feat: add factory enum

factory-enum/README.md

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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
+
+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.
+
+This pattern is ideal when the set of possible objects is known and fixed (e.g., file processors, coin types, document types).
+
+## Real-World Analogy
+
+> 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.
+
+## Programmatic Example Using FileProcessor
+
+In this example, we want to process different file types: Excel and PDF. Each processor implements the `FileProcessor` interface.
+
+### FileProcessor Interface
+
+```java
+public interface FileProcessor {
+  String getDescription();
+}
+```
+Implementations
+```java
+@NoArgsConstructor(access = AccessLevel.PRIVATE)
+public class ExcelProcessor implements FileProcessor {
+
+  static final String DESCRIPTION = "This is an Excel processor.";
+
+  @Override
+  public String getDescription() {
+    return DESCRIPTION;
+  }
+
+  public static class InstanceHolder {
+    public static final ExcelProcessor INSTANCE = new ExcelProcessor();
+  }
+}
+```
+```java
+@NoArgsConstructor(access = AccessLevel.PRIVATE)
+public class PDFProcessor implements FileProcessor {
+
+  static final String DESCRIPTION = "This is a PDF processor.";
+
+  @Override
+  public String getDescription() {
+    return DESCRIPTION;
+  }
+
+  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);
+
+  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
+
+* When object types are known in advance and won't change frequently.
+
+* When you want singleton instances per type.
+
+* To avoid traditional factory classes and keep the logic self-contained within an enum.
+
+* When readability and simplicity are preferred over extensibility.
+
+## 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.
+
+## 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.
+
+## 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.
+
+## 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)

factory-enum/etc/factory.urm.puml

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+@startuml
+package com.iluwatar.factory {
+
+  class App {
+    - LOGGER : Logger {static}
+    + App()
+    + main(args : String[]) {static}
+  }
+
+  interface FileProcessor {
+    + getDescription() : String {abstract}
+  }
+
+  enum FileProcessorType {
+    + EXCEL {static}
+    + PDF   {static}
+    - instance : FileProcessor
+    + getInstance() : FileProcessor
+    + valueOf(name : String) : FileProcessorType {static}
+    + values() : FileProcessorType[] {static}
+  }
+
+  class ExcelProcessor {
+    ~ DESCRIPTION : String {static}
+    + ExcelProcessor()
+    + getDescription() : String
+  }
+
+  class PDFProcessor {
+    ~ DESCRIPTION : String {static}
+    + PDFProcessor()
+    + getDescription() : String
+  }
+}
+
+ExcelProcessor ..|> FileProcessor
+PDFProcessor  ..|> FileProcessor
+@enduml

factory-enum/pom.xml

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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>

factory-enum/src/main/java/com/iluwatar/factory/App.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;
+
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * 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 {
+
+  /** Program main entry point. */
+  public static void main(String[] args) {
+    LOGGER.info("The alchemist begins his work.");
+    FileProcessor excelProcessor = FileProcessorType.EXCEL.getInstance();
+    FileProcessor pdfProcessor = FileProcessorType.PDF.getInstance();
+    LOGGER.info(excelProcessor.getDescription());
+    LOGGER.info(pdfProcessor.getDescription());
+  }
+}

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;
+
+import lombok.AccessLevel;
+import lombok.NoArgsConstructor;
+
+/** ExcelProcessor implementation. */
+@NoArgsConstructor(access = AccessLevel.PRIVATE)
+public class ExcelProcessor implements FileProcessor {
+
+  static final String DESCRIPTION = "This is a excel processor.";
+
+  @Override
+  public String getDescription() {
+    return DESCRIPTION;
+  }
+
+  @NoArgsConstructor(access = AccessLevel.PRIVATE)
+  public static class InstanceHolder {
+    public static final ExcelProcessor INSTANCE = new ExcelProcessor();
+  }
+}

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;
+
+/** FileProcessor interface. */
+public interface FileProcessor {
+
+  String getDescription();
+}