#590 add explanation for Dependency Injection

dependency-injection/README.md

@@ -9,12 +9,78 @@ tags:
 ---
 
 ## Intent
-Dependency Injection is a software design pattern in which one or
-more dependencies (or services) are injected, or passed by reference, into a
-dependent object (or client) and are made part of the client's state. The
-pattern separates the creation of a client's dependencies from its own
-behavior, which allows program designs to be loosely coupled and to follow the
-inversion of control and single responsibility principles.
+Dependency Injection is a software design pattern in which one or more dependencies (or services) are injected, or 
+passed by reference, into a dependent object (or client) and are made part of the client's state. The pattern separates 
+the creation of a client's dependencies from its own behavior, which allows program designs to be loosely coupled and 
+to follow the inversion of control and single responsibility principles.
+
+## Explanation
+Real world example
+
+> The old wizard likes to fill his pipe and smoke tobacco once in a while. However, he doesn't want to depend on a single tobacco brand only but likes to be able to enjoy them all interchangeably.    
+
+In plain words
+
+> Dependency Injection separates creation of client's dependencies from its own behavior. 
+
+Wikipedia says
+
+> In software engineering, dependency injection is a technique in which an object receives other objects that it depends on. These other objects are called dependencies.
+
+**Programmatic Example**
+
+Let's first introduce the tobacco brands.
+
+```java
+public abstract class Tobacco {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Tobacco.class);
+
+  public void smoke(Wizard wizard) {
+    LOGGER.info("{} smoking {}", wizard.getClass().getSimpleName(),
+        this.getClass().getSimpleName());
+  }
+}
+
+public class SecondBreakfastTobacco extends Tobacco {
+}
+
+public class RivendellTobacco extends Tobacco {
+}
+
+public class OldTobyTobacco extends Tobacco {
+}
+```
+
+Next here's the wizard class hierarchy.
+
+```java
+public interface Wizard {
+
+  void smoke();
+}
+
+public class AdvancedWizard implements Wizard {
+
+  private Tobacco tobacco;
+
+  public AdvancedWizard(Tobacco tobacco) {
+    this.tobacco = tobacco;
+  }
+
+  @Override
+  public void smoke() {
+    tobacco.smoke(this);
+  }
+}
+```
+
+And lastly we can show how easy it is to give the old wizard any brand of tobacco.
+
+```java
+    var advancedWizard = new AdvancedWizard(new SecondBreakfastTobacco());
+    advancedWizard.smoke();
+```
 
 ## Class diagram
 ![alt text](./etc/dependency-injection.png "Dependency Injection")
@@ -22,5 +88,12 @@ inversion of control and single responsibility principles.
 ## Applicability
 Use the Dependency Injection pattern when
 
-* when you need to remove knowledge of concrete implementation from object
-* to enable unit testing of classes in isolation using mock objects or stubs
+* When you need to remove knowledge of concrete implementation from object
+* To enable unit testing of classes in isolation using mock objects or stubs
+
+## Credits
+
+* [Dependency Injection Principles, Practices, and Patterns](https://www.amazon.com/gp/product/161729473X/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=161729473X&linkId=57079257a5c7d33755493802f3b884bd)
+* [Clean Code: A Handbook of Agile Software Craftsmanship](https://www.amazon.com/gp/product/0132350882/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0132350882&linkCode=as2&tag=javadesignpat-20&linkId=2c390d89cc9e61c01b9e7005c7842871)
+* [Java 9 Dependency Injection: Write loosely coupled code with Spring 5 and Guice](https://www.amazon.com/gp/product/1788296257/ref=as_li_tl?ie=UTF8&tag=javadesignpat-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=1788296257&linkId=4e9137a3bf722a8b5b156cce1eec0fc1)
+* [Google Guice Tutorial: Open source Java based dependency injection framework](https://www.amazon.com/gp/product/B083P7DZ8M/ref=as_li_tl?ie=UTF8&tag=javadesignpat-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=B083P7DZ8M&linkId=04f0f902c877921e45215b624a124bfe)