Does anyone have a good analogy for dependency injection? [closed]

Answer 1

The best analogy I can think of is that of hiring a mechanic.

Without dependency injection, you hire a mechanic and the mechanic brings his own tools. He may have lousy tools, he may have great tools, he may be using a pipe wrench when he should be using a socket. You don't know, and may not care, so long as he gets the work done.

With dependency injection, you hire a mechanic and you provide him with the tools that you want him to do his work with. You get to choose what you consider to be the best or most appropriate tools for the work you are hiring him to do.

Answer 2

Maybe focus on the "injection" part? When I see that term, I think of syringes. The process of pushing the dependencies of a component to the component can be thought of as injecting into the component.

Just like with the body, when there is something that it needs in the way of medicine (a component that it needs) you can inject it into the body.

Answer 3

A magician's sleight of hand! What you may think you see may be secretly manipulated or replaced.

Answer 4

Another analogy: let us say you are a developer and whenever you like you order computer science books from the market directly - you know the sellers and their prices. In fact your company might have a preferred seller and you contact them directly. All this works fine but may be a new seller is now offering better prices and your company wants to change the 'preferred' seller.

At this point you have to make the following changes - update the contact details (and other stuff) so as to use the new seller. You still place the order directly.

Now consider we introduce a new step in between, there is a 'library' officer in the company and you have to go through him to get the books. While there is a new dependency, you are now immune to any changes to the seller: either the seller changes mode of payment or the seller himself is changed, you now simply put an order to the librarian and he gets the books for you.

Answer 5

Life is full of dependency injection analogies:

• printer - cartridge
• digital device - battery
• letter - stamp
• musician - instrument
• bus - driver
• sickness - pill

Answer 6

In their 2003 JavaPolis presentation (slides), Jon Tirsén & Aslak Hellesøy had an amusing analogy with a Girl object that needs a Boy to kiss. I seem to remember that the BoyFactory is sometimes known as a 'nightclub', but that's not in the slides.