The Most Concise Guide to Dependency Injection for Beginners

I want to explain dependency injection in the easiest way for beginners to understand.

This will be the first of a 3-part series to dependency injection and Dagger 2 for Android.

Prerequisites

1. Java
2. Android (optional for Part 1)

Lessons

1. Part 1: What’s dependency injection?
2. Part 2: Dagger 2
3. Part 3: Dagger-Android

Part 1: Dependency Injection

Suppose we have the following:

interface Healer
{
    void heal();
}
class Mercy implements Healer 
{
    ...
}

BAD

The following is NOT dependency injection. Any time you write new, you’re creating a hard dependency that will make the code difficult to reuse or mock out in a test. You cannot control what the Overwatch constructor does.

public Overwatch()
{
     this.healer = new Mercy(); //hard dependency
}

GOOD

The following IS dependency injection—we now have control (stub out, substitute etc.) over the injected variable.

public Overwatch(@NonNull Healer healer) //inject a healer instead
{
    this.healer = healer;
}

Now wasn’t that simple?