Dependency Injection

Dependency injection (Wikipedia) is an OOP design pattern where classes declare their dependencies as arguments instead of directly creating them.

This pattern lets us easily create flexible and testable objects, following the Liskov Substitution Principle

Without Dependency Injection

Here is an example where we do not use dependency injection:

public class UserDao {
    
    private final Database database = new SQLiteDatabase("/myfile.dat");
    
    // ...
    
}

In this example, you can't replace the database implementation or file path, which makes it hard to test and re-use

With Dependency Injection

Here is an example where we do use dependency injection:

public class UserDao {
    
    private final Database database;
    
    public UserDao(Database database) {
        this.database = database;
    }
    
    // ...
    
}

In this example, we can set any database implementation, so it is testable and re-usable

With unnamed/inject

Here is an example that uses dependency injection with unnamed/inject

import team.unnamed.inject.Inject;

public class UserDao {
    
    @Inject private Database database;
    
}

In this example, we just declare a dependency using the @Inject annotation instead of as an argument in a constructor. However, the @Inject annotation is not magic and this will not work using new UserDao() since the object is not processed.