java-design-patterns/factory/README.md

---
layout: pattern
title: Factory
folder: factory
permalink: /patterns/factory/
categories: Creational
language: en
tags:
 - Gang of Four
---

## Also known as

* Simple Factory
* Static Factory Method

## Intent

Providing a static method encapsulated in a class called the factory, to hide the implementation 
logic and make client code focus on usage rather than initializing new objects.

## Explanation

Real-world example

> Imagine an alchemist who is about to manufacture coins. The alchemist must be able to create both 
> gold and copper coins and switching between them must be possible without modifying the existing 
> source code. The factory pattern makes it possible by providing a static construction method which 
> can be called with relevant parameters.

Wikipedia says

> Factory is an object for creating other objects – formally a factory is a function or method that 
> returns objects of a varying prototype or class.

**Programmatic Example**

We have an interface `Coin` and two implementations `GoldCoin` and `CopperCoin`.

```java
public interface Coin {
  String getDescription();
}

public class GoldCoin implements Coin {

  static final String DESCRIPTION = "This is a gold coin.";

  @Override
  public String getDescription() {
    return DESCRIPTION;
  }
}

public class CopperCoin implements Coin {
   
  static final String DESCRIPTION = "This is a copper coin.";

  @Override
  public String getDescription() {
    return DESCRIPTION;
  }
}
```

Enumeration above represents types of coins that we support (`GoldCoin` and `CopperCoin`).

```java
@RequiredArgsConstructor
@Getter
public enum CoinType {

  COPPER(CopperCoin::new),
  GOLD(GoldCoin::new);

  private final Supplier<Coin> constructor;
}
```

Then we have the static method `getCoin` to create coin objects encapsulated in the factory class 
`CoinFactory`.

```java
public class CoinFactory {

  public static Coin getCoin(CoinType type) {
    return type.getConstructor().get();
  }
}
```

Now on the client code we can create different types of coins using the factory class.

```java
LOGGER.info("The alchemist begins his work.");
var coin1 = CoinFactory.getCoin(CoinType.COPPER);
var coin2 = CoinFactory.getCoin(CoinType.GOLD);
LOGGER.info(coin1.getDescription());
LOGGER.info(coin2.getDescription());
```

Program output:

```java
The alchemist begins his work.
This is a copper coin.
This is a gold coin.
```

## Class Diagram

![alt text](./etc/factory.urm.png "Factory pattern class diagram")

## Applicability

Use the factory pattern when you only care about the creation of a object, not how to create 
and manage it.

Pros

* Allows keeping all objects creation in one place and avoid of spreading 'new' keyword across codebase.
* Allows to write loosely coupled code. Some of its main advantages include better testability, easy-to-understand code, swappable components, scalability and isolated features.

Cons

* The code becomes more complicated than it should be. 

## Known uses

* [java.util.Calendar#getInstance()](https://docs.oracle.com/javase/8/docs/api/java/util/Calendar.html#getInstance--)
* [java.util.ResourceBundle#getBundle()](https://docs.oracle.com/javase/8/docs/api/java/util/ResourceBundle.html#getBundle-java.lang.String-)
* [java.text.NumberFormat#getInstance()](https://docs.oracle.com/javase/8/docs/api/java/text/NumberFormat.html#getInstance--)
* [java.nio.charset.Charset#forName()](https://docs.oracle.com/javase/8/docs/api/java/nio/charset/Charset.html#forName-java.lang.String-)
* [java.net.URLStreamHandlerFactory#createURLStreamHandler(String)](https://docs.oracle.com/javase/8/docs/api/java/net/URLStreamHandlerFactory.html) (returns different singleton objects, depending on a protocol)
* [java.util.EnumSet#of()](https://docs.oracle.com/javase/8/docs/api/java/util/EnumSet.html#of(E))
* [javax.xml.bind.JAXBContext#createMarshaller()](https://docs.oracle.com/javase/8/docs/api/javax/xml/bind/JAXBContext.html#createMarshaller--) and other similar methods.

## Related patterns

* [Factory Method](https://java-design-patterns.com/patterns/factory-method/)
* [Factory Kit](https://java-design-patterns.com/patterns/factory-kit/)
* [Abstract Factory](https://java-design-patterns.com/patterns/abstract-factory/)