What is SOLID?

SOLID is a set of five object-oriented design principles that can help you write more maintainable, flexible, and scalable code based on well-designed, cleanly structured classes. These principles are a fundamental part of object-oriented design best practices.

Principles usage

You can see here a example of usage of the principles with some DO’s and DONT’s.

Single responsibility

Class has one job to do. Each change in requirements can be done by changing just one class.

DON’T

// Single Responsibility Principle Violation
class SystemManager { 
	fun addUser(user: User) {...} 
	fun deleteUser(user: User) {...} 
	fun sendNotification(notification:String) {...}
}

DO

// Single Responsibility Principle Correct Usage
class NotificationManager() { 
	fun sendNotification(notification: String) {...}
}
class UserManager { 
	fun addUser(user: User) {...} 
	fun deleteUser(user: User) {...}
}

Open/closed principle

Class is happy (open) to be used by others. Class is not happy (closed) to be changed by others.

DON’T

// Open/Closed Principle Violation
class Shape(val type: String, val width: Double, val height: Double)
fun calculateArea(shape: Shape): Double { 
	if (shape.type == "rectangle") { 
		return shape.width * shape.height } 
	else if (shape.type == "circle") { 
		return Math.PI * shape.width * shape.width 
	} 
	return 0.0
}

DO

// Open/Closed Principle Correct Usage
interface Shape { 
	fun area(): Double
}

class Rectangle(val width: Double, val height: Double) : Shape { 
	override fun area() = width * height
}

class Circle(val radius: Double) : Shape { 
	override fun area() = Math.PI * radius * radius
}

fun calculateArea(shape: Shape) = shape.area()