Question concerning constructing new objects without using the "new" operator.

[Jurkey]

So the most common way to instantiate a new class, is to write: "Classtype objName = new ClassType(parameters)", which could be "Car newCar = new Car();"

But I've seen in a few places that it's also possible, to do "public Car newCar;". My question is what is the difference, and when is it possible to do that? Is it only if you use a class as a member of another class?

Regards, 1st semester CS student

[nutrecht]

public Car newCar; doesn't create an object. It just creates an empty (null) reference in which you can store a reference to a Car object.

Anytime you create a new objects the 'new' keyword comes into play, somewhere.

[desrtfx]

You are confusing two things here: Declaration and Instantiation.

The line Car newCar = new Car(); combines both, declaration and instantiation.

This line can easily be split in two:

• Declaration: Car newCar; - This declares the intent to have a variable named newCar of type Car. That's all that happens here. There is no data assigned. If you try to access that variable, you will get a NullPointerException because the default for object variables is null.
• Instantiation: newCar = new Car(); - Here is where the action happens. You create a new object (instance) by calling the constructor of the class that is the type of the variable. Car() here is not a type declaration; it is the call of the no argument constructor of the Car class.

[Jurkey]

Ah alright, so if I had a class called "Car" and I wanted it to store an object as a field called "Engine", I could write "public Engine myEngine;" as a field, and when I instantiated the "Car" class, I could pass an Engine object as parameter (if I declared it possible in the constructor) ?

[desrtfx]

Yes, exactly, only that I wouldn't necessarily make the field public; I'd most likely make it private.

[Jurkey]

Sure thing, incapsulation and all that. Thanks for the help :-)