99 lines
3.2 KiB
Markdown
99 lines
3.2 KiB
Markdown
---
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layout: pattern
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title: Value Object
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folder: value-object
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permalink: /patterns/value-object/
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categories: 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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