Java : Super class array object assigned with sub class array object

Question

I'm trying to assign a sub class object array to its super class. The program compiles successfully, but I' getting an ArrayStoreException. I know that arrays parent and child are references to same array, but shouldn't I be able to access method func at least?

class Pclass
{
    Pclass()
    {
        System.out.println("constructor : Parent class");
    }

    public void func()
    { 
        System.out.println("Parent class");
    }
}

class Cclass extends Pclass
{
    Cclass()
    { 
        System.out.println("Constructor : Child class");
    }

    public void func2()
    {  
        System.out.println("It worked");
    }

    public void func()
    { 
        System.out.println("Common");
    }
}

public class test
{     
    public static void main(String ab[])
    {    
        Cclass[] child = new Cclass[10];
        Pclass[] parent = child;
        parent[0]=new Pclass();
        parent[0].func();
    }
}

Solution 1

You can't do this:

Cclass[] child = new Cclass[10];
Pclass[] parent = child;
parent[0]=new Pclass();

You should try doing this:

Cclass[] child = new Cclass[10];
Pclass[] parent = child;
parent[0]=new Cclass();

That's because, You first assigned the Pclass array to the child reference that can only have Cclass objects, then you are trying to assign Pclass object to the parent reference, that's not allowed!

See, what happens is that you have created a Cclass object on the heap when you wrote new Cclass, though the Cclass objects were null in the array but now they would accept only Cclass objects or it's subclass's objects

so assigning the Pclass object would be illegal!

Reason for getting a runtime exception and not compile time:

The compiler only checks whether the classes are in the same inheritance hierarchy or not, since they are in the same hierarchy you get a Runtime exception.

Solution 2

If you read the spec of ArrayStoreException, you'd find out it is thrown to indicate that an attempt has been made to store the wrong type of object into an array of objects.

You created an instance of a Cclass array, so only instances of Cclass (or sub-classes of it) may be stored in this array. The fact that you store the reference of that instance in a variable of type Pclass[] doesn't change that.

Comments

[drew moore]

OP knows he can't do that - the question is why not? Your alternative suggestion has nothing to do with the question whatsoever. -1

[Sarthak Mittal]

@drewmoore it takes time to write the full answer :)

[drew moore]

downvote rescinded, but you still haven't really answered this question. The underlying confusion is a reference-vs-value issue

[Sarthak Mittal]

because compiler doesn't checks that, what the compiler checks is whether those classes are in the same inheritance hierarchy or not, since they are in the same hierarchy you get a runtime exception, i think i should mention this in the answer too :)