Question

Computer[] labComputers = new Computer[10];

with

public class Computer {
...
     void toString(){
     // print computer specs
     }
}
public class Notebook extends Computer{
...
     void toString(){
     // print computer specs + laptop color
     }
}

each subscripted variable labComputers[i] can reference either a Computer object or a Notebook object because Notebook is a subclass of Computer. For the method call labComputers[i].toString(), polymorphism ensures that the correct toString method is called.

I wonder what if we do

Notebook[] labComputers = new Notebook[10];

what kind or error would I get if I reference with Computer object and a Notebook object

Answer 1

Since the question specifically asks about kind of error I will explain them with below scenarios

If you do below

Notebook[] labComputers = new Notebook[10];

Now you can only set Notebook objects in array.

labComputers[0]  = new Notebook(); // Fine
labComputers[1]  = new Computer(); // Compilation error 

Now if you do

Computer[] notebooks = new Notebook[10];
notebooks[0] = new Notebook();
notebooks[1] = new Computer(); // <--- ArrayStoreException

Because arrays are covarant,reified in nature ie. if Sub is a subtype of Super, then the array type Sub[] is a subtype of Super[], and arrays enforce their element types at run time it will cause ArrayStoreException

You can read oracle docs about Polymorphism to know more how it works.