Introduction To SOLID Principles

SOLID is an acronym for 5 important design principles when doing OOP (Object Oriented Programming).

These 5 principles were introduced by Robert C. Martin (Uncle Bob), in his 2000 paper Design Principles and Design Patterns.

The actual SOLID acronym was, however, identified later by Michael Feathers.

The intention of these principles is to make software designs more understandable, easier to maintain and easier to extend. As a software engineer, these 5 principles are essential to know!

This tutorial will take you through step by step approach and examples using Java while learning Design Pattern concepts.

What is SOLID?

 SOLID stands for

  1. S- Single Responsible Principle (SRP).
  2. O- Open Closed Principle (OSP).
  3. L- Liskov Substitute Principle (LSP).
  4. I- Interface Segregation Principle (ISP).
  5. D- Dependency Inversion Principle (DIP).

Benefits of SOLID

  1. It makes software design more understandable, flexible, and maintainable.
  2. Best suitable principle can be used as per project requirement.
  3. Loosely coupled.
  4. Parallel Development.
  5. Testability.
  6. Code becomes smaller and cleaner
  7. Maintainability – Large Systems or Growing systems become complicated and difficult to maintain. This Principle helps us to create a maintainable system. A maintainable system is very important in industries.

S – Single Responsible Principle (SRP)

In programming, the Single Responsibility Principle states that every module or class should have responsibility over a single part of the functionality provided by the software.

A class should only have one responsibility. Furthermore, it should only have one reason to change

How does this principle help us to build better software? Let’s see a few of its benefits:

  1. Testing – A class with one responsibility will have far fewer test cases
  2. Lower coupling – Less functionality in a single class will have fewer dependencies
  3. Organization – Smaller, well-organized classes are easier to search than monolithic ones

Problem

To understand the SRP principle, let’s assume we have working on an application which involve working with employees. We have an interface IEmployeeStore and it’s implementation  EmployeeStore  which have following methods.

public class Employee 
{
    public string Name { get; set;}
    public string Address { get; set;}
    public string Organization { get; set;}
}
public interface IEmployeeStore 
{
    public Employee GetEmployeeById(Long id);
     
    public void AddEmployee(Employee employee);
     
    public void SendEmail(Employee employee, String content);
}
public class EmployeeStore : IEmployeeStore 
{
    public Employee GetEmployeeById(Long id) 
    {
        return null;
    } 

    public void AddEmployee(Employee employee)
    {        
    }

    public void SendEmail(Employee employee, String content)
    {
    }
}

Above class seems good on any normal application. using EmployeeStore, are able to get/add employees and send email to them.

Now suppose after product release, we got requirement that email content can be of two types i.e. HTML and text. Above class supprt only text content. What you will do?

One way to solve this issue is create another method sendHtmlEmail() – but what happens when we are asked to support different protocols for sending emails for both content types. Overall class will look very ugly and difficult to read and maintain.

And there is always a chance that during modification, some developer can change the logic used for get/add employee methods if they are shared.

Solution

public interface IEmployeeStore 
{
    public Employee GetEmployeeById(Long id);
     
    public void AddEmployee(Employee employee);
}
public class EmployeeStore : IEmployeeStore 
{
    public Employee GetEmployeeById(Long id) 
    {
        return null;
    } 

    public void AddEmployee(Employee employee)
    {        
    }
}
public interface IEmailSender 
{
    public void SendEmail(Employee employee, IEmailContent content);
}
public class EmailSender : IEmailSender
{
    public void SendEmail(Employee employee, IEmailContent content) 
    {       
        //logic
    }
}
public interface IEmailContent
{
    public String type;
    public String content;
}
public class EmailContent : IEmailContent 
{
    public String type;
    public String content;
}

Now if we want to change the email functionality, we will change EmailSender class only. Any change to employee CRUD operations will happen in EmployeeStore only. Any change in one capability will not change other one by mistake. They are not more easy to read and maintain as well.

O- Open Closed Principle (OSP)

Bertrand Meyer defines OCP in 1988, “Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.”

Let’s now discuss on what are the problems this principle solved, 

Simply put, classes should be open for extension, but closed for modification. In doing so, we stop ourselves from modifying existing code and causing potential new bugs in an otherwise happy application.

Of course, the one exception to the rule is when fixing bugs in existing code

OCP says a class should be,

  1. Open for extension – means we need to create a base class and that class will be available for extension. This class should have common functionality.
  2. Close for Modification – Instead of changing the base class, we will extend the base class and add/modify type-specific coding in the derived class.
O- Open Closed Principle (OSP)

Let’s explore the concept further with a quick code example. As part of a new project, imagine we’ve implemented a Guitar class.

It’s fully fledged and even has a volume knob:

public class Guitar {

    private String make;
    private String model;
    private int volume;

    //Constructors, getters & setters
}

We launch the application, and everyone loves it. However, after a few months, we decide the Guitar is a little bit boring and could do with an awesome flame pattern to make it look a bit more ‘rock and roll’.

At this point, it might be tempting to just open up the Guitar class and add a flame pattern – but who knows what errors that might throw up in our application.

Instead, let’s stick to the open-closed principle and simply extend our Guitar class:

public class SuperCoolGuitarWithFlames extends Guitar {

    private String flameColor;

    //constructor, getters + setters
}

By extending the Guitar class we can be sure that our existing application won’t be affected.

L-Liskov Substitution Principle (LSK)

Barbara Liskov introduced this principle in 1987 in the conference (Data abstraction and hierarchy) hence it is called the Liskov Substitution Principle (LSK). This principle is just an extension of the Open Close principle.

Robert C Martin also defines this principle. His definition of LSK is “Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it”.

Let’s just jump straight to the code to help wrap our heads around this concept:

public interface Car {

    void turnOnEngine();
    void accelerate();
}

Above, we define a simple Car interface with a couple of methods that all cars should be able to fulfill – turning on the engine, and accelerating forward.

Let’s implement our interface and provide some code for the methods:

public class MotorCar implements Car {

    private Engine engine;

    //Constructors, getters + setters

    public void turnOnEngine() {
        //turn on the engine!
        engine.on();
    }

    public void accelerate() {
        //move forward!
        engine.powerOn(1000);
    }
}

As our code describes, we have an engine that we can turn on, and we can increase the power. But wait, its 2019, and Elon Musk has been a busy man.

We are now living in the era of electric cars:

public class ElectricCar implements Car {

    public void turnOnEngine() {
        throw new AssertionError("I don't have an engine!");
    }

    public void accelerate() {
        //this acceleration is crazy!
    }
}

By throwing a car without an engine into the mix, we are inherently changing the behavior of our program. This is a blatant violation of Liskov substitution and is a bit harder to fix than our previous 2 principles.

One possible solution would be to rework our model into interfaces that take into account the engine-less state of our Car.

I-  Interface Segregation

Robert C Martin’s definition of ISP, “Clients should not be forced to depend upon interfaces that they do not use.”

The ‘I ‘ in SOLID stands for interface segregation, and it simply means that larger interfaces should be split into smaller ones. By doing so, we can ensure that implementing classes only need to be concerned about the methods that are of interest to them.

For this example, we’re going to try our hands as zookeepers. And more specifically, we’ll be working in the bear enclosure.

Let’s start with an interface that outlines our roles as a bear keeper:

public interface BearKeeper {
    void washTheBear();
    void feedTheBear();
    void petTheBear();
}

As avid zookeepers, we’re more than happy to wash and feed our beloved bears. However, we’re all too aware of the dangers of petting them. Unfortunately, our interface is rather large, and we have no choice than to implement the code to pet the bear.

Let’s fix this by splitting our large interface into 3 separate ones:

public interface BearCleaner {
    void washTheBear();
}

public interface BearFeeder {
    void feedTheBear();
}

public interface BearPetter {
    void petTheBear();
}

Now, thanks to interface segregation, we’re free to implement only the methods that matter to us:

public class BearCarer implements BearCleaner, BearFeeder {

    public void washTheBear() {
        //I think we missed a spot...
    }

    public void feedTheBear() {
        //Tuna Tuesdays...
    }
}

And finally, we can leave the dangerous stuff to the crazy people:

public class CrazyPerson implements BearPetter {

    public void petTheBear() {
        //Good luck with that!
    }
}

Going further, we could even split our BookPrinter class from our example earlier to use interface segregation in the same way. By implementing a Printer interface with a single print method, we could instantiate separate ConsoleBookPrinter and OtherMediaBookPrinter classes.

D – Dependency Inversion

Robert C Martin has defined DIP as, “High-level modules should not depend on low-level modules. Both should depend on abstractions. Abstractions should not depend on details. Details should depend on abstractions”. 

Let’s try to understand this principle with an example.

To demonstrate this, let’s go old-school and bring to life a Windows 98 computer with code:

public class Windows98Machine {}

But what good is a computer without a monitor and keyboard? Let’s add one of each to our constructor so that every Windows98Computer we instantiate comes pre-packed with a Monitor and a StandardKeyboard:

public class Windows98Machine {

    private final StandardKeyboard keyboard;
    private final Monitor monitor;

    public Windows98Machine() {
        monitor = new Monitor();
        keyboard = new StandardKeyboard();
    }

}

This code will work, and we’ll be able to use the StandardKeyboard and Monitor freely within our Windows98Computer class. Problem solved? Not quite. By declaring the StandardKeyboard and Monitor with the new keyword, we’ve tightly coupled these 3 classes together.

Not only does this make our Windows98Computer hard to test, but we’ve also lost the ability to switch out our StandardKeyboard class with a different one should the need arise. And we’re stuck with our Monitor class, too.

Let’s decouple our machine from the StandardKeyboard by adding a more general Keyboard interface and using this in our class:

public interface Keyboard { }
public class Windows98Machine{

    private final Keyboard keyboard;
    private final Monitor monitor;

    public Windows98Machine(Keyboard keyboard, Monitor monitor) {
        this.keyboard = keyboard;
        this.monitor = monitor;
    }
}

Here, we’re using the dependency injection pattern here to facilitate adding the Keyboard dependency into the Windows98Machine class.

Let’s also modify our StandardKeyboard class to implement the Keyboard interface so that it’s suitable for injecting into the Windows98Machine class:

public class StandardKeyboard implements Keyboard { }

Now our classes are decoupled and communicate through the Keyboard abstraction. If we want, we can easily switch out the type of keyboard in our machine with a different implementation of the interface. We can follow the same principle for the Monitor class.

Excellent! We’ve decoupled the dependencies and are free to test our Windows98Machine with whichever testing framework we choose.

Conclusion

In this tutorial, we’ve taken a deep dive into the SOLID principles of object-oriented design.

We started with a quick bit of SOLID history and the reasons these principles exist.

Letter by letter, we’ve broken down the meaning of each principle with a quick code example that violates it. We then saw how to fix our code and make it adhere to the SOLID principles.

Special Thanks to : Baeldung

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