Dependency injection (DI) is a design pattern used in object-oriented programming that involves injecting or providing dependencies to an object, rather than having the object create or find its dependencies itself. A dependency is an object that a class or method relies on to perform its functionality.

With dependency injection, dependencies are passed to an object through its constructor, method parameters, or through a specialized framework or container that manages the dependencies. This approach allows for a more loosely coupled architecture, making it easier to manage dependencies, test code, and change the implementation of a dependency without impacting the rest of the system.

The benefits of dependency injection include increased modularity, testability, and maintainability of code. By separating the concerns of a class or method from its dependencies, it becomes easier to modify and test the code. Additionally, it makes it easier to switch out dependencies with alternate implementations or mock objects for testing.

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For example, let's say you have a class that needs to access a database to retrieve and manipulate data. Rather than having the class create a new instance of the database object, which would create a tight coupling between the class and the database implementation, you can pass the database object to the class through its constructor or method parameters. This allows the class to use the database object without having to know how it was created or initialized, making it easier to modify and test the class.

Overall, dependency injection is a powerful technique for creating modular and testable code, and is widely used in modern software development.