What does “program to interfaces, not implementations” mean?

Question

One stumbles upon this phrase when reading about design patterns.

But I don\'t understand it, could someone explain this for me?

Answer 1

This statement is about coupling. One potential reason for using object oriented programming is reuse. So for example you can split your algorithm among two collaborating objects A and B. This might be useful for later creation of another algorithm, which might reuse one or another of the two objects. However, when those objects communicate (send messages - call methods), they create dependencies among each other. But if you want to use one without the other, you need to specify what should do some other object C do for object A if we replace B. Those descriptions are called interfaces. This allows object A to communicate without change with different object relying on the interface. The statement you mentioned says that if you plan to reuse some part of an algorithm (or more generally a program), you should create interfaces and rely on them, so you might change the concrete implementation any time without changing other objects if you use the declared interface.