What is Dependency Injection and why is it useful?

Dependency Injection provides dependencies to objects rather than having objects create them. Types: Constructor injection (dependencies in constructor), Setter injection (via setter methods), Interface injection (dependency provides injector method). Benefits: loose coupling, easier testing (mock dependencies), follows Dependency Inversion Principle, supports configuration over code. Frameworks: Spring (Java), .NET Core DI, Angular, Guice. Container manages object lifecycle. DI enables modular, testable, maintainable code.