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Ilkka Seppälä
4108f86177
* Changed database implementation. Removed static objects. * Fix Logs * Fix 40 errors from checkstyle plugin run. 139 left)) * Fix CacheStore errors from checkstyle plugin 107 left * Fix last errors in checkstyle. * Fix sonar issues * Fix issues in VALIDATE phase * Fix Bug with mongo connection. Used "Try with resources" * Add test * Added docker-compose for mongo db. MongoDb db work fixed. * Provided missing tests * Comments to start Application with mongo. * Fix some broken links * Remove extra space * Update filename * Fix some links in localization folders * Fix link * Update frontmatters * Work on patterns index page * Work on index page * Fixes according PR comments. Mainly Readme edits. * fix frontmatter * add missing png * Update pattern index.md * Add index.md for Chinese translation * update image paths * update circuit breaker image paths * Update image paths for localizations * add generated puml * Add missing image * Update img file extensions * Update the rest of the EN and ZH patterns to conform with the new website Co-authored-by: Victor Zalevskii <zvictormail@gmail.com>
96 lines
3.1 KiB
Markdown
96 lines
3.1 KiB
Markdown
---
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title: Value Object
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category: Creational
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language: en
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tags:
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- Instantiation
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---
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## Intent
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Provide objects which follow value semantics rather than reference semantics.
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This means value objects' equality is not based on identity. Two value objects are
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equal when they have the same value, not necessarily being the same object.
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## Explanation
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Real-world example
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> There is a class for hero statistics in a role-playing game. The statistics contain attributes
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> such as strength, intelligence, and luck. The statistics of different heroes should be equal
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> when all the attributes are equal.
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In plain words
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> Value objects are equal when their attributes have the same value
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Wikipedia says
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> In computer science, a value object is a small object that represents a simple entity whose
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> equality is not based on identity: i.e. two value objects are equal when they have the same
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> value, not necessarily being the same object.
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**Programmatic Example**
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Here is the `HeroStat` class that is the value object. Notice the use of
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[Lombok's `@Value`](https://projectlombok.org/features/Value) annotation.
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```java
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@Value(staticConstructor = "valueOf")
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class HeroStat {
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int strength;
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int intelligence;
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int luck;
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}
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```
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The example creates three different `HeroStat`s and compares their equality.
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```java
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var statA = HeroStat.valueOf(10, 5, 0);
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var statB = HeroStat.valueOf(10, 5, 0);
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var statC = HeroStat.valueOf(5, 1, 8);
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LOGGER.info(statA.toString());
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LOGGER.info(statB.toString());
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LOGGER.info(statC.toString());
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LOGGER.info("Is statA and statB equal : {}", statA.equals(statB));
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LOGGER.info("Is statA and statC equal : {}", statA.equals(statC));
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```
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Here's the console output.
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```
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20:11:12.199 [main] INFO com.iluwatar.value.object.App - HeroStat(strength=10, intelligence=5, luck=0)
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20:11:12.202 [main] INFO com.iluwatar.value.object.App - HeroStat(strength=10, intelligence=5, luck=0)
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20:11:12.202 [main] INFO com.iluwatar.value.object.App - HeroStat(strength=5, intelligence=1, luck=8)
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20:11:12.202 [main] INFO com.iluwatar.value.object.App - Is statA and statB equal : true
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20:11:12.203 [main] INFO com.iluwatar.value.object.App - Is statA and statC equal : false
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```
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## Class diagram
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## Applicability
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Use the Value Object when
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* The object's equality needs to be based on the object's value
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## Known uses
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* [java.util.Optional](https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html)
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* [java.time.LocalDate](https://docs.oracle.com/javase/8/docs/api/java/time/LocalDate.html)
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* [joda-time, money, beans](http://www.joda.org/)
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## Credits
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* [Patterns of Enterprise Application Architecture](http://www.martinfowler.com/books/eaa.html)
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* [ValueObject](https://martinfowler.com/bliki/ValueObject.html)
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* [VALJOs - Value Java Objects : Stephen Colebourne's blog](http://blog.joda.org/2014/03/valjos-value-java-objects.html)
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* [Value Object : Wikipedia](https://en.wikipedia.org/wiki/Value_object)
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* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=f27d2644fbe5026ea448791a8ad09c94)
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