enhancement: Refactor and add explanation for value object (#1942)

Co-authored-by: Subhrodip Mohanta <[email protected]>

Author: iluwatar

value-object/README.md

@@ -10,19 +10,80 @@ tags:
 ---
 
 ## Intent
+
 Provide objects which follow value semantics rather than reference semantics.
-This means value objects' equality are not based on identity. Two value objects are
+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`](https://projectlombok.org/features/Value) annotation.
+
+```java
+@Value(staticConstructor = "valueOf")
+class HeroStat {
+
+    int strength;
+    int intelligence;
+    int luck;
+}
+```
+
+The example creates three different `HeroStat`s and compares their equality.
+
+```java
+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](./etc/value-object.png "Value Object")
 
 ## Applicability
+
 Use the Value Object when
 
-* You need to measure the objects' equality based on the objects' value
+* The object's equality needs to be based on the object's value
 
-## Real world examples
+## Known uses
 
 * [java.util.Optional](https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html)
 * [java.time.LocalDate](https://docs.oracle.com/javase/8/docs/api/java/time/LocalDate.html)
@@ -31,6 +92,7 @@ Use the Value Object when
 ## Credits
 
 * [Patterns of Enterprise Application Architecture](http://www.martinfowler.com/books/eaa.html)
+* [ValueObject](https://martinfowler.com/bliki/ValueObject.html)
 * [VALJOs - Value Java Objects : Stephen Colebourne's blog](http://blog.joda.org/2014/03/valjos-value-java-objects.html)
 * [Value Object : Wikipedia](https://en.wikipedia.org/wiki/Value_object)
 * [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)

value-object/src/main/java/com/iluwatar/value/object/App.java

@@ -43,14 +43,16 @@
 public class App {
 
   /**
-   * This practice creates three HeroStats(Value object) and checks equality between those.
+   * This example creates three HeroStats (value objects) and checks equality between those.
    */
   public static void main(String[] args) {
     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));

value-object/src/main/java/com/iluwatar/value/object/HeroStat.java

@@ -23,10 +23,7 @@
 
 package com.iluwatar.value.object;
 
-import lombok.EqualsAndHashCode;
-import lombok.Getter;
-import lombok.RequiredArgsConstructor;
-import lombok.ToString;
+import lombok.Value;
 
 /**
  * HeroStat is a value object.
@@ -35,23 +32,10 @@
  *     http://docs.oracle.com/javase/8/docs/api/java/lang/doc-files/ValueBased.html
  *     </a>
  */
-@Getter
-@ToString
-@EqualsAndHashCode
-@RequiredArgsConstructor
-public class HeroStat {
-
-  // Stats for a hero
-
-  private final int strength;
-  private final int intelligence;
-  private final int luck;
-
-  // Static factory method to create new instances.
-  public static HeroStat valueOf(int strength, int intelligence, int luck) {
-    return new HeroStat(strength, intelligence, luck);
-  }
-
-  // The clone() method should not be public. Just don't override it.
+@Value(staticConstructor = "valueOf")
+class HeroStat {
 
+  int strength;
+  int intelligence;
+  int luck;
 }