Classes, constructors, instantiating objects and methods. What are they?

"A constructor is the method that you will run when you instantiate the class before the object is returned to the caller. Again, instantiation happens when you create the new instance of the class using the new keyword. This is a good place to stick information about the initial setup of the object."

Is it that that's confusing you?

OK. So, we have a class which is a blueprint for creating instances of that class. Doing this is called instantiation, it is also referred to as 'creating an object', 'making an instance' etc. There's lots of terminology that can be used. Let's worry about what an object or instance is. An object or instance is a unique version of the class, it is an example of what the class creates, as a blueprint or template. It is the real thing rather than the template itself.

Example required .. erm ... let's say you've got a machine that stamps metal into plates. So, the shape of that stamp is like the class. It isn't a plate but it knows how to create one. The object/instance is an actual plate that gets created when the stamp presses the sheet metal. There can be many, many individual plates produced from this one stamp and they can be stamped out of different thicknesses of metal; that depends what sheet metal you put against the template, but they are all, basically, made the same way. OK - that's one example lamely overdone.

We now have some idea what an instance is and that means we know that instantiate is the verb used to describe creating an instance. So, what's this constructor? Like Craig said, it is a method. That's like a bit of code that does something rather than a bit of code that stores something. A method produces something. A constructor is a specific bit of code that every class has and what it does is build the instance/object. Let's get back to plates ... like that example isn't old already!

In our plate factory, the constructor is the process that gets some metal, puts it in the right place then moves the stamp to press out a plate-shaped object. Craig mentioned that "This is a good place to stick information about the initial setup of the object". So, our constructor could select what type of metal to use (I dunno ... copper, tin, steel, whatever!) and its thickness, perhaps (thin, thick, something else(?!)), then create a plate by applying the template to the metal The end result is a plate that matches the "initial setup of the object" delivered to whoever requested it to be made. Or, as Craig put it, "returned to the caller".

Let's make this even less clear by trying to create some dummy code for our dodgy plate-making example ...

public class Plate{

  private int thickness; 
  private Metal madeOf;

  // This is the constructor:
  public Plate(int howThick, Metal useMetal){
    thickness = howThick;
    madeOf = useMetal;
  }
}

Plate myPlate = new Plate(2, copper)

(Yes, I made up a Metal class - forgive me!)

This is probably pointless as Craig's course will do a far better job of explaining all this, but it's worth a shot. Here we've replicated the lame plate example in (probably incorrect) Java code - it isn't supposed to be perfect! The person wanting the plate, "the caller", can pass details of what type of plate he wants, "initial setup", into the constructor which will create a plate-shaped object, "instantiate", and give that to the user, "return to the caller".

We can see how, in code, we can create unique objects with differing properties but by using the same template. We could add more properties; color, diameter, shape etc. I didn't as I've done the example to death already. But the key concepts here are the stamp itself - it makes plates, as many plates as you want - and the constructor which allows you to tailor the properties of your plate(s).

We've covered the class, methods, the constructor, instances & objects, instantiation, the caller and we've learned I know nothing about the manufacture of plates, or any other form of crockery, for that matter. But I hope that has shed a little light on the part of the video that was causing you a problem. If not, that's my fault; sorry! Honestly, just crack on with the courses, you will get the hang of all this in no time.

Have a great day!

Steve.

cc. Craig Dennis - I may have confused one of your students ... sorry!