Value Object
- Creational
Intent
Provide objects which follow value semantics rather than reference semantics. This means value objects' equality is not based on identity. Two value objects are equal when they have the same value, not necessarily being the same object.
Explanation
Real-world example
There is a class for hero statistics in a role-playing game. The statistics contain attributes such as strength, intelligence, and luck. The statistics of different heroes should be equal when all the attributes are equal.
In plain words
Value objects are equal when their attributes have the same value
Wikipedia says
In computer science, a value object is a small object that represents a simple entity whose equality is not based on identity: i.e. two value objects are equal when they have the same value, not necessarily being the same object.
Programmatic Example
Here is the HeroStat class that is the value object. Notice the use of Lombok's @Value annotation.
@Value(staticConstructor = "valueOf")
class HeroStat {
int strength;
int intelligence;
int luck;
}
The example creates three different HeroStats and compares their equality.
var statA = HeroStat.valueOf(10, 5, 0);
var statB = HeroStat.valueOf(10, 5, 0);
var statC = HeroStat.valueOf(5, 1, 8);
LOGGER.info(statA.toString());
LOGGER.info(statB.toString());
LOGGER.info(statC.toString());
LOGGER.info("Is statA and statB equal : {}", statA.equals(statB));
LOGGER.info("Is statA and statC equal : {}", statA.equals(statC));
Here's the console output.
20:11:12.199 [main] INFO com.iluwatar.value.object.App - HeroStat(strength=10, intelligence=5, luck=0)
20:11:12.202 [main] INFO com.iluwatar.value.object.App - HeroStat(strength=10, intelligence=5, luck=0)
20:11:12.202 [main] INFO com.iluwatar.value.object.App - HeroStat(strength=5, intelligence=1, luck=8)
20:11:12.202 [main] INFO com.iluwatar.value.object.App - Is statA and statB equal : true
20:11:12.203 [main] INFO com.iluwatar.value.object.App - Is statA and statC equal : false
Class diagram
Applicability
Use the Value Object when
- The object's equality needs to be based on the object's value