Should every single object have an interface and all objects loosely coupled?

Question

From what I have read best practice is to have classes based on an interface and loosely couple the objects, in order to help code re-use and unit test.

Is this corr

Answer 1

Interfaces should be used when you want to clearly define the interaction between two different sections of your software. Especially when it is possible that you want to rip out either end of the connection and replace it with something else.

For example in my CAM application I have a CuttingPath connected to a Collection of Points. It makes no sense to have a IPointList interface as CuttingPaths are always going to be comprised of Points in my application.

However I uses the interface IMotionController to communicate with the machine because we support many different types of cutting machine each with their own commend set and method of communications. So in that case it makes sense to put it behind a interface as one installation may be using a different machine than another.

Our applications has been maintain since the mid 80s and went to a object oriented design in late 90s. I have found that what could change greatly exceeded what I originally thought and the use of interfaces has grown. For example it used to be that our DrawingPath was comprised of points. But now it is comprised of entities (splines, arcs, ec) So it is pointed to a EntityList that is a collection of Object implementing IEntity interface.

But that change was propelled by the realization that a DrawingPath could be drawn using many different methods. Once that it was realized that a variety of drawing methods was needed then the need for a interface as opposed to a fixed relationship to a Entity Object was indicated.

Note that in our system DrawingPaths are rendered down to a low level cutting path which are always series of point segments.