Dagger 2 for Android Beginners — Introduction

Dagger is a fully static, compile-time dependency injection framework for both Java and Android. It is an adaptation of an earlier version created by Square and now maintained by Google. (source)

Who should read?

If you’re a budding Android developer, who started with Android directly and learning Java parallely — this is for you. Or if you’re one who tried learning Dagger 2 and you’re finding it little difficult to understand the other online content — don’t worry, I was having a tough time too (we are little special, we need to be explained differently) and this is definitely for you. If you’re already experienced in DI and Dagger, you may pick up something new or get more clarity (or may sound very beginner).

Series Pitstops

Prerequisites

I assume that you’re familiar with Java Programming language, OOP principles and Android development.

What is Dependency?

Note : In order to explain the following concept, I’m using the analogy with Game of Thrones. If you’re not familiar with that, please replace the following class names — Targaryens and Dragons with A and B as you proceed. And, you should definitely start watching that series though :-P

Understanding dependency/coupling is the first step in achieving more clarity in object oriented programming concepts. So, what is a dependency? For example, we have a class called Targaryens which uses another class or interface called Dragons inside the Targaryens class — then class Targaryens depends upon the class or interface Dragons.

Targaryens dependant on Dragons

This means that Targaryens cannot work without Dragons. This also means that where ever Targaryens are, Dragons needs to be there i.e, we cannot reuse Targaryens without reusing Dragons. So in this scenario, the Targaryens are called the dependants and the Dragons are called the dependency. A dependant depends on its dependencies.

The two classes that use each other are termed as ‘coupled’. The coupling between classes are not binary — it’s either ‘tight’(strong) or ‘loose’(weak). These dependencies are always directional i.e. Targaryens depend upon Dragons but Dragons may not depend upon Targaryens.

Explaining dependency

Why dependencies are bad?

When there are many dependencies floating around the class, it leads to hard dependency problems — which are bad because of the following reasons

  • Hard dependencies reduce the reusability
  • Hard dependencies make it hard for testing
  • Hard dependencies hinders code maintainability when the project scales up

#Reusablility

When classes and methods are loosely or not coupled or not dependant on many things, it’s reusable nature increases. Reusability is one of the core commandment of object-oriented programming.

#Testing

For testing, you will mock certain objects. But if there are so many dependencies within the method or the class, it will be hard to test. If a java class creates an instance of another class via the new operator, then it cannot be used and tested independently from that class.

#Maintainability

When your code cannot be tested properly and the components are not reusable and if your project continues to grow, it will be very difficult to maintain. Maintainability includes a lot of other factors, but as long as new developers in your team understands the system and makes the work of developers comfortable, you are good to go.

Types of Dependencies

Dependencies are of many types, the common ones are

  • Class dependency
  • Interface dependency
  • Method/field dependency
  • Direct and indirect dependency

And few more dependencies are there, which are not included here.

#Class Dependency

Class dependencies are the dependencies on classes. We have seen the class dependency example earlier, the constructor method Targaryens depends on or needs the Dragons class to invoke callForWar().

#Interface Dependency

Interface dependencies are the dependencies of the interfaces. The method executePlan takes the interface WarStrategy as the dependency. Where WarDependency will be implemented by all the houses (Targaryens, Starks, Lannisters etc..)

#Method or Field Dependency

Method or field dependencies are dependencies on concrete methods or fields of an object. The following method takes a class object and the method getKing() is the method dependency, which cannot be identified with method declaration.

#Direct and indirect dependency

Let me break this down with another example. Class Targaryens are dependant on class Starks for conquering the throne. But Starks are dependant on other Kingdoms in the North say, class Mormont — so now, Targaryens are indirectly dependant on Mormont and directly dependant on Starks.

TL;DR

If a java class creates an instance of another class via the new operator, then it cannot be used and tested independently from that class and is called a dependency.

Dependencies are bad because it prevents the code reusability throughout the project and also makes the job of testing difficult — because of this it also hinders the code maintenance.

Dependencies are of many types like class, interface, method or field, direct and indirect etc..

What’s Next?

On the next pitstop, we’ll be talking on the solution for this hard dependency problem — yes, you’re right : dependency injection. We’ll go through a real scenario, identify hard dependency and analyse it.

Plus, checkout my other stories.

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