What is a class diagram?

 

In software engineering, a class diagram is a type of UML diagram that shows the system's structure in terms of its classes, attributes, and method. It also shows us the relationships that exist among the classes. 

It was primarily made for developers to understand software requirements and describe detailed designs. For example, during the analysis and design phases of the development cycle, we can create class diagrams to capture the class structure and define the relationships between the classes and classifiers. 

We use various components when we draw a class diagram. Let's discuss some basic ones in this Answer.

Classes

A class is a central object in a system. We represent it by a rectangle with three sections. 

These sections are as follows:

  • Class name: We write the class name in the first compartment.

  • Class attributes: These are the object's attributes and characteristics. We write them in the second compartment. We show the attribute type after the colon.

  • Class operations (methods): The class operations or methods represent the class's behavior. We show them in the third compartment. We write the return type of the method after the colon.

Note: The last two compartments of the class are optional. A class with only the class name is known as a simple class.

Member access modifiers

The "++" and "-" symbols before the class attributes or methods are access modifiers or class visibility notations. The various access levels with their corresponding symbols are as follows:

  • Public (++)

  • Private (-)

  • Protected (#\#)

  • Package (~)

The package access level

Note: We use the package symbol (~) to group classes that are similar or related to each other. This helps make the diagram more readable. 

Relationships

In a class diagram, we require a relationship to exist between classes. The relationships that exist in a class diagram are as follows:

Association

Association shows the communication between two classes and is drawn by a continuous connecting line. We can specify the association's name and multiplicityThe number of objects that can take part in the relationship  at the top or bottom of the line.

The association between two classes

Inheritance

Inheritance or generalization is a directed relationship between a parent class and its descendants. The arrowhead is always directed towards the parent class. For example, a dog is a type of animal. So, the "dog" class has a generalization relationship with the "animal" class. 

An inheritance or generalization relationship between two classes

Realization

Realization refers to a relationship between an interface and the objects that implement the interface. We represent this by a dotted line with an empty arrowhead. We draw this from the class that defines the functionality to the class that implements the function. 

Two classes realizing an interface

Dependency

Dependency refers to when one class uses an object from another in its method, and that object is not stored in any field. In this case, changes in one class also entail changes in the other. 

A company is dependent on its employee

Aggregation

Aggregation is the relationship between classes where one class is a composite part of another. We show this by a line and an arrow between a parent and a child class, with a diamond pointing at the parent. For example, a professor may work in more than one department, but a department can't work for more than one university.

Aggregation between classes

Composite

Composite is a type of aggregation where classes that form the aggregator class are destroyed when the aggregator class is destroyed. We show this similarly to aggregation, but with a filled diamond. For example, we have two classes: a house, and a room. A room can only exist if a house exists, and vice versa. If the house is destroyed, so is the room.

Composite aggression

Conclusion

A class diagram is the backbone of object-oriented modeling and is a great way to show a system's structure. Making a class diagram makes the development process easier. This is because we can make changes to our development process early by looking at the different attributes and relationships between the objects.

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