Adventures in Machine Learning

Mastering SOLID Principles: Building Maintainable and Scalable Code

Introduction to Object-Oriented Design and SOLID Principles

Object-oriented programming (OOP) facilitates creating software applications using a system of classes, objects, and methods. Each object type is designed to mimic real-world objects.

These objects encapsulate data and functions, making them a fundamental part of OOP. However, without a specific planning process, OOP may result in complex, convoluted code that is challenging to maintain, scale, and reuse.

This is where object-oriented design (OOD) comes in. The purpose of OOD is to design classes and objects that are well-structured, highly maintainable, and reusable.

SOLID principles are a set of guidelines that help achieve this goal.

Importance of Planning OOD and Using SOLID Principles

Planning OOD using SOLID principles has become a crucial part of writing maintainable, flexible, and scalable code. By leveraging design patterns, developers can structure their project in a way that facilitates easy maintenance and reuse.

OOD best practices, such as separating responsibility, create cleanly structured classes that alleviate the need for making indirect changes, which can impact other parts of the codebase. A well-defined OOD also leads to a more modular codebase, which, in turn, facilitates testing and debugging.

Single-Responsibility Principle (SRP)

The single-responsibility principle (SRP) states that a class should only have one responsibility and one reason to change. It is one of the fundamental SOLID principles.

A class should do one thing only, and it should do it well. The rationale is to keep the class well-focused and decoupled from other code.

The separation of concerns makes a code base easier to maintain, change, and reuse.

With the SRP, each class does only one thing, and it delegates related responsibilities to other classes.

This means that class code is cleaner, simpler, and more reusable. It also means that a change in responsibility can only affect one class, rather than multiple classes.

Breaking down a class based on this principle can push developers towards creating a more modular and flexible codebase.

SRP Example

One example of SRP violation is a FileManager class that has to read, write, compress, and decompress a file. The responsibilities of reading and writing the file are related, but the compression and decompression of files are separate responsibilities.

Suppose the FileManager class needs to support another compression method. In that case, it would violate the SRP since adding a new feature means changing the FileManager class, which should only be responsible for file manipulation.

The solution is to create a ZipFileManager class that deals only with the compression and decompression of files.

ZipFileManager class handles all requests related to file compression and decompression instead of FileManager class taking on multiple responsibilities.

Splitting out responsibilities in this manner simplifies maintenance since different areas of concern have been separated out via the SRP principle.


Incorporating SOLID principles into OOD has become critical in the modern era of software development. The SRP is just one of the facets of object-oriented best practices.

Other techniques include open-closed, dependency inversion, interface segregation, and Liskov substitution principles. By applying these principles correctly, developers can create cleaner, more maintainable, and more scalable codebases.

Ultimately, adhering to these principles will ensure that developer teams create successful software projects that meet the needs of their clients.

Open-Closed Principle (OCP)

The open-closed principle (OCP) states that software entities (classes, modules, functions) should be open for extension but closed for modification. The idea behind OCP is that once a specific component has been released, it should not be modified.

Instead, new features should be added via extension. This allows for existing code to remain untouched while new features and functionality get added.

By resisting changes to a component, we can maintain stability in the code, keep the existing codebase well-tested, and prevent the introduction of new bugs. At the same time, by leaving a component open to extension, we can integrate new functionality without having to modify the existing code.

The use of multiple interfaces and implementation inheritance are techniques that help apply OCP in OOD.

OCP Example

An example of OCP violation is a Shape class that has a .calculate_area() method. Suppose we now want to add a Circle and Rectangle class, which inherit the shape class.

This works fine until we add a Square class since it has the same properties as a rectangle and needs to share the same .calculate_area() method. If we modify the Rectangle class to allow for this, we are violating the OCP.

To correct this OCP violation, we introduce an Abstract Base Class (ABC), which defines an interface that the classes should implement. If the classes implement the interface, the .calculate_area() function can run independent of the actual implementation.

Additionally, we use an implementation inheritance, which creates a template for any class that implements the ABC so that creating new classes that share common properties can be more streamlined.

Liskov Substitution Principle (LSP)

The Liskov substitution principle (LSP) states that a derived class must be a substitute for its base class. This means that a derived class should be interchangeable with its base class, regardless of the calling code.

The principle is essential for ensuring that inheritance functions correctly.

To practically apply LSP, you must ensure that an object of a derived class must behave in precisely the same way as an object of its base class.

Anything else will break code that assumes that a base class object is in use. LSP is a critical aspect of ensuring code correctness and reliability, especially when using polymorphism.

LSP Example

An example of an LSP violation is using a square class derived from a Rectangle class. Since a square’s height and width should always be equal, setting a width value will also set the height value, and vice versa, thereby violating the rectangle class’ LSP.

If a calling function expects a rectangle, it may not anticipate encountering a square since it violates the LSP.

To correct this LSP violation, we could instead avoid using a derived class of the rectangle class.

Alternatively, we could use a sibling class type, where the square and rectangle classes inherit from a shared superclass, rather than via direct inheritance. By using this method, we avoid the transformation of a base shape into another that violates the defining characteristics of the base class.


OCP and LSP are some of the key principles that guide object-oriented programming. These principles, when applied correctly, help developers create maintainable, flexible, and scalable code bases.

Adhering to OCP ensures that developers can add new functionality without modifying existing code, thereby maintaining the stability of the codebase. LSP helps ensure that derived classes correctly inherit base class attributes and exhibit the same behavior as their base classes.

By using OCP and LSP, developers can create efficient and stable codebases that stand the test of time.

Interface Segregation Principle (ISP)

The interface segregation principle (ISP) is an OOD principle that states that no client should be forced to implement methods that it does not use. The key idea of ISP is to keep interfaces focused and specific to what is needed in a particular scenario while reducing the amount of coupling between classes.

In practice, ISP removes unnecessary methods and splits larger interfaces into smaller, more specialized ones, which are more likely to be used.

The goal of ISP is to reduce interface “bloat,” which can occur when a class has a large number of methods that are not used in all scenarios.

Reducing interface bloat makes it easier to change the interfaces without affecting the client code. This, in turn, enables better reuse of code and fewer potential problems with implementation errors.

Using the right design patterns and techniques can help enforce ISP.

ISP Example

One example of an ISP violation is a multi-function interface that defines a set of methods that are not always needed in the client code. Suppose there is a Shape interface that defines .draw() and .erase() methods.

A Square class that implements this Shape interface is expected to have both of these methods. However, a Line class that also implements this Shape interface only needs the .draw() method.

This introduces extra code complexity since the Line class now has to implement an unneeded method, which will have no effect on client code. To correct this ISP violation, we introduce a Light interface, which is tailored to a specific method set, to complement the originally created heavy interface.

Through the use of Abstract Base Class (ABC) and implementation inheritance, we’re able to achieve the benefits of an interface method without creating coupling issues. Light interfaces require clients to only use the specific methods that they require, rather than requiring the implementation of superfluous methods.

This helps keep our codebase clean and concise, creating proper separation of interfaces for each method.


In summary, the interface segregation principle is a crucial aspect of OOD. It enables better reuse of code and fewer implementation errors.

By appropriately separating interfaces, we ensure that clients are not dependent on unneeded methods and only use what is relevant to their use case. By reducing interface bloat, we can make it easier to change interfaces, maintain code stability, and reduce potential problems that arise from unnecessary methods.

Applying ISP improves the overall code quality and makes it easier to modify as required, ensuring that developers can create effective codebases that can scale with time and maintain quality. In this article, we have covered the basic principles of Object-Oriented Design, specifically SOLID principles.

We started by defining OOD and the purpose of SOLID principles, which make our codebase well-structured, highly maintainable, and reusable. We went in-depth into each SOLID principle, including Single-Responsibility Principle, Open-Closed Principle, Liskov Substitution Principle, and Interface Segregation Principle, with examples of violations and solutions to refactoring them.

The overall takeaway is that applying OOD best practices is critical to creating maintainable, flexible, and scalable codebases. Adhering to these principles will ensure that developer teams can create successful software projects that meet the needs of their clients.

The ultimate goal of SOLID principles is to make sure that code is intuitive, clean, and effective.