Design patterns: Factory Method

This is a creational pattern, the factory method pattern is a design pattern that allows for the creation of objects without specifying the type of object that is to be created in code. A factory class contains a method that allows determination of the created type at run-time.

The factory pattern is used to replace class constructors, abstracting the process of object generation so that the type of the object instantiated can be determined at run-time.

Elements involved:

  • FactoryBase: This is an abstract base class for the concrete factory classes that will actually generate new objects.
  • ConcreteFactory: Inheriting from the FactoryBase class, the concrete factory classes inherit the actual factory method. This is overridden with the object generation code unless already implemented in full in the base class.
  • ProductBase: This abstract class is the base class for the types of object that the factory can create. It is also the return type for the factory method.
  • ConcreteProduct: Multiple subclasses of the Product class are defined, each containing specific functionality. Objects of these classes are generated by the factory method.

We should use the Factory Method design pattern when:

  • when a class can’t anticipate the type of the objects it is supposed to create.
  • when a class wants its subclasses to be the ones to specific the type of a newly created object.

Let´s see some code:

public abstract class FactoryBase {
    public abstract ProductBase build(int type);
}
public abstract class ProductBase {
    public abstract void whoIAm();
}
public class ConcreteFactory extends FactoryBase {

    @Override
    public ProductBase build(int type) {
        final ProductBase productBase;

        switch (type) {
            case 1:
                productBase = new ConcreteProduct1();
                break;

            case 2:
                productBase = new ConcreteProduct2();
                break;

            default:
                throw new IllegalArgumentException(String.format("Illegal type %s", type));
        }

        return productBase;
    }
}
public class ConcreteProduct1 extends ProductBase {

    @Override
    public void whoIAm() {
        System.out.println("I am ConcreteProduct1");
    }
}
public class ConcreteProduct2 extends ProductBase {

    @Override
    public void whoIAm() {
        System.out.println("I am ConcreteProduct2");
    }
}
public class Main {
    public static void main(String[] args) {
        final FactoryBase factoryBase = new ConcreteFactory();

        factoryBase.build(1).whoIAm();
        factoryBase.build(2).whoIAm();
    }
}

The output after the execution should be something like that:

I am ConcreteProduct1
I am ConcreteProduct2

You can find the code in my GitHub repository “design-apperns“.

Design patterns: Factory Method

Design Patterns: Builder

This is a creational pattern, as it is used to control class instantiation. The builder pattern is a design pattern that allows for the step-by-step creation of complex objects using the correct sequence of actions. The construction is controlled by a director object that only needs to know the type of object it is to create.

Elements involved:

  • Product: The product class defines the type of the complex object that is to be generated by the builder pattern.
  • Builder: This abstract base class defines all of the steps that must be taken in order to correctly create a product. Each step is generally abstract as the actual functionality of the builder is carried out in the concrete subclasses. The getProduct method is used to return the final product. The builder class is often replaced with a simple interface.
  • ConcreteBuilder: There may be any number of concrete builder classes inheriting from Builder. These classes contain the functionality to create a particular complex product.
  • Director: The director class controls the algorithm that generates the final product object. A director object is instantiated and its construct method is called. The method includes a parameter to capture the specific concrete builder object that is to be used to generate the product. The director then calls methods of the concrete builder in the correct order to generate the product object. On completion of the process, the getProduct method of the builder object can be used to return the product.

We should use the Builder design pattern when:

When object creation algorithms should be decoupled from the system, and multiple representations of creation algorithms are required. This decoupling is useful as you can add new creation functionality to your system without affecting the core code. You also get control over the creation process at runtime with this approach.

Let´s see some code:

public abstract class Builder {
    public abstract void buildA();
    public abstract void buildB();
    public abstract void buildC();
    public abstract Product getProduct();
}
public class ConcreteBuilder extends Builder {

    private final Product product = new Product();

    @Override
    public void buildA() { this.product.setA("A"); }

    @Override
    public void buildB() { this.product.setB("B"); }

    @Override
    public void buildC() { this.product.setC("C"); }

    @Override
    public Product getProduct() { return this.product; }
}
public class Product {

    public String a;
    public String b;
    public String c;

    public String getA() { return a; }
    public void setA(String a) { this.a = a; }
    public String getB() { return b; }
    public void setB(String b) { this.b = b; }
    public String getC() { return c; }
    public void setC(String c) { this.c = c; }
}
public class Director {

    public Product construct(final Builder builder) {
        builder.buildA();
        builder.buildB();
        builder.buildC();

        return builder.getProduct();
    }
}
public class Main {

    public static void main(String[] args) {
        final Director director = new Director();
        final Product product = director.construct(new ConcreteBuilder());

        System.out.println(String.format("Product: A = %s, B = %s, C = %s",
                product.getA(), product.getB(), product.getC()));
    }
}

You can find the code in my GitHub repository “design-apperns“.

Not exactly matching the pattern described in the book wrote by the GoF, we can see a couple more definitions/implementations of the Builder pattern.

The first one is used to build immutable objects and, in addition, it makes easier to add new properties to your object without having a constructor with a huge number of parameters. It is similar to a fluent interface usually implemented by using method cascading or method chaining.

Let´s see an example.

public class Product {

    private final String a;
    private final String b;
    private final String c;
    private final String d;
    private final String e;

    private Product(ProductBuilder productBuilder) {
        this.a = productBuilder.a;
        this.b = productBuilder.b;
        this.c = productBuilder.c;
        this.d = productBuilder.d;
        this.e = productBuilder.e;
    }

    public String getA() { return a; }

    public String getB() { return b; }

    public String getC() { return c; }

    public String getD() { return d; }

    public String getE() { return e; }

    @Override
    public String toString() {
        return "Product{" + "a=" + a + ", b=" + b + ", c=" + c + ", d=" + d + ", e=" + e + '}';
    }

    public static class ProductBuilder {

        private final String a;
        private final String b;
        private String c; // Optional
        private String d; // Optional
        private String e; // Optional

        public ProductBuilder(String a, String b) {
            this.a = a;
            this.b = b;
        }

        public ProductBuilder setC(final String c) {
            this.c = c;
            return this;
        }

        public ProductBuilder setD(final String d) {
            this.d = d;
            return this;
        }

        public ProductBuilder setE(final String e) {
            this.e = e;
            return this;
        }

        public Product build() {
            return new Product(this);
        }
    }
}
public class Main {

    public static void main(String[] args) {
        System.out.print(new Product.ProductBuilder("A", "B")
                .setC("C")
                .setD("D")
                .setE("E")
                .build()
                .toString());
    }
}

You can find the example in the repository, in the builder project in the package labeled as “variant1”.

The second example, it is basically the same, but applied to POJOs to make easier to build them. It is a much more simplified version of the previous example.

Let´s see how this is:

public class Product {

    private String a;
    private String b;

    public String getA() { return this.a; }

    public Product setA(final String a) {
        this.a = a;
        return this;
    }

    public String getB() { return this.b; }

    public Product setB(final String b) {
        this.b = b;
        return this;
    }

    @Override
    public String toString() {
        return "Product{" + "a=" + a + ", b=" + b + '}';
    }
}
public class Main {

    public static void main(String[] args) {
        System.out.println(new Product()
                .setA("A")
                .setB("B")
                .toString());
    }
}

You can find the example in the repository, in the builder project in the package labeled as “variant2”.

Design Patterns: Builder

Design patterns: Abstract Factory

This is a creational pattern, as it is used to control class instantiation. The abstract factory pattern is used to provide a client with a set of related or dependant objects. The set of objects created by the factory is determined at run-time according to the selection of concrete factory class.

Provides a level of indirection that abstracts the creation of families of related or dependent objects without directly specifying their concrete classes. The factory object has the responsibility for providing creation services for the entire platform family. Clients never create platform objects directly, they ask the factory to do that for them.

This mechanism makes exchanging product families easy because the specific class of the factory object appears only once in the application – where it is instantiated. The application can wholesale replace the entire family of products simply by instantiating a different concrete instance of the abstract factory.

Elements involved:

  • AbstractFactory: This is an abstract base class for the concrete factory classes that will generate new sets of related objects. A method is included for each type of object that will be instantiated.
  • ConcreteFactory: Inheriting from the AbstractFactory class, the concrete factory classes override the methods that generate the suite of objects required by the client.
  • AbstractProduct: This abstract class is the base class for the types of object that a factory can create. One base type exists for each of the distinct types of product required by the client.
  • ConcreteProduct: Multiple subclasses of the ConcreteProduct classes are defined, each containing specific functionality. Objects of these classes are generated by the abstract factory to populate the client.
  • Client: This class uses the factories to generate a family of related objects. In the UML diagram, the client has two private fields that hold instances of the abstract product classes.

We should use the Abstract Factory design pattern when:

  • the system needs to be independent from the way the products it works with are created.
  • the system is or should be configured to work with multiple families of products.
  • a family of products is designed to work only all together.
  • the creation of a library of products is needed, for which is relevant only the interface, not the implementation, too.

Let´s see some code:

public abstract class AbstractFactory {
    public abstract AbstractProduct createProduct();
}
public abstract class AbstractProduct {
    public abstract void operation1();
    public abstract void operation2();
}
public class ConcreteFactory1 extends AbstractFactory {
    @Override
    public AbstractProduct createProduct() {
        return new ConcreteProduct1();
    }
}
public class ConcreteFactory2 extends AbstractFactory {
    @Override
    public AbstractProduct createProduct() {
        return new Product2();
    }
}
public class ConcreteProduct1 extends AbstractProduct {
    public ConcreteProduct1() {
        System.out.println("Creating product 1...");
    }

    @Override
    public void operation1() {
        System.out.println("Executing ConcreteProduct1::operation1...");
    }

    @Override
    public void operation2() {
        System.out.println("Executing ConcreteProduct1::operation2...");
    }
}
public class ConcreteProduct2 extends AbstractProduct {
    public ConcreteProduct2() {
        System.out.println("Creating product 2...");
    }

    @Override
    public void operation1() {
        System.out.println("Executing Product2::operation1...");
    }

    @Override
    public void operation2() {
        System.out.println("Executing Product2::operation2...");
    }
}
public enum ProductType {
    PRODUCT_1,
    PRODUCT_2
}
public class FactoryMaker {

    public static AbstractFactory getFactory(final ProductType productType) {
        final AbstractFactory abstractFactory;

        switch (productType) {
            case PRODUCT_1:
                abstractFactory = new ConcreteFactory1();
                break;

            case PRODUCT_2:
                abstractFactory = new ConcreteFactory2();
                break;

            default:
                throw new IllegalArgumentException("The product type does not exist.");
        }

        return abstractFactory;
    }
}
public class Client {

    public static void main(String[] args) {
        Arrays.stream(ProductType.values())
            .forEach(productType -> {
                AbstractFactory abstractFactory = FactoryMaker.getFactory(productType);
                AbstractProduct product = abstractFactory.createProduct();

                product.operation1();
                product.operation2();
            });
    }
}

The output after the execution should be something like that:

Creating product 1...
Executing ConcreteProduct1::operation1...
Executing ConcreteProduct1::operation2...
Creating product 2...
Executing Product2::operation1...
Executing Product2::operation2...

You can find the code in my GitHub repository “design-apperns“.

Design patterns: Abstract Factory

Design Patterns

Design patterns provide solutions to common software design problems. In the case of object-oriented programming, design patterns are generally aimed at solving the problems of object generation and interaction, rather than the larger scale problems of overall software architecture. They give generalised solutions in the form of templates that may be applied to real-world problems.

In the next series of articles we are going to be focus in the Gang of Four design patterns. They wrote a book called “Design Patterns: Elements of Reusable Object-Oriented Software”. The four authors are:

They included in their book thirty-three design patterns divided in three main categories:

  • Creational patterns: Creational patterns provide ways to instantiate single objects or groups of related objects.
  • Structural patterns: Structural patterns provide a manner to define relationships between classes or objects.
  • Behavioral patterns: Behavioural patterns define manners of communication between classes and objects.

In the next articles we will go deeper in each one of the types and in the design patterns included in each one of the categories.

Design Patterns