What is the point of interfaces??? Confused!!!!

Question

I have obviously missed something big out because I know that interfaces are very important but I don't get the point of them.
I underdstand an interface as a class which declared methods but does not implement them i.e.: public void hello(int num);.
Then you implement it in a class and you implement the hello method. Why don't you just implement the hello method in the class, why would you want to create an interface and then implement it.
I have obviously missed something here so could someone give me an example and quickly show me how to use an interface and explain why I would want to use one.
Manny thanks,

Answer 1

I'm brand new to Java myself, and haven't yet written anything worthwhile, so take what I say with a grain of salt. BUT... my understanding of interfaces is as follows:
An interface allows you define a "way" in which something should be done without specifying "how" that thing should be done. Say you have a computer, and you want to write some software that controls how the cursor is moved around on the screen. So, you could write your software so that it watches for activity coming from the mouse, and then moves the cursor according to how the mouse was moved. Great, right? Not quite.
What happens when someone comes along and wants to install your software on a laptop, which uses a touchpad instead of a mouse? Well, unfortunately, you wrote your software so that it watched for mouse activity, and not touchpad activity, so the cursor doesn't respond correctly. (Yes this is a theoretical example, I understand that this sort of thing is really abstracted away by the operating system, but let's just pretend that we're writing operating system-level software.)
Now you can dig deep into your code and change it so that it works for both mice and touchpads, but this requires you to carefully check all the surrounding layers of code to make sure that you didn't screw anything else up. And then what happens if you find that someone has invented a light-pen that lets a stylus be used on the screen to move the mouse? You have to go through it all again!
If instead you had defined an interface to define how input devices should move the cursor, you might have an easier time of it. You can write a new class that handles touchpad or light-pen input and moves the cursor appropriately, and as long as it implements the interface that you wrote, your code can still take advantage of polymorphism -- because the way in which the input devices are manipulated was "officially" defined by the interface ahead of time. You still have to write the code to handle the touchpad or light-pen, but it plugs into your code more easily.
Now the question of when to use an interface and when to use an abstract class... I'm not totally clear on that yet. It seems that an interface is more general, whereas an abstract class is when you know that you're going to be writing a class that descends ("is a") from another class....
HTH
Erik