Creational Design Patterns tutorial example - Factory Method - Java vs Kotlin

This design pattern is all about the class instantiation. These design patterns tries to create objects  in a manner suitable to the situation.

Again there are two sub-categories into it

1) Object-Creational Design Pattern

2)Class-Creational Design Pattern

Object-Creational DP                                                                            Class-Creational DP

1) Abstract Factory DP                                                                         1)Factory Method DP

2)Builder DP

3)Prototype DP

4)Singleton DP

Factory Method Design Pattern:

It is used to instantiate an object from one among a set of classes based on some logic. It is also used instead of constructor or in addition to existing constructor.

eg .sample program for it

public interface Pet{ public String petSound();}

public class Dog implements Pet{  public String petSound(){return "Bow Bow" } }

public class Cat implements Pet{  public String petSound(){return "Meaw Meaw" } }

public class PetFactory{ public Pet getPet(String petType){ if("Bow Bow".equals(petType)){pet=new Dog(); else if("Meaw Meaw.equals(petType){ pet=new Cat();}") }  return pet}}

publi class SampleFactoryMethodTest

{ 

  public static void main(String args[])

  {

   PetFactory factory=new PetFattory();

   Pet pet=factory.getPet("Bow");

   System.out.println(pet.petSound());

  }

}

When to use Factory Method Design Pattern

1) when a class cant guess the type of the object it is supposed to create.

2)When a class wants its subclass to be the ones to specific the type of a newly created object.

3)when we want to localize the knowledge of which class gets created.

Example in kotlin : 

fun main() {
    val queen = createPiece("qa5")
    println(queen)
}

interface ChessPiece {
    val file: Char
    val rank: Char
}

data class Pawn(
    override val file: Char,
    override val rank: Char
) : ChessPiece


data class Queen(
    override val file: Char,
    override val rank: Char
) : ChessPiece

fun createPiece(notation: String): ChessPiece {
    val (type, file, rank) = notation.toCharArray()
    return when (type) {
        'q' -> Queen(file, rank)
        'p' -> Pawn(file, rank)
        else -> throw RuntimeException("Unknown piece: $type")
    }
}

Output : Queen(file=a, rank=5)