Difference in the type of "Array of Objects" AND "ArrayList of same Objects"? [duplicate]

Question

This question already has answers here:

Is List<Dog> a subclass of List<Animal>? Why are Java generics not implicitly polymorphic? (19 answers)

Closed 9 years ago.

I am unable to understand the difference in the type of "Array of Objects" AND "ArrayList of same Objects". For example I have Interface X:

public interface X {

    public void implemntX();
}

Class XYZ implementing X:

public class XYZ implements X{

@Override
public void implemntX() {
    // TODO Auto-generated method stub

}
}

Following codes works perfect:

X arraryOfX[] = new XYZ[10];

if(arraryOfX instanceof XYZ[]){
        System.out.println("arraryOfX is instanceOF XYZ[]");
    }else{
        System.out.println("arraryOfX is NOT instanceOF XYZ[]");
    }

Output: arraryOfX is instanceOF XYZ[]

But following line of code gives me compile time error:

ArrayList<X> arrayListOFX = new ArrayList<XYZ>();

ERROR: Type mismatch: cannot convert from ArrayList<XYZ> to ArrayList<X>

To the best of my understanding, ArrayList internally uses array then why Type mismatch error is bubbling up?

Answer 1

A list of XYZ is not a list of X, even if XYZ inherits from or implements X.Java Generics do not allow that. To be more explicit, change XYZ and X by real objects (cats and animals) like it was said in the so link

Answer 2

Suppose you had class ABC implements X you CAN add an instance of ABC to List<X> but you CAN'T add it to ArrayList<XYZ>.

Just because ArrayList is backed by an array, don't confuse the 2.