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feat: add factory enum #3300
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--- | ||
title: "Factory Enum Pattern in Java: Simplifying Object Access" | ||
shortTitle: Factory Enum | ||
description: "Learn the Factory Enum Pattern in Java using practical examples. Understand how to use enums to manage object instantiation in a clean, scalable, and singleton-safe way." | ||
category: Creational | ||
language: en | ||
tag: | ||
- Abstraction | ||
- Enum Singleton | ||
- Gang of Four | ||
- Factory Pattern | ||
- Object Instantiation | ||
--- | ||
|
||
## Intent of Factory Enum Pattern | ||
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The **Factory Enum Pattern** is a creational pattern that leverages Java's `enum` type to manage object instantiation. It provides a centralized, type-safe, and singleton-friendly way to access various implementations of a common interface. | ||
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This pattern is ideal when the set of possible objects is known and fixed (e.g., file processors, coin types, document types). | ||
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## Real-World Analogy | ||
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> Imagine a toolbox where each tool has a fixed slot. You don’t create a new tool each time — you just access the correct one from the right compartment. Similarly, with Factory Enum, each `enum` constant maps to a specific object instance that can be reused or constructed once. | ||
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## Programmatic Example Using FileProcessor | ||
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In this example, we want to process different file types: Excel and PDF. Each processor implements the `FileProcessor` interface. | ||
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### FileProcessor Interface | ||
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```java | ||
public interface FileProcessor { | ||
String getDescription(); | ||
} | ||
``` | ||
Implementations | ||
```java | ||
@NoArgsConstructor(access = AccessLevel.PRIVATE) | ||
public class ExcelProcessor implements FileProcessor { | ||
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static final String DESCRIPTION = "This is an Excel processor."; | ||
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@Override | ||
public String getDescription() { | ||
return DESCRIPTION; | ||
} | ||
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public static class InstanceHolder { | ||
public static final ExcelProcessor INSTANCE = new ExcelProcessor(); | ||
} | ||
} | ||
``` | ||
```java | ||
@NoArgsConstructor(access = AccessLevel.PRIVATE) | ||
public class PDFProcessor implements FileProcessor { | ||
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static final String DESCRIPTION = "This is a PDF processor."; | ||
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@Override | ||
public String getDescription() { | ||
return DESCRIPTION; | ||
} | ||
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public static class InstanceHolder { | ||
public static final PDFProcessor INSTANCE = new PDFProcessor(); | ||
} | ||
} | ||
``` | ||
Factory Enum: FileProcessorType | ||
```java | ||
@RequiredArgsConstructor | ||
@Getter | ||
public enum FileProcessorType { | ||
EXCEL(ExcelProcessor.InstanceHolder.INSTANCE), | ||
PDF(PDFProcessor.InstanceHolder.INSTANCE); | ||
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private final FileProcessor instance; | ||
} | ||
``` | ||
Client Code | ||
```java | ||
public static void main(String[] args) { | ||
LOGGER.info("Start processing files."); | ||
var excel = FileProcessorType.EXCEL.getInstance(); | ||
var pdf = FileProcessorType.PDF.getInstance(); | ||
LOGGER.info(excel.getDescription()); | ||
LOGGER.info(pdf.getDescription()); | ||
} | ||
``` | ||
Output | ||
``` | ||
INFO: Start processing files. | ||
INFO: This is an Excel processor. | ||
INFO: This is a PDF processor. | ||
``` | ||
## When to Use the Factory Enum Pattern | ||
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* When object types are known in advance and won't change frequently. | ||
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* When you want singleton instances per type. | ||
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* To avoid traditional factory classes and keep the logic self-contained within an enum. | ||
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* When readability and simplicity are preferred over extensibility. | ||
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## Benefits | ||
* Type-safe and readable – enum provides compiler-checked safety. | ||
* Singleton-friendly – each instance is created once and reused. | ||
* No need for external factory class – the enum holds both the type and instance. | ||
* Thread-safe by nature of enum and static holders. | ||
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## Trade-offs | ||
* Not suitable for dynamic or extensible object types. | ||
* Violates the Open/Closed Principle if new types are added frequently. | ||
* Slightly unconventional compared to traditional Factory pattern. | ||
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## Related Patterns | ||
* [Traditional Factory Method](https://java-design-patterns.com/patterns/factory/): Separates creation logic into a factory class. | ||
* [Enum Singleton](https://www.baeldung.com/java-singleton#enum-singleton): Ensures one instance per type via enum. | ||
* [Abstract Factory](https://java-design-patterns.com/patterns/abstract-factory/): Can be considered a kind of Factory that works with groups of products. | ||
* [Service Locator](https://java-design-patterns.com/patterns/service-locator/): If you combine enum with lookup behavior. | ||
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## References and Credits | ||
* [Design Patterns: Elements of Reusable Object-Oriented Software](https://amzn.to/3w0Rk5y) | ||
* [Effective Java](https://amzn.to/4cGk2Jz) | ||
* [Head First Design Patterns: Building Extensible and Maintainable Object-Oriented Software](https://amzn.to/3UpTLrG) |
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@startuml | ||
package com.iluwatar.factory { | ||
|
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class App { | ||
- LOGGER : Logger {static} | ||
+ App() | ||
+ main(args : String[]) {static} | ||
} | ||
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interface FileProcessor { | ||
+ getDescription() : String {abstract} | ||
} | ||
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enum FileProcessorType { | ||
+ EXCEL {static} | ||
+ PDF {static} | ||
- instance : FileProcessor | ||
+ getInstance() : FileProcessor | ||
+ valueOf(name : String) : FileProcessorType {static} | ||
+ values() : FileProcessorType[] {static} | ||
} | ||
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class ExcelProcessor { | ||
~ DESCRIPTION : String {static} | ||
+ ExcelProcessor() | ||
+ getDescription() : String | ||
} | ||
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class PDFProcessor { | ||
~ DESCRIPTION : String {static} | ||
+ PDFProcessor() | ||
+ getDescription() : String | ||
} | ||
} | ||
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ExcelProcessor ..|> FileProcessor | ||
PDFProcessor ..|> FileProcessor | ||
@enduml |
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<?xml version="1.0" encoding="UTF-8"?> | ||
<!-- | ||
This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt). | ||
The MIT License | ||
Copyright © 2014-2022 Ilkka Seppälä | ||
Permission is hereby granted, free of charge, to any person obtaining a copy | ||
of this software and associated documentation files (the "Software"), to deal | ||
in the Software without restriction, including without limitation the rights | ||
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | ||
copies of the Software, and to permit persons to whom the Software is | ||
furnished to do so, subject to the following conditions: | ||
The above copyright notice and this permission notice shall be included in | ||
all copies or substantial portions of the Software. | ||
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | ||
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
THE SOFTWARE. | ||
--> | ||
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd"> | ||
<modelVersion>4.0.0</modelVersion> | ||
<parent> | ||
<groupId>com.iluwatar</groupId> | ||
<artifactId>java-design-patterns</artifactId> | ||
<version>1.26.0-SNAPSHOT</version> | ||
</parent> | ||
<artifactId>factory-enum</artifactId> | ||
<dependencies> | ||
<dependency> | ||
<groupId>org.slf4j</groupId> | ||
<artifactId>slf4j-api</artifactId> | ||
</dependency> | ||
<dependency> | ||
<groupId>ch.qos.logback</groupId> | ||
<artifactId>logback-classic</artifactId> | ||
</dependency> | ||
<dependency> | ||
<groupId>org.junit.jupiter</groupId> | ||
<artifactId>junit-jupiter-engine</artifactId> | ||
<scope>test</scope> | ||
</dependency> | ||
</dependencies> | ||
<build> | ||
<plugins> | ||
<!-- Maven assembly plugin is invoked with default setting which we have | ||
in parent pom and specifying the class having main method --> | ||
<plugin> | ||
<groupId>org.apache.maven.plugins</groupId> | ||
<artifactId>maven-assembly-plugin</artifactId> | ||
<executions> | ||
<execution> | ||
<configuration> | ||
<archive> | ||
<manifest> | ||
<mainClass>com.iluwatar.factory.App</mainClass> | ||
</manifest> | ||
</archive> | ||
</configuration> | ||
</execution> | ||
</executions> | ||
</plugin> | ||
</plugins> | ||
</build> | ||
</project> |
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/* | ||
* This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt). | ||
* | ||
* The MIT License | ||
* Copyright © 2014-2022 Ilkka Seppälä | ||
* | ||
* Permission is hereby granted, free of charge, to any person obtaining a copy | ||
* of this software and associated documentation files (the "Software"), to deal | ||
* in the Software without restriction, including without limitation the rights | ||
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | ||
* copies of the Software, and to permit persons to whom the Software is | ||
* furnished to do so, subject to the following conditions: | ||
* | ||
* The above copyright notice and this permission notice shall be included in | ||
* all copies or substantial portions of the Software. | ||
* | ||
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | ||
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
* THE SOFTWARE. | ||
*/ | ||
package com.iluwatar.factory; | ||
|
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import lombok.extern.slf4j.Slf4j; | ||
|
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/** | ||
* The Factory Enum Pattern is a creational design pattern that leverages Java enums to encapsulate | ||
* the creation logic of different object types. It promotes cleaner code by removing the need for | ||
* separate factory classes and centralizing instantiation within type-safe enum constants. | ||
* | ||
* <p>In this example, different file processors (Excel, PDF) implement the same interface. The | ||
* FileProcessorType enum holds the corresponding instance for each processor, allowing the client | ||
* code to retrieve them in a clean and consistent way. | ||
*/ | ||
@Slf4j | ||
public class App { | ||
|
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/** Program main entry point. */ | ||
public static void main(String[] args) { | ||
LOGGER.info("The alchemist begins his work."); | ||
var excelProcessor = FileProcessorType.EXCEL.getInstance(); | ||
var pdfProcessor = FileProcessorType.PDF.getInstance(); | ||
LOGGER.info(excelProcessor.getDescription()); | ||
LOGGER.info(pdfProcessor.getDescription()); | ||
} | ||
} |
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45
factory-enum/src/main/java/com/iluwatar/factory/ExcelProcessor.java
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/* | ||
* This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt). | ||
* | ||
* The MIT License | ||
* Copyright © 2014-2022 Ilkka Seppälä | ||
* | ||
* Permission is hereby granted, free of charge, to any person obtaining a copy | ||
* of this software and associated documentation files (the "Software"), to deal | ||
* in the Software without restriction, including without limitation the rights | ||
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | ||
* copies of the Software, and to permit persons to whom the Software is | ||
* furnished to do so, subject to the following conditions: | ||
* | ||
* The above copyright notice and this permission notice shall be included in | ||
* all copies or substantial portions of the Software. | ||
* | ||
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | ||
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
* THE SOFTWARE. | ||
*/ | ||
package com.iluwatar.factory; | ||
|
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import lombok.AccessLevel; | ||
import lombok.NoArgsConstructor; | ||
|
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/** ExcelProcessor implementation. */ | ||
@NoArgsConstructor(access = AccessLevel.PRIVATE) | ||
public class ExcelProcessor implements FileProcessor { | ||
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static final String DESCRIPTION = "This is a excel processor."; | ||
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@Override | ||
public String getDescription() { | ||
return DESCRIPTION; | ||
} | ||
|
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@NoArgsConstructor(access = AccessLevel.PRIVATE) | ||
public static class InstanceHolder { | ||
public static final ExcelProcessor INSTANCE = new ExcelProcessor(); | ||
} | ||
} |
31 changes: 31 additions & 0 deletions
31
factory-enum/src/main/java/com/iluwatar/factory/FileProcessor.java
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/* | ||
* This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt). | ||
* | ||
* The MIT License | ||
* Copyright © 2014-2022 Ilkka Seppälä | ||
* | ||
* Permission is hereby granted, free of charge, to any person obtaining a copy | ||
* of this software and associated documentation files (the "Software"), to deal | ||
* in the Software without restriction, including without limitation the rights | ||
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | ||
* copies of the Software, and to permit persons to whom the Software is | ||
* furnished to do so, subject to the following conditions: | ||
* | ||
* The above copyright notice and this permission notice shall be included in | ||
* all copies or substantial portions of the Software. | ||
* | ||
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | ||
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
* THE SOFTWARE. | ||
*/ | ||
package com.iluwatar.factory; | ||
|
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/** FileProcessor interface. */ | ||
public interface FileProcessor { | ||
|
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String getDescription(); | ||
} |
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minor suggestion-
this better be private and class property should be accesses only through
getDescription