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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
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.
> 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
@ -25,11 +29,12 @@ In plain words
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**
Let's first introduce the tobacco brands.
Let's first introduce the `Tobacco` interface and the concrete brands.
```java
public abstract class Tobacco {
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}
```
Next here's the wizard class hierarchy.
Next here's the `Wizard` class hierarchy.
```java
public interface Wizard {
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```
## Class diagram
![alt text](./etc/dependency-injection.png "Dependency Injection")
## 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
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.
## Credits