I was browsing reddit this afternoon and came across this post. It’s an interesting concept to explain. I mean, what is an interface in Java?

I thought through the best way to explain this to someone who doesn’t understand OO or Java in general, and I came up with the following.

The Letterbox Analogy
Think of a letterbox.

Doesn’t matter who makes the letterbox, it’ll always operate the same way. Doesn’t matter what country you go to, if you want to post a letter, you just put it through a letter box.

public interface Letterbox {
void post(Letter letter);
}

If I’m a manufacturer, I might want to make exquisite letterboxes. I’ll make ‘em out of gold, and they’ll be sturdy and look outstanding. The shape, though, will remain the same as any other. So I’ll implement the Letterbox interface:

public class GoldLetterBox implements Letterbox {
public void post(Letter letter) {
// some code...
}
}

and then people will buy them, and they’ll put them in doors, and their postal employees will put their letters and packages through the door. It’ll still be Super Special Gold Letterbox Co’s letterbox, but everyone will know how to use it.