223 lines
7.1 KiB
Markdown
223 lines
7.1 KiB
Markdown
---
|
||
layout: pattern
|
||
title: Abstract Factory
|
||
folder: abstract-factory
|
||
permalink: /patterns/abstract-factory/
|
||
categories: Creational
|
||
tags:
|
||
- Gang of Four
|
||
---
|
||
|
||
## 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. 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.
|
||
|
||
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.
|
||
|
||
```java
|
||
public interface Castle {
|
||
String getDescription();
|
||
}
|
||
|
||
public interface King {
|
||
String getDescription();
|
||
}
|
||
|
||
public interface Army {
|
||
String getDescription();
|
||
}
|
||
|
||
// Elven implementations ->
|
||
public class ElfCastle implements Castle {
|
||
static final String DESCRIPTION = "This is the Elven castle!";
|
||
@Override
|
||
public String getDescription() {
|
||
return DESCRIPTION;
|
||
}
|
||
}
|
||
public class ElfKing implements King {
|
||
static final String DESCRIPTION = "This is the Elven king!";
|
||
@Override
|
||
public String getDescription() {
|
||
return DESCRIPTION;
|
||
}
|
||
}
|
||
public class ElfArmy implements Army {
|
||
static final String DESCRIPTION = "This is the Elven Army!";
|
||
@Override
|
||
public String getDescription() {
|
||
return DESCRIPTION;
|
||
}
|
||
}
|
||
|
||
// Orcish implementations similarly -> ...
|
||
|
||
```
|
||
|
||
Then we have the abstraction and implementations for the kingdom factory
|
||
|
||
```java
|
||
public interface KingdomFactory {
|
||
Castle createCastle();
|
||
King createKing();
|
||
Army createArmy();
|
||
}
|
||
|
||
public class ElfKingdomFactory implements KingdomFactory {
|
||
public Castle createCastle() {
|
||
return new ElfCastle();
|
||
}
|
||
public King createKing() {
|
||
return new ElfKing();
|
||
}
|
||
public Army createArmy() {
|
||
return new ElfArmy();
|
||
}
|
||
}
|
||
|
||
public class OrcKingdomFactory implements KingdomFactory {
|
||
public Castle createCastle() {
|
||
return new OrcCastle();
|
||
}
|
||
public King createKing() {
|
||
return new OrcKing();
|
||
}
|
||
public Army createArmy() {
|
||
return new OrcArmy();
|
||
}
|
||
}
|
||
```
|
||
|
||
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.
|
||
|
||
```java
|
||
var factory = new ElfKingdomFactory();
|
||
var castle = factory.createCastle();
|
||
var king = factory.createKing();
|
||
var army = factory.createArmy();
|
||
|
||
castle.getDescription();
|
||
king.getDescription();
|
||
army.getDescription();
|
||
```
|
||
|
||
Program output:
|
||
|
||
```java
|
||
This is the Elven castle!
|
||
This is the Elven king!
|
||
This 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 (Army, King, Castle).
|
||
In this example, we also used an enum to parameterize which type of kingdom factory the client will ask for.
|
||
|
||
```java
|
||
public static class FactoryMaker {
|
||
|
||
public enum KingdomType {
|
||
ELF, ORC
|
||
}
|
||
|
||
public static KingdomFactory makeFactory(KingdomType type) {
|
||
switch (type) {
|
||
case ELF:
|
||
return new ElfKingdomFactory();
|
||
case ORC:
|
||
return new OrcKingdomFactory();
|
||
default:
|
||
throw new IllegalArgumentException("KingdomType not supported.");
|
||
}
|
||
}
|
||
}
|
||
|
||
public static void main(String[] args) {
|
||
var app = new App();
|
||
|
||
LOGGER.info("Elf Kingdom");
|
||
app.createKingdom(FactoryMaker.makeFactory(KingdomType.ELF));
|
||
LOGGER.info(app.getArmy().getDescription());
|
||
LOGGER.info(app.getCastle().getDescription());
|
||
LOGGER.info(app.getKing().getDescription());
|
||
|
||
LOGGER.info("Orc Kingdom");
|
||
app.createKingdom(FactoryMaker.makeFactory(KingdomType.ORC));
|
||
-- similar use of the orc factory
|
||
}
|
||
```
|
||
|
||
## Class diagram
|
||
|
||
|
||
|
||
|
||
## 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 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.
|
||
|
||
## Tutorial
|
||
|
||
* [Abstract Factory Pattern Tutorial](https://www.journaldev.com/1418/abstract-factory-design-pattern-in-java)
|
||
|
||
## Known uses
|
||
|
||
* [javax.xml.parsers.DocumentBuilderFactory](http://docs.oracle.com/javase/8/docs/api/javax/xml/parsers/DocumentBuilderFactory.html)
|
||
* [javax.xml.transform.TransformerFactory](http://docs.oracle.com/javase/8/docs/api/javax/xml/transform/TransformerFactory.html#newInstance--)
|
||
* [javax.xml.xpath.XPathFactory](http://docs.oracle.com/javase/8/docs/api/javax/xml/xpath/XPathFactory.html#newInstance--)
|
||
|
||
## Related patterns
|
||
|
||
[Factory Method](https://java-design-patterns.com/patterns/factory-method/)
|
||
[Factory Kit](https://java-design-patterns.com/patterns/factory-kit/)
|
||
|
||
## Credits
|
||
|
||
* [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)
|
||
* [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)
|