Factory Method Pattern

Factory Method Pattern - Example Implementation

First, create a Polygon interface

interface Polygon {
    String getType();
}

Next, create a few Polygon implementations like Square, Triangle, etc

class Square implements Polygon {
    String getType() { return "square"; }
}

class Triangle implements Polygon {
    String getType() { return "triangle"; }
}

interface PolygonFactoryMethod {
	Polygon polygonFactoryMethod(final int numberOfSides);
}

Factory Method Implementations and Uses Cases

class PolygonFactoryMethodImpl1 implements PolygonFactoryMethod {  	
	@Override
	Polygon polygonFactoryMethod(final int numberOfSides) {
    	if (numberOfSides == 3) {
        	Triangle triangle = new Triangle();
			// custom logic
			return triangle;
    	} else if (numberOfSides == 4) {
        	Square square = new Square();
			// customm logic
			return square
    	} else {
        	throw new RuntimeException("invalid input");
    	}
	}
}

class PolygonFactoryMethodImpl2 implements PolygonFactoryMethod {
	@Override
	Polygon polygonFactoryMethod(final int numberOfSides) {
    	if (numberOfSides == 3) {
        	Triangle triangle = new Triangle();
			// custom logic
			return triangle;
    	} else if (numberOfSides == 4) {
        	Square square = new Square();
			// customm logic
			return square
    	} else {
        	throw new RuntimeException("invalid input");
    	}
	}
}

static void main(String[] args) {
    var polygonFactoryMethod = new PolygonFactoryMethodImpl1();
    Polygon triangle = polygonFactoryMethod.polygonFactoryMethod(3);
 	Polygon square   = polygonFactoryMethod.polygonFactoryMethod(4);
    // ...
    System.out.println(triangle.getType()); // triangle
    System.out.println(square.getType());   // square
}

static void main(String[] args) {
    var polygonFactoryMethod = new PolygonFactoryMethodImpl2();
    Polygon triangle = polygonFactoryMethod.polygonFactoryMethod(3);
 	Polygon square   = polygonFactoryMethod.polygonFactoryMethod(4);
    // ...
    System.out.println(triangle.getType()); // triangle
    System.out.println(square.getType());   // square
}

Comparisons

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Abstract Factory Pattern vs Factory Method Pattern

• the main difference is that the factory method is a method, and an abstract factory is an object
  • the intended purpose of the class containing a factory method is NOT to create objects
  • the intended purpose of an abstract factory should ONLY be used to create objects
• abstract factory is implemented by composition, but factory method is implemented by Inheritance
• goals:
  • abstract factory’s goal is to create a family of objects that are designed to work together
  • factory method’s goal is to encapsulate object creation in a separate method, allowing subclasses to provide the implementation for creating specific objects
• one should take care when using factory methods since it’s easy to break the Liskov Substitution Principle (LSP) when creating objects

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Factory Method Pattern vs Static Factory Method Pattern

• Static Factory Method Pattern does not allow overriding
• Factory Method Pattern allows overriding for flexibility before compile time

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Factory Method Pattern vs Template Method Pattern

• Factory Method is a specialization of Template Method used specifically for creating objects rather than containing business logic
• At the same time, a Factory Method may serve as a step in a large Template Method

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Other

• Utilizes Strategy Pattern for object creation
• Factory Kit Pattern - used to create/define factory methods during runtime