a data item that is at a higher level than an array and an array is at a higher level than a bit.

An example of different levels of control abstraction: execute(Game) is at a higher level than print(Char) which is at a higher than an Assembly language instruction MOV.

Abstraction is a general concept that is not limited to just data or control abstractions.

Some more general examples of abstraction:

  • An OOP class is an abstraction over related data and behaviors.
  • An architecture is a higher-level abstraction of the design of a software.
  • Models (e.g., UML models) are abstractions of some aspect of reality.

A class diagram captures the structure of the software design but not the behavior.

Multiple models of the same entity may be needed to capture it fully.

In addition to a class diagram (or even multiple class diagrams), a number of other diagrams may be needed to capture various interesting aspects of the software.

How

In software development, models are useful in several ways:

a) To analyze a complex entity related to software development.

Some examples of using models for analysis:

  1. Models of the problem domain (i.e. the environment in which the software is expected to solve a problem) can be built to aid the understanding of the problem to be solved.
  2. When planning a software solution, models can be created to figure out how the solution is to be built. An architecture diagram is such a model.

b) To communicate information among stakeholders. Models can be used as a visual aid in discussions and documentations.

Some examples of using models to communicate:

  1. We can use an architecture diagram to explain the high-level design of the software to developers.
  2. A business analyst can use a use case diagram to explain to the customer the functionality of the system.
  3. A class diagram can be reverse-engineered from code so as to help explain the design of a component to a new developer.

An architecture diagram depicts the high-level design of a software.

c) As a blueprint for creating software. Models can be used as instructions for building software.

Some examples of using models to as blueprints:

  1. A senior developer draws a class diagram to propose a design for an OOP software and passes it to a junior programmer to implement.
  2. A software tool allows users to draw UML models using its interface and the tool automatically generates the code based on the model.
 Model Driven Development tangential

Model-driven development (MDD), also called Model-driven engineering, is an approach to software development that strives to exploit models as blueprints. MDD uses models as primary engineering artifacts when developing software. That is, the system is first created in the form of models. After that, the models are converted to code using code-generation techniques (usually, automated or semi-automated, but can even involve manual translation from model to code). MDD requires the use of a very expressive modeling notation (graphical or otherwise), often specific to a given problem domain. It also requires sophisticated tools to generate code from models and maintain the link between models and the code. One advantage of MDD is that the same model can be used to create software for different platforms and different languages. MDD has a lot of promise, but it is still an emerging technology

Further reading:

UML Models

The following diagram uses the class diagram notation to show the different types of UML diagrams.


source:https://en.wikipedia.org/

Modeling Structures

OO Structures

An OO solution is basically a network of objects interacting with each other. Therefore, it is useful to be able to model how the relevant objects are 'networked' together inside a software  i.e. how the objects are connected together.

Given below is an illustration of some objects and how they are connected together.  Note: the diagram uses an ad-hoc notation.

Note that these object structures within the same software can change over time.

Given below is how the object structure in the previous example could have looked like at a different time.

However, object structures do not change at random; they change based on a set of rules, as was decided by the designer of that software. Those rules that object structures need to follow can be illustrated as a class structure  i.e. a structure that exists among the relevant classes.

Here is a class structure (drawn using an ad-hoc notation) that matches the object structures given in the previous two examples. For example, note how this class structure does not allow any connection between Genre objects and Author objects, a rule followed by the two object structures above.

UML Object Diagrams are used to model object structures and UML Class Diagrams are used to model class structures of an OO solution.

Here is an object diagram for the above example:

And here is the class diagram for it:

Class Diagrams (Basics)

Classes form the basis of class diagrams.

Associations are the main connections among the classes in a class diagram.

The most basic class diagram is a bunch of classes with some solid lines among them to represent associations, such as this one.

An example class diagram showing associations between classes.

In addition, associations can show additional decorations such as association labels, association roles, multiplicity and navigability to add more information to a class diagram.

Here is the same class diagram shown earlier but with some additional information included:

Adding More Info to UML Models

UML notes can be used to add more info to any UML model.

Class Diagrams - Intermediate

A class diagram can also show different types of associations: inheritance, compositions, aggregations, dependencies.

Modeling inheritance

Modeling composition

Modeling aggregation

Modeling dependencies

A class diagram can also show different types of class-like entities:

Modeling enumerations

Modeling abstract classes

Modeling interfaces

Object Diagrams

Object diagrams can be used to complement class diagrams. For example, you can use object diagrams to model different object structures that can result from a design represented by a given class diagram.

Modeling Behaviors

Sequence Diagrams - Basic

Sequence Diagrams - Intermediate