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Ilkka Seppälä
2020-08-29 16:56:52 +03:00
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dependency-injection/README.md
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--- ---
## Intent ## 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 Dependency Injection is a software design pattern in which one or more dependencies (or services)
the creation of a client's dependencies from its own behavior, which allows program designs to be loosely coupled and are injected, or passed by reference, into a dependent object (or client) and are made part of the
to follow the inversion of control and single responsibility principles. 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 ## Explanation
Real world example 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. > 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 In plain words
@@ -25,11 +29,12 @@ In plain words
Wikipedia says 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. > 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** **Programmatic Example**
Let's first introduce the tobacco brands. Let's first introduce the `Tobacco` interface and the concrete brands.
```java ```java
public abstract class Tobacco { public abstract class Tobacco {
@@ -52,7 +57,7 @@ public class OldTobyTobacco extends Tobacco {
} }
``` ```
Next here's the wizard class hierarchy. Next here's the `Wizard` class hierarchy.
```java ```java
public interface Wizard { public interface Wizard {
@@ -83,13 +88,15 @@ And lastly we can show how easy it is to give the old wizard any brand of tobacc
``` ```
## Class diagram ## Class diagram
![alt text](./etc/dependency-injection.png "Dependency Injection") ![alt text](./etc/dependency-injection.png "Dependency Injection")
## Applicability ## Applicability
Use the Dependency Injection pattern when
* When you need to remove knowledge of concrete implementation from object Use the Dependency Injection pattern when:
* 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 ## Credits