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Value Object

CreationalInstantiationAbout 1 min

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 @Valueopen in new window 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

alt text
Value Object

Applicability

Use the Value Object when

  • The object's equality needs to be based on the object's value

Known uses

Credits