Understand Complex Programming Concepts

Thank you for sharing the details about your learning requirements. Let's delve into Dependency Injection (DI) using your context.
Dependency Injection is a powerful design pattern that allows us to remove hard-coded dependencies and make our applications loosely coupled, extendable, and maintainable. We use the term "Injection" because the dependencies are pushed by an outside entity (an injector) into the class we want to decouple.
In Java, this is often done using frameworks like Spring, but the core principles can be applied even without any frameworks.
Suppose we have a class WebApplication that depends on a DatabaseService.
public class WebApplication {

    private DatabaseService dbService;

    public WebApplication() {
        this.dbService = new DatabaseService();
    }

    public void start() {
        this.dbService.connect();
        // ... other logic
    }
}

In the above code, WebApplication is tightly coupled with DatabaseService. This means that any change to DatabaseService can directly impact WebApplication, and it's also hard to unit test WebApplication without a real DatabaseService.
This is where Dependency Injection comes in. Instead of creating the DatabaseService inside WebApplication, we "inject" it, usually via a constructor or a setter method.
public class WebApplication {

    private DatabaseService dbService;

    public WebApplication(DatabaseService dbService) {
        this.dbService = dbService;
    }

    public void start() {
        this.dbService.connect();
        // ... other logic
    }
}

Now, we can create a DatabaseService outside of WebApplication and pass it in. This decouples the two classes, making it easier to swap out the DatabaseService for a different implementation or a mock in our tests.
When to use Dependency Injection:

When a class has a dependency on another class with a complex setup or configuration, DI helps to simplify this process.
When a class depends on an interface, and we want to be able to switch between different implementations of this interface.
When we want to make our application more modular and testable.

So in the context of developing a Java-based web application, using DI would be a good choice when you have components with dependencies that you'd like to be able to switch out, perhaps for different environments (like development and production), or for testing purposes. It also helps in creating code that adheres to the SOLID principles, thus making the application more flexible and maintainable.
Remember, like any design pattern, DI is a tool that's helpful in some situations but not all. Use your judgment about when it's appropriate based on your specific needs.