What does it mean to "program to an interface"?

Question

I have seen this mentioned a few times and I am not clear on what it means. When and why would you do this? I know what interfaces do, but the fact I am not clear on this makes me think I am missing out on using them correctly. Is it just so if you were to do: IInterface classRef = new ObjectWhatever()You could use any class that implements `IInterface`? When would you need to do that? The only thing I can think of is if you have a method and you are unsure of what object will be passed except for it implementing `IInterface`. I cannot think how often you would need to do that. Also, how could you write a method that takes in an object that implements an interface? Is that possible?

Answer 1

You are defining the methods in the interface...

The methods shall work with a specific type of data..

For example, consider the quick sort routine.

The algorithm works the same way regardless of whatever type of data is being sorted.

So an interface that does the comparison is passed to the quicksort routine.