πŸ”₯ Low-Level Design: Class-Level Diagrams

πŸ”₯ Low-Level Design: Class-Level Diagrams

A Beginner Friendly approach to understand Class-Level Diagrams

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Lovepreet Singh
Β·Dec 15, 2022Β·

3 min read

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Table of contents

  • πŸ“ Class-Level diagram Representation
  • πŸ“ Association (Has-a Relation)
  • Aggregation
  • Composition
  • πŸ“ Generalization(Is-a Relation)
  • πŸ“ Dependency
  • πŸ‘‰ Multiplicity

πŸ™‚ Well this is the Second blog of our Series on Low-Level Design. Here, we will look into how to draw class-level diagrams. Whenever we design an application, we must draw a class-level diagram before implementation. While designing we do follow SOLID Principles too.

🀜Today we'll see the different components of a class diagram followed by some projects on LLD.

What Are The Best Software Engineering Principles? - Blog | luminousmen

πŸ“ Class-Level diagram Representation

πŸ‘‰ Above diagram represents a class that includes Class Name, Attributes, and Methods with Access Modifiers like public, protected, private, etc.

  • Due to this diagram we can directly code the class and can define the methods easily

Now, we'll see different links representing the relationship between different classes.

πŸ“ Association (Has-a Relation)

πŸ™‚ We have two types of Associations, Bidirectional and Unidirectional (The first two in the above Diagram). Bidirectional means both classes can call each other (A----B) while directed or unidirectional association means, One class has access to the other but vice versa is not true. Forex. A--->B, A can access B but B couldn't. In Association We have Aggregation and Composition.

Note:- Aggregation and Composition are both types of Association. Both differ only due to the lifecycle dependency.

UML Association vs Aggregation vs Composition - Javatpoint


Bank has an instance of a Customer and It is mostly called as a loose association because the life cycle of both classes is independent or when Bank dies customer may live on. Aggregation is a has-a relationship. It is represented by a hollow diamond arrow.


Here, Kindly note the difference. Whenever we are initializing a Car Instance Engine will also be initialized which means the Engine can't exist without Car and both are highly dependent on each other. It is represented by a filled diamond arrow.

πŸ“ Generalization(Is-a Relation)

πŸ”₯ Here, you can see that we have two is-a relations (Inheritance and Realization) But it is quite clear from the diagram that the Left side is inheritance and the right side is an interface inheritance.

πŸ“ Dependency

Here, we can see that the Player state is dependent on the state of Die i.e. Die roll can change the Player state. Which means the Player has a dependency on the Die.

πŸ‘‰ Multiplicity

It tells us how many instances of a class participating in a relationship.

The above example is a one-to-many relationship, meaning One State can have many Cities.

Similarly, we have:-

  • One-One

  • One-Many & Many-One

  • Many-Many

πŸ™‚ Below is the summary of our Class-level diagram and its components:-

πŸ™‚ Follow for more, We'll have some Projects too involving Low-Level Design.