Explaining Interfaces to Students [closed]

Question

For a few years I was a teaching assistant for an introduction to programming module - Java for first year undergraduates.

Mostly it went well and we managed to get object-oriented programming across to the students quite well, but one thing that students rarely saw the point of was interfaces.

Pretty much any explanation we gave either came across as too contrived to be useful for learning, or too far removed from their position as beginners. The reaction we tended to get was "I... see," translated as "I don't understand and they don't sound useful".

Anyone here have a way of successfully teaching students about interfaces? I'm not a teaching assistant any more, but it's always nagged at me.

Answer 1

If you are trying to explain it to beginners I would stick with the idea that interfaces can promote code reuse and modularity within the code:

For example lets say we are going to paint some objects:

public class Painter {     private List<Paintable> paintableObjects;      public Painter(){        paintableObjects = new ArrayList<Paintable>();     }      public void paintAllObjects(){         for(Paintable paintable : paintableObjects){             paintable.paint();         }     } }  public interface Paintable {      public void paint(); } 

Now you could explain to the students that without Paintable interface the Painter object would need to have methods to paint certain types of objects, like a method called paintFences() and paintRocks() and we would need to have a new Collection for each type of objects we want the painter to be able to paint.

But thankfully we have interfaces which make painting objects a breeze and how objects are painted is left entirely up to classes that implement the Paintable interface.

EDIT

Another benefit that I forgot to mention is that if you ever need to add new object to paint to your code base, all you need to do is create a new class that implements Paintable and the Painter class never has to change. In this sense the Painter class is never dependent upon the objects it is going to paint, it only needs to be able to paint them.

EDIT 2

James Raybould reminded me of a key use of interfaces I forgot to mention: Having an interface between your components, like the Paintable objects and Painter objects, allows you to more easily develop with other people. One developer can work on the Painter objects and another can work on the Paintable objects and all they have to do to function properly together is define a common interface beforehand that they will both use. I know when I've worked on projects with other people in college level projects its really helpful when you are trying to have everyone work on different parts of the project and still have all components come together nicely in the end.

Answer 2

In explaining interfaces and object oriented concepts in general to non-programmers, I always use the home entertainment system analogy.

The DVD player, TV, Cable Box, Remote Control are all objects that encapsulate complicated and sophisticated functionality. However, they have interfaces to each other and to the Humans that operate them that largely hide the lion share of that complexity.

The video in jack of a TV is an interface that is implemented by the DVD player and the cable box and a number of other types of devices.

I suspect it would be possible and perhaps an educational exercise for a student to describe their own home entertainment system entirely using Java code.