Repository: iluwatar/java-design-patterns
Issue: Explanation for Data Locality
Labels: type: feature
epic: documentation

In this issue, let's add a proper explanation for the pattern.

Acceptance criteria

- The pattern's README.md has been amended with an explanation
- The explanation consists of
  - Real world example
  - In plain words
  - Wikipedia says
  - Programmatic example


Issue comments:
@iluwatar I can try this as well!
This issue has been automatically marked as stale because it has not had recent activity. The issue will be unassigned if no further activity occurs. Thank you for your contributions.


Pull request review comments:

Patch that resolved the issue:
diff --git a/data-locality/README.md b/data-locality/README.md
index 212bd663a08d..c44ba1a34fa5 100644
--- a/data-locality/README.md
+++ b/data-locality/README.md
@@ -1,29 +1,135 @@
 ---
 title: Data Locality
-category: Behavioral
+category: Performance optimization
 language: en
 tag:
- - Game programming
- - Performance
+    - Caching
+    - Data access
+    - Game programming
+    - Memory management
+    - Performance
 ---
 
+## Also known as
+
+* Cache-Friendly Design
+* Data-Oriented Design
+
 ## Intent
-Accelerate memory access by arranging data to take advantage of CPU caching.
 
-Modern CPUs have caches to speed up memory access. These can access memory adjacent to recently accessed memory much quicker. Take advantage of that to improve performance by increasing data locality keeping data in contiguous memory in the order that you process it.
+The Data Locality design pattern aims to minimize data access times and improve cache utilization by arranging data in memory to take advantage of spatial locality. This pattern is particularly useful in high-performance computing and game development where access speed is crucial.
+
+## Explanation
+
+Real-world example
+
+> Consider a supermarket where items are arranged based on purchase patterns and categories for efficiency. Just like the Data Locality pattern organizes data in memory for quick access, the supermarket places frequently bought items together and in easily accessible areas. This layout minimizes the time shoppers spend searching for items, enhancing their shopping experience by ensuring that related and popular items are close at hand, much like how data locality improves cache utilization and reduces access latency in computing.
+
+In plain words
+
+> The Data Locality pattern organizes data in memory to reduce access times and improve performance by ensuring that data frequently accessed together is stored close together.
+
+Programmatic Example
+
+The Data Locality pattern is a design pattern that aims to improve performance by arranging data in memory to take advantage of spatial locality. This pattern is particularly useful in high-performance computing and game development where access speed is crucial.
+
+In the data-locality module, the pattern is demonstrated using a game loop that processes a bunch of game entities. These entities are decomposed into different domains: AI, physics, and rendering.
+
+The GameEntity class is the main class that represents a game entity. It contains an array of AiComponent, PhysicsComponent, and RenderComponent objects. These components represent different aspects of a game entity.
+
+```java
+public class GameEntity {
+    private final AiComponent[] aiComponents;
+    private final PhysicsComponent[] physicsComponents;
+    private final RenderComponent[] renderComponents;
+// ...
+}
+```
+
+The GameEntity class has a start method that initializes all the components.
+
+```java
+public void start() {
+  for (int i = 0; i < numEntities; i++) {
+    aiComponents[i] = new AiComponent();
+    physicsComponents[i] = new PhysicsComponent();
+    renderComponents[i] = new RenderComponent();
+  }
+}
+```
+
+The GameEntity class also has an update method that updates all the components. This method demonstrates the data locality pattern. Instead of updating all aspects of a single entity at a time (AI, physics, and rendering), it updates the same aspect (e.g., AI) for all entities first, then moves on to the next aspect (e.g., physics). This approach improves cache utilization because it's more likely that the data needed for the update is already in the cache.
+
+```java
+public void update() {
+  for (int i = 0; i < numEntities; i++) {
+    aiComponents[i].update();
+  }
+  for (int i = 0; i < numEntities; i++) {
+    physicsComponents[i].update();
+  }
+  for (int i = 0; i < numEntities; i++) {
+    renderComponents[i].update();
+  }
+}
+```
+
+The Application class contains the main method that creates a GameEntity object and starts the game loop.
+
+```java
+public class Application {
+  public static void main(String[] args) {
+    var gameEntity = new GameEntity(NUM_ENTITIES);
+    gameEntity.start();
+    gameEntity.update();
+  }
+}
+```
+
+In this way, the data-locality module demonstrates the Data Locality pattern. By updating all components of the same type together, it increases the likelihood that the data needed for the update is already in the cache, thereby improving performance.
 
 ## Class diagram
+
 ![alt text](./etc/data-locality.urm.png "Data Locality pattern class diagram")
 
 ## Applicability
 
-* Like most optimizations, the first guideline for using the Data Locality pattern is when you have a performance problem.
-* With this pattern specifically, you’ll also want to be sure your performance problems are caused by cache misses.
+This pattern is applicable in scenarios where large datasets are processed and performance is critical. It's particularly useful in:
+
+* Game development for efficient rendering and physics calculations.
+* High-performance computing tasks that require rapid access to large data sets.
+* Real-time data processing systems where latency is a critical factor.
+
+## Known Uses
+
+* Game engines (e.g., Unity, Unreal Engine) to optimize entity and component data access.
+* High-performance matrix libraries in scientific computing to optimize matrix operations.
+* Real-time streaming data processing systems for efficient data manipulation and access.
+
+## Consequences
+
+Benefits:
+
+* Improved Cache Utilization: By enhancing spatial locality, data frequently accessed together is stored close together in memory, improving cache hit rates.
+* Reduced Access Latency: Minimizes the time taken to fetch data from memory, leading to performance improvements.
+* Enhanced Performance: Overall system performance is improved due to reduced memory access times and increased efficiency in data processing.
+
+Trade-offs:
+
+* Complexity in Implementation: Managing data layout can add complexity to the system design and implementation.
+* Maintenance Overhead: As data access patterns evolve, the layout may need to be re-evaluated, adding to the maintenance overhead.
+* Less Flexibility: The tight coupling of data layout to access patterns can reduce flexibility in how data structures are used and evolved over time.
 
-## Real world example
+## Related Patterns
 
-* The [Artemis](http://gamadu.com/artemis/) game engine is one of the first and better-known frameworks that uses simple IDs for game entities.
+* [Flyweight](https://java-design-patterns.com/patterns/flyweight/): Can be used in conjunction with Data Locality to share data efficiently among multiple objects.
+* [Object Pool](https://java-design-patterns.com/patterns/object-pool/): Often used together to manage a group of initialized objects that can be reused, further optimizing memory usage and access.
+* [Iterator](https://java-design-patterns.com/patterns/iterator/): Facilitates navigation through a collection of data laid out with data locality in mind.
 
 ## Credits
 
-* [Game Programming Patterns Optimization Patterns: Data Locality](http://gameprogrammingpatterns.com/data-locality.html)
\ No newline at end of file
+* [Game Programming Patterns](https://amzn.to/3vK8c0d)
+* [High-Performance Java Persistence](https://amzn.to/3TMc8Wd)
+* [Java Performance: The Definitive Guide](https://amzn.to/3Ua392J)
+* [Effective Java](https://amzn.to/4cGk2Jz)
+* [Game Programming Patterns Optimization Patterns: Data Locality](http://gameprogrammingpatterns.com/data-locality.html)


Test patch:
diff --git a/data-locality/src/test/java/com/iluwatar/data/locality/ApplicationTest.java b/data-locality/src/test/java/com/iluwatar/data/locality/ApplicationTest.java
index 1f5986c9f2bc..1f1689425f41 100644
--- a/data-locality/src/test/java/com/iluwatar/data/locality/ApplicationTest.java
+++ b/data-locality/src/test/java/com/iluwatar/data/locality/ApplicationTest.java
@@ -36,7 +36,6 @@ class ApplicationTest {
 
   /**
    * Issue: Add at least one assertion to this test case.
-   *
    * Solution: Inserted assertion to check whether the execution of the main method in {@link Application#main(String[])}
    * throws an exception.
    */