How to explain an object?

Question

It\'s been years since I thought of this, but I am training some real juniors soon and need to explain what an object is to someone who doesn\'t know what it is.

B

Answer 1

Stick with Booch's definition: An object has state, exhibits some well-defined behavior, and has a unique identity. I noticed that others posted this already, but people made comments regarding some objects not needing state, behavior, or identity which I disagree with. An object has state in the sense that some programmer defined amount of memory has been allocated for it, behavior in the sense that it can react to messages being sent to it, and identity in the sense that it can be assigned an identifier. Which actually sounds a lot like Kay's explanation of a lot of little specialized computers communicating with one another.

I also agree with the few posts that mention first understanding the concepts of procedural programming, because the work in an object-oriented program is still done at the procedure level. The difference is the level of abstraction at which the programmer can write the program. In object-oriented programming, programming at a higher level of abstraction is the ability for the programmer to make concepts that span state and behavior, in any combination, explicit. Which obviously requires an understanding of how to implement concepts using state and behavior.

The next major concept I would tackle is generalization. An object choosing which behavior to invoke in response to being sent a message, at run time, is what allows for generalization. Generalization occurs when a programmer hides the details of how to accomplish a similar concept in two distinct ways behind one uniform message.

From here I would move on to encapsulation, inheritance, and polymorphism. And from there on to more high level concepts. I just feel that keeping everything focused specifically around how these concepts help you solve problems is important for retaining them.