Dependency Injection (DI) in Android is a software design pattern that allows the separation of object creation and its dependencies from the class that uses them. It is a way to provide the required dependencies to an object from the outside, rather than having the object create or find its dependencies itself. In simple words, DI helps in achieving loose coupling between classes by removing the responsibility of creating and managing dependencies from the dependent class. Instead, dependencies are provided or “injected” into the class from an external source.

Let’s say we have a class called Car that requires an instance of the Engine class to function. Without using DI, the Car class would have to create an instance of Engine internally, like this

public class Car {
private Engine engine;

public Car() {
this.engine = new Engine();
}

// Car class methods that use the engine
}

However, with Dependency Injection, the Car class’s dependency on Engine is injected from the outside

public class Car {
private Engine engine;

public Car(Engine engine) {
this.engine = engine;
}

// Car class methods that use the engine
}

the Engine instance is provided to the Car class through its constructor. This way, the Car class is not responsible for creating the Engine object, and it can work with any implementation of the Engine interface.

By using DI, we can easily swap out the Engine implementation or mock it during testing, without modifying the Car class. It improves modularity, testability, and makes the code more flexible and maintainable.