223 lines
7.1 KiB
Markdown
223 lines
7.1 KiB
Markdown
---
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layout: pattern
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title: Abstract Factory
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folder: abstract-factory
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permalink: /patterns/abstract-factory/
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categories: Creational
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tags:
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- Gang of Four
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---
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## Also known as
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Kit
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## Intent
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Provide an interface for creating families of related or dependent
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objects without specifying their concrete classes.
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## Explanation
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Real world example
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> To create a kingdom we need objects with a common theme. Elven kingdom needs an Elven king, Elven castle and Elven army whereas Orcish kingdom needs an Orcish king, Orcish castle and Orcish army. There is a dependency between the objects in the kingdom.
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In plain words
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> A factory of factories; a factory that groups the individual but related/dependent factories together without specifying their concrete classes.
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Wikipedia says
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> The abstract factory pattern provides a way to encapsulate a group of individual factories that have a common theme without specifying their concrete classes
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**Programmatic Example**
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Translating the kingdom example above. First of all we have some interfaces and implementation for the objects in the
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kingdom.
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```java
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public interface Castle {
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String getDescription();
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}
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public interface King {
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String getDescription();
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}
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public interface Army {
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String getDescription();
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}
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// Elven implementations ->
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public class ElfCastle implements Castle {
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static final String DESCRIPTION = "This is the Elven castle!";
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@Override
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public String getDescription() {
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return DESCRIPTION;
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}
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}
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public class ElfKing implements King {
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static final String DESCRIPTION = "This is the Elven king!";
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@Override
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public String getDescription() {
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return DESCRIPTION;
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}
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}
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public class ElfArmy implements Army {
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static final String DESCRIPTION = "This is the Elven Army!";
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@Override
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public String getDescription() {
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return DESCRIPTION;
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}
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}
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// Orcish implementations similarly -> ...
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```
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Then we have the abstraction and implementations for the kingdom factory
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```java
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public interface KingdomFactory {
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Castle createCastle();
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King createKing();
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Army createArmy();
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}
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public class ElfKingdomFactory implements KingdomFactory {
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public Castle createCastle() {
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return new ElfCastle();
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}
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public King createKing() {
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return new ElfKing();
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}
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public Army createArmy() {
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return new ElfArmy();
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}
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}
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public class OrcKingdomFactory implements KingdomFactory {
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public Castle createCastle() {
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return new OrcCastle();
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}
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public King createKing() {
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return new OrcKing();
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}
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public Army createArmy() {
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return new OrcArmy();
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}
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}
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```
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Now we have our abstract factory that lets us make family of related objects i.e. Elven kingdom factory creates Elven castle, king and army etc.
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```java
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var factory = new ElfKingdomFactory();
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var castle = factory.createCastle();
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var king = factory.createKing();
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var army = factory.createArmy();
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castle.getDescription();
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king.getDescription();
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army.getDescription();
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```
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Program output:
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```java
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This is the Elven castle!
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This is the Elven king!
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This is the Elven Army!
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```
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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.
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The client can use FactoryMaker to create the desired concrete factory which, in turn, will produce different concrete objects (Army, King, Castle).
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In this example, we also used an enum to parameterize which type of kingdom factory the client will ask for.
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```java
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public static class FactoryMaker {
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public enum KingdomType {
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ELF, ORC
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}
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public static KingdomFactory makeFactory(KingdomType type) {
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switch (type) {
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case ELF:
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return new ElfKingdomFactory();
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case ORC:
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return new OrcKingdomFactory();
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default:
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throw new IllegalArgumentException("KingdomType not supported.");
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}
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}
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}
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public static void main(String[] args) {
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var app = new App();
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LOGGER.info("Elf Kingdom");
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app.createKingdom(FactoryMaker.makeFactory(KingdomType.ELF));
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LOGGER.info(app.getArmy().getDescription());
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LOGGER.info(app.getCastle().getDescription());
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LOGGER.info(app.getKing().getDescription());
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LOGGER.info("Orc Kingdom");
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app.createKingdom(FactoryMaker.makeFactory(KingdomType.ORC));
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-- similar use of the orc factory
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}
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```
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## Class diagram
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## Applicability
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Use the Abstract Factory pattern when
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* The system should be independent of how its products are created, composed and represented
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* The system should be configured with one of multiple families of products
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* The family of related product objects is designed to be used together, and you need to enforce this constraint
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* You want to provide a class library of products, and you want to reveal just their interfaces, not their implementations
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* The lifetime of the dependency is conceptually shorter than the lifetime of the consumer.
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* You need a run-time value to construct a particular dependency
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* You want to decide which product to call from a family at runtime.
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* You need to supply one or more parameters only known at run-time before you can resolve a dependency.
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* When you need consistency among products
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* You don’t want to change existing code when adding new products or families of products to the program.
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Example use cases
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* Selecting to call to the appropriate implementation of FileSystemAcmeService or DatabaseAcmeService or NetworkAcmeService at runtime.
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* Unit test case writing becomes much easier
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* UI tools for different OS
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## Consequences:
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* Dependency injection in java hides the service class dependencies that can lead to runtime errors that would have been caught at compile time.
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* While the pattern is great when creating predefined objects, adding the new ones might be challenging.
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* The code becomes more complicated than it should be, since a lot of new interfaces and classes are introduced along with the pattern.
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## Tutorial
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* [Abstract Factory Pattern Tutorial](https://www.journaldev.com/1418/abstract-factory-design-pattern-in-java)
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## Known uses
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* [javax.xml.parsers.DocumentBuilderFactory](http://docs.oracle.com/javase/8/docs/api/javax/xml/parsers/DocumentBuilderFactory.html)
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* [javax.xml.transform.TransformerFactory](http://docs.oracle.com/javase/8/docs/api/javax/xml/transform/TransformerFactory.html#newInstance--)
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* [javax.xml.xpath.XPathFactory](http://docs.oracle.com/javase/8/docs/api/javax/xml/xpath/XPathFactory.html#newInstance--)
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## Related patterns
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* [Factory Method](https://java-design-patterns.com/patterns/factory-method/)
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* [Factory Kit](https://java-design-patterns.com/patterns/factory-kit/)
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## Credits
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* [Design Patterns: Elements of Reusable Object-Oriented Software](https://www.amazon.com/gp/product/0201633612/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0201633612&linkCode=as2&tag=javadesignpat-20&linkId=675d49790ce11db99d90bde47f1aeb59)
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* [Head First Design Patterns: A Brain-Friendly Guide](https://www.amazon.com/gp/product/0596007124/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596007124&linkCode=as2&tag=javadesignpat-20&linkId=6b8b6eea86021af6c8e3cd3fc382cb5b)
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