Today's mission involves using multiple Object-Oriented Programming (OOP) principles to tackle complex tasks. When principles like Encapsulation, Abstraction, Polymorphism, and Composition are blended, the resulting code becomes streamlined and easier to manage.
Our goal is to dissect two real-world examples, gaining insights into how these principles can seamlessly orchestrate solutions.
Let's design an online library system to reinforce our understanding of Encapsulation and Polymorphism. Encapsulation will help us protect the attributes of books, members, and transactions, ensuring they are accessible in a controlled manner. Polymorphism will demonstrate its power by enabling a single interface to represent different underlying forms, such as digital and print versions of books.
Kotlin1// Data class for different types of library users 2data class Member(val name: String) { 3 fun checkOutBook(book: Book) { 4 println("$name checked out ${book.getBookType()} book ${book.title}.") 5 } 6} 7 8// Base class for different types of books 9abstract class Book(open val title: String) { 10 abstract fun getBookType(): String 11} 12 13// Inherits from Book, represents a digital book 14class DigitalBook(override val title: String) : Book(title) { 15 override fun getBookType() = "Digital" 16} 17 18// Inherits from Book, represents a physical book 19class PhysicalBook(override val title: String) : Book(title) { 20 override fun getBookType() = "Physical" 21} 22 23// Library class that manages members and books 24class Library { 25 private val members = mutableListOf<Member>() 26 private val books = mutableListOf<Book>() 27 28 fun addMember(member: Member) { 29 members.add(member) 30 } 31 32 fun addBook(book: Book) { 33 books.add(book) 34 } 35} 36 37fun main() { 38 val myLibrary = Library() 39 40 val alice = Member("Alice") 41 val bob = Member("Bob") 42 43 myLibrary.addMember(alice) 44 myLibrary.addMember(bob) 45 46 val digitalBook = DigitalBook("The Kotlin Handbook") 47 val physicalBook = PhysicalBook("Learning Kotlin Design Patterns") 48 49 myLibrary.addBook(digitalBook) 50 myLibrary.addBook(physicalBook) 51 52 alice.checkOutBook(digitalBook) // Prints: Alice checked out Digital book The Kotlin Handbook. 53 bob.checkOutBook(physicalBook) // Prints: Bob checked out Physical book Learning Kotlin Design Patterns. 54}
In this code snippet, Encapsulation is clearly observed through the class structures and the controlled access to their attributes. Polymorphism is vividly illustrated by how both DigitalBook and PhysicalBook classes inherit from the Book class but provide their own implementations of the getBookType method. This setup allows objects of DigitalBook and PhysicalBook to be used interchangeably when a book's type needs to be identified, demonstrating polymorphism's capability to work with objects of different classes through a common interface.
Encapsulationensures that details about members and books are well-contained within their respective classes.Polymorphismshowcases flexibility by treating different book types uniformly, making the system more adaptive and scalable.
Next, we'll develop a shape-drawing application capable of drawing various shapes. For this, we'll employ the principles of Abstraction and Composition.
Abstractionsimplifies the complexity associated with drawing different shapes.Compositiontakes care of composite shapes.
Here's how we translate these principles into our shape-drawing application:
Kotlin1// Define the basic Shape class 2abstract class Shape { 3 // Abstract method that will be implemented in each subclass 4 abstract fun draw() 5} 6 7// Define the Circle class 8class Circle : Shape() { 9 // Implement the draw method for circle 10 override fun draw() { 11 println("Drawing a circle.") 12 } 13} 14 15// Define the Square class 16class Square : Shape() { 17 // Implement the draw method for square 18 override fun draw() { 19 println("Drawing a square.") 20 } 21} 22 23// Define the ShapeComposite class 24class ShapeComposite : Shape() { 25 // Initialize with an empty list of shapes 26 private val shapes = mutableListOf<Shape>() 27 28 // Add a new shape to the composite 29 fun addShape(shape: Shape) { 30 shapes.add(shape) 31 } 32 33 // Implement the draw method to draw each shape in composite 34 override fun draw() { 35 for (shape in shapes) { 36 shape.draw() 37 } 38 } 39} 40 41fun main() { 42 val circle = Circle() 43 val square = Square() 44 45 // Drawing individual shapes 46 circle.draw() // Output: Drawing a circle. 47 square.draw() // Output: Drawing a square. 48 49 // Create a ShapeComposite instance for composite shapes 50 val compositeShape = ShapeComposite() 51 52 // Add individual shapes to the composite 53 compositeShape.addShape(circle) 54 compositeShape.addShape(square) 55 56 // Drawing the composite shape 57 compositeShape.draw() 58 // Output: 59 // Drawing a circle. 60 // Drawing a square. 61}
- Abstraction: In this example, the
Shapeclass is abstract. We don’t care about the specific details of how each shape is drawn here, but we know that each shape must have adraw()method. The abstract class helps us define this rule for all shapes. - Composition: The
ShapeCompositeclass demonstrates composition by combining multiple shapes. It can hold and draw multiple shapes together. Composition is used when one object (a composite shape) contains other objects (individual shapes).
Well done! You combined multiple OOP principles to respond to complex tasks. By dissecting real-world examples, we understood how these principles found their applications. Now, it's time to put this knowledge to work. Practice fortifies concepts, transforming knowledge into expertise. So, let's get coding!