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