Browser docs

Abstract Factory

April 14, 2023 5 min read Gang of Four Design Patterns Creational
On this page

Also known as

Kit

Intent

Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

Explanation

Real-world example

To create a kingdom we need objects with a common theme. The Elven Kingdom needs Elven King, Elven Castle, and Elven Army, whereas The Orcish Kingdom needs an Orcish King, Orcish Castle, and Orcish Army. There is a dependency between the objects in the kingdom.

In plain words

A factory of factories; a factory that groups the individual but related/dependent factories together without specifying their concrete classes.

Wikipedia says

The abstract factory pattern provides a way to encapsulate a group of individual factories that have a common theme without specifying their concrete classes

Programmatic Example

Translating the kingdom example above. First of all, we have some interfaces and implementation for the objects in the kingdom.

 1public interface Castle {
 2  String getDescription();
 3}
 4
 5public interface King {
 6  String getDescription();
 7}
 8
 9public interface Army {
10  String getDescription();
11}
12
13// Elven implementations ->
14public class ElfCastle implements Castle {
15  static final String DESCRIPTION = "This is the elven castle!";
16  @Override
17  public String getDescription() {
18    return DESCRIPTION;
19  }
20}
21public class ElfKing implements King {
22  static final String DESCRIPTION = "This is the elven king!";
23  @Override
24  public String getDescription() {
25    return DESCRIPTION;
26  }
27}
28public class ElfArmy implements Army {
29  static final String DESCRIPTION = "This is the elven Army!";
30  @Override
31  public String getDescription() {
32    return DESCRIPTION;
33  }
34}
35
36// Orcish implementations similarly -> ...

Then we have the abstraction and implementations for the kingdom factory.

 1public interface KingdomFactory {
 2  Castle createCastle();
 3  King createKing();
 4  Army createArmy();
 5}
 6
 7public class ElfKingdomFactory implements KingdomFactory {
 8
 9  @Override
10  public Castle createCastle() {
11    return new ElfCastle();
12  }
13
14  @Override
15  public King createKing() {
16    return new ElfKing();
17  }
18
19  @Override
20  public Army createArmy() {
21    return new ElfArmy();
22  }
23}
24
25public class OrcKingdomFactory implements KingdomFactory {
26
27  @Override
28  public Castle createCastle() {
29    return new OrcCastle();
30  }
31
32  @Override
33  public King createKing() {
34    return new OrcKing();
35  }
36  
37  @Override
38  public Army createArmy() {
39    return new OrcArmy();
40  }
41}

Now we have the abstract factory that lets us make a family of related objects i.e. elven kingdom factory creates elven castle, king and army, etc.

1var factory = new ElfKingdomFactory();
2var castle = factory.createCastle();
3var king = factory.createKing();
4var army = factory.createArmy();
5
6castle.getDescription();
7king.getDescription();
8army.getDescription();

Program output:

1This is the elven castle!
2This is the elven king!
3This is the elven Army!

Now, we can design a factory for our different kingdom factories. In this example, we created FactoryMaker, responsible for returning an instance of either ElfKingdomFactory or OrcKingdomFactory.
The client can use FactoryMaker to create the desired concrete factory which, in turn, will produce different concrete objects (derived from Army, King, Castle).
In this example, we also used an enum to parameterize which type of kingdom factory the client will ask for.

 1public static class FactoryMaker {
 2
 3    public enum KingdomType {
 4        ELF, ORC
 5    }
 6
 7    public static KingdomFactory makeFactory(KingdomType type) {
 8        return switch (type) {
 9            case ELF -> new ElfKingdomFactory();
10            case ORC -> new OrcKingdomFactory();
11            default -> throw new IllegalArgumentException("KingdomType not supported.");
12        };
13    }
14}
15
16    public static void main(String[] args) {
17        var app = new App();
18
19        LOGGER.info("Elf Kingdom");
20        app.createKingdom(FactoryMaker.makeFactory(KingdomType.ELF));
21        LOGGER.info(app.getArmy().getDescription());
22        LOGGER.info(app.getCastle().getDescription());
23        LOGGER.info(app.getKing().getDescription());
24
25        LOGGER.info("Orc Kingdom");
26        app.createKingdom(FactoryMaker.makeFactory(KingdomType.ORC));
27        --similar use of the orc factory
28    }

Class diagram

alt text

Applicability

Use the Abstract Factory pattern when

  • The system should be independent of how its products are created, composed, and represented
  • The system should be configured with one of the multiple families of products
  • The family of related product objects is designed to be used together, and you need to enforce this constraint
  • You want to provide a class library of products, and you want to reveal just their interfaces, not their implementations
  • The lifetime of the dependency is conceptually shorter than the lifetime of the consumer.
  • You need a run-time value to construct a particular dependency
  • You want to decide which product to call from a family at runtime.
  • You need to supply one or more parameters only known at run-time before you can resolve a dependency.
  • When you need consistency among products
  • You don’t want to change existing code when adding new products or families of products to the program.

Example use cases

  • Selecting to call to the appropriate implementation of FileSystemAcmeService or DatabaseAcmeService or NetworkAcmeService at runtime.
  • Unit test case writing becomes much easier
  • UI tools for different OS

Consequences

  • Dependency injection in java hides the service class dependencies that can lead to runtime errors that would have been caught at compile time.
  • While the pattern is great when creating predefined objects, adding the new ones might be challenging.
  • The code becomes more complicated than it should be since a lot of new interfaces and classes are introduced along with the pattern.

Tutorials

Known uses

Credits