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oop questions 1 ~ 27

1) What are the Object Oriented Principles (OOPs)?

Answer:

Class
Object
Encapsulation
Inheritance
Abstraction
Polymorphism

2) What is Class? How to declare it?

Answer:

Class is a Blueprint or Template / Logical Structure / Skeleton to create Objects
Class doesn’t own any memory. Once class is created we can create any number of objects out of that class.

class TV {

	var model: Int = 2023

	func start() {

	}

	func changeChannel() {

	}

}

3) What is Instance / Object?

Answer:

Object is an instance of a class.
Object is a Physical live implementation of the Class.
Collection of Properties and Methods.

class Example {

	// Properties
	var var1: String = ""
	var var2: String = ""

	// Methods
	func method1() {}
	func method2() {}

}

// instance, Object
let object: Example = Example()

4) What is Property?

Answer:

Properties are instance members of a Class / Structure which stores information about that Class / Structure

class ExampleClass {

	// Properties, Instance Members
	var var1: String = ""
	var var2: String = ""

	// Methods, Instance Members
	func method1() {}
	func method2() {}

}

// instance, Object
let object: ExampleClass = ExampleClass()

struct ExampleStruct {

	// Properties, Instance Members
	var var1: String = ""
	var var2: String = ""

	// Methods, Instance Members
	mutating func method1() {
		var1 = "Hell World"
	}
	func method2() {}

}

5) What are the 4 types of Properties?

Answer:

Stored Properties
Computed Properties
Lazy Properties
Static / Type Properties

class Person {

		// Stored Properties
		let firstName: String = "Paige"
		let lastName: String = "Shin"

		// Computed Properties
		var fullName: String {
			get {
				return "\(firstName), \(lastName)"
			}
		}

		// Lazy Properties
		lazy var hasGirlfriend: Bool = {
			return true
		}()

		// Static Properties
		static let Nationality: String = "Korea"

		// Type Properties
		class var age: Int = 32

}

6) What is stored Property?

Answer:

A property which holds direct value is known as Store Properties.

class Person {

		// Stored Properties
		let firstName: String = "Paige"
		let lastName: String = "Shin"

}

7) What is Computed Property?

Answer:

The value of computed property gets calculated when it is accessed and the value of computed property depends on the other properties of the Class / Structure.

class Person {

		// Stored Properties
		let firstName: String = "Paige"
		let lastName: String = "Shin"

		// Computed Properties
		var fullName: String {
				return "\(firstName), \(lastName)"
		}

}

8) What is Static Property or Type Property? How to declare them?

Answer:

Type properties are class level properties whose memory will be allocated only once.
Access type properties using ClassName.typePropertyName

class Person {

		// Type & Static Properties
		static let Nationality: String = "Korea"

}

print(Person.Nationality) // Korea

9) What is lazy property?

Answer:

A property whose memory will be allocated when the variable actually used.

class TV {

	lazy var logo: String = "Apple"

}

let tv: TV = TV()
print(tv.logo) // memory is allocated when called

10) static keyword vs class keyword

Answer:

static is used to declare Type Properties and Type Methods in Structures and Classes.
class keyword is only used in Classes to create Type methods but not Type Properties.

class Person {

    // Class Properties are not allowed..
    #warning("Compile Error")
    class var bloodType: String = "A"

		// Work around..
		class var born: Int {
			return 1991
		}

    // Works!
    class func sayMyName() {

    }

}

11) What are Property Observers?

Answer:

willSet and didSet are the property observers.
These blocks get triggered when there is a change in the value.

class Person {

 var age: Int? {
 	 willSet {
 		 print("Age is about to change: \(newValue)")
	 }
	 didSet {
		 print("Age is changed: \(oldValue)")
	 }
 }

}

let person: Person = Person()
person.age = 30
// Age is about to change: Optional(30)
// Age is changed: nil
person.age = 31
// Age is about to change: Optional(31)
// Age is changed: Optional(30)

12) What is get and set? How to declare get only or set property?

Answer:

The read and write access of a property can be declared by using get and set keywords.
You can access the value of get only property but cannot set the value.
A property must contain at least get.

class Person {

 var age = 0
 var days: Int {
		get {
			return age * 365
		}
 }

}

let person: Person = Person()
person.age = 5
print(person.days) // 1825

// Compile Error.. because it's declared with get only
person.days = 3000

class Person {

 var age = 0
 var days: Int {
		get {
			return age * 365
		}
		set {
			age = newValue / 365
		}
 }

}

let person: Person = Person()
person.age = 5
print(person.days) // 1825

person.days = 3000
print(person.days) // 2920

13) Can a property has only setter(set)?

Answer:

No, Every property must contain a getter. We can declare get only property but not set only property.

14) Difference between Class vs Structures?

Answer:

Structures are value types.
Classes are reference types.
Structures don’t support inheritance.
classes support inheritance.
Structures don’t support de-initializers. ( deinit )
Classes support deinitializers.
Structures don’t follow Reference Counting.
Classes follow Reference Counting.
Mutating Keyword is needed to modify the property values in Structure’s instance methods.
No need of mutating keyword to modify the class variable’s value.

15) What is Encapsulation? Example? Advantages?

Answer:

Encapsulation is process of binding instance variables / properties and methods together into a single unit.
Class is an example of encapsulation.

16) What is Inheritance?

Answer:

Deriving a class from another class or extending the functionality of a class by subclassing is known as Inheritance.

17) What are the advantages of Inheritance?

Answer:

Reusing the existing functionality.

18) What is Polymorphism?

Answer:

Polymorphism is the ability to appear in many forms.
There are two kinds of polymorphism:

Static Polymorphism / Overloading
Dynamic Polymorphism / Overriding

19) What is Dynamic Polymorphism / Overriding?

Answer)

In inheritance relationship, having the same method with the same name with difference in implementation in subclass and superclass is known as Overriding / Dynamic Polymorphism.

class Person {

 func run() {
     print("Running with speed of 10")
 }

}

class Man: Person {

 // Dynamic Polymorphism
 override func run() {
			//super.run() => this will run Person::class.run()
     print("Running with speed of 10")
 }

}

20) What is Static Polymorphism / Overloading?

Answer)

In a class having multiple methods with same name but different in
1. number of Parameters (or)
2. Order of Parameters (or)
3. Type of Parameters

class Person {

 func run() {
     print("Running with speed of 10")
 }

 // Overloading
 func run(speed: Int) {
     print("Running with \(speed)")
 }

 // Overloading
 func run(speed: Float) {
		 print("Running with \(speed)")
 }

 // Overloading
 func run(speed: Int, weight: Float) {

 }

 // Overloading
 func run(speed: Float, weight: Float) {

 }

}

21) What is Abstraction? How to achieve it?

Answer:

Abstraction is the process of hiding unnecessary functionality and exposing necessary functionality.
Abstraction can be achieved using Access Specifiers.

22) Does swift support multiple inheritances?

Answer:

No, Use protocols to achieve Multiple Inheritance.

23) How to call a method with some delay?

Answer:

self.perform(#selector(methodName), with nil, afterDelay: 5.0)
DispatchQueue

class Perosn: NSObject {

  func executeSomeMethod() {
		self.perform(#selector(callAfter5Seconds), with: nil, afterDelay: 5)
	}

  @objc func callAfter5Seconds() {

	}

}

class Perosn {

  func executeSomeMethod() {
		DispatchQueue.main.asyncAfter(deadline: .now() + 5) { [weak self] in
			self?.callAfter5Seconds()
		}
	}

  func callAfter5Seconds() {

	}

}

24) What is Base / Root class in Swift?

Answer:

Swift has no base class as in Objective-C.
Classes can be created without any base class / parent class. In such cases, the current class itself is the base class.
If a class is derived from any class, then the super class is its base class.

25) What is self and super keywords?

Answer:

self and super are keywords used in instance methods of a class.
self refers to the current class object in which the method is present. To refer the current class properties and methods use self.
super refers to the Parent class object of the current class. To refer its Parent class properties and methods use super.

class Person {

  var age: Int
	var name: String

	init(age: String, name: String) {
    self.age = age
    self.name = name
  }

  func sayMyName() {
			print("\(self.name)")
	}

  func printMyInfo() {
      self.sayMyName()
      print("and I'm \(self.age)")
  }

}

class Woman: Person {

  override func sayMyName() {
		super.sayMyName()
  }

}

26) Can we achieve Overriding without inheritance?

Answer:

No, overriding needs at least two classes and those two must be in inheritance relationship.

27) When do you prefer struct over class?

Answer:

In Swift, structs and classes give you value and reference-based constructs for your objects.
struct is preferred for objects designed for data storage like Array.
struct also helps remove memory issues when passing objects in a multi-threaded environment.
class, unlike struct, supports inheritance and is used more for containing logic like UIViewController.
Most standard library data objects in Swift, like String, Array, Dictionary, Int, Float, Boolean, are all structs, therefore value objects.