Need help in questions about casting and assignment from ExamLab final test (answers included)

[Emanuele Ghe]

Hi,
I did examlab final test, scoring 63%, but there are 2 questions about assignment and casting that I failed, and I can't understand why things work the way they work ;)
I am really struggling about this, since I will attempt my exam the next week.

Here is the first question with answer:



I can't understand why the last 4 assignments are legal/illegal, as indicated in the figure.
Here are my thoughts, I hope someone can confirm them to me:

A r2 = (A) r1;
is legal because, at runtime, r1 could refer to some subclasses of A (that also implements Runnable and got assigned to the Runnable reference variable r1) ?

A[] aa1 = (A[]) r1;
is illegal because you can never cast a "normal object reference" to an "array object reference" ?

A[] aa2 = (A[]) ra1;
legal for the same reason of { A r2 = (A) r1; } ?

ra1 =(Runnable[]) ca;
is illegal because C is a final class and so is not possible that there will be some subclasses that implements runnable ?


Can someone explain to me ?


Here is the second question:

  
class A {}
final class B extends A {}
interface I {}

class UC {
	public static void main(String args[]) {
		A a = new A();
		B b = (B) a;
		A a2 = (A) b;

		I[] i1 = (I[]) new A();   //LINE 1
		I[] i2 = (I[]) new A[] {};
		I[] i3 = (I[]) new B();   //LINE 2
		I[] i4 = (I[]) new B[] {};//LINE 3

		A[] gred = (A[]) new I[4];
		B[] bik = new B[2];
		gred = bik;
		bik = (B[]) gred;  //LINE 4
	}
}
  

code at LINE 1, LINE 2, LINE 3 do not compile, and LINE 4 does.

Here are my thoughts:
LINE 1 do not compile because you can never cast a "normal object reference" to an "array object reference" ?

LINE 2 do not compile for the same reason of LINE 1 ?

LINE 3 do not compile because B is final, so it is impossible that there will be some subclass of B implementing I ?

LINE 4 compiles ... Why ?


I understand this is a long question to answer, and I appreciate everyone that will help me.


Thanks in advance.

[Prasad Kharkar]

hmm
terribly mistaken
sorry for that
will look into it deeper

[Emanuele Ghe]

now in the line
A[] gred = (B[]) gread ; the compilation fails

This is wrong, code at LINE 4 compiles succesfully.

[Prasad Kharkar]

well well
really sorry for answering the question wrong totally
may I ask you something?
why do you think that the lines should not compile?
just asking I am not cross questioning you
because I am here to learn also
if you can put your opinions
then we can discuss more

according to me
the answer is a little simple and I did not think about it before
I think

A and B are in the same inheritance tree and hence we can cast the arrays of superclass to type of subclass at COMPILE TIME but it throws the ClassCastException
as follows
I edited the code a little bit and compiled and run

    
class A {}
final class B extends A {}
interface I {}

class UC {
	public static void main(String args[]) {
		A a = new A();
		B b = (B) a;
		A a2 = (A) b;

		//I[] i1 = (I[]) new A();   //LINE 1
		//I//[] i2 = (I[]) new A[] {};
		//I[] i3 = (I[]) new B();   //LINE 2
		//I[] i4 = (I[]) new B[] {};//LINE 3

		A[] gred = (A[]) new I[4];
		B[] bik = new B[2];
		gred = bik;
		bik = (B[]) gred;  //LINE 4
	}
}
    

and this gives the runtime exception
ClassCastException
as we cannot convert an A[] type into B[] type

[Emanuele Ghe]

Prasad Kharkar wrote:well well
really sorry for answering the question wrong totally
may I ask you something?
why do you think that the lines should not compile?
just asking I am not cross questioning you
because I am here to learn also
if you can put your opinions
then we can discuss more

according to me
the answer is a little simple and I did not think about it before
I think

A and B are in the same inheritance tree and hence we can cast the arrays of superclass to type of subclass at COMPILE TIME but it throws the ClassCastException
as follows
I edited the code a little bit and compiled and run

    
class A {}
final class B extends A {}
interface I {}

class UC {
	public static void main(String args[]) {
		A a = new A();
		B b = (B) a;
		A a2 = (A) b;

		//I[] i1 = (I[]) new A();   //LINE 1
		//I//[] i2 = (I[]) new A[] {};
		//I[] i3 = (I[]) new B();   //LINE 2
		//I[] i4 = (I[]) new B[] {};//LINE 3

		A[] gred = (A[]) new I[4];
		B[] bik = new B[2];
		gred = bik;
		bik = (B[]) gred;  //LINE 4
	}
}
    

and this gives the runtime exception
ClassCastException
as we cannot convert an A[] type into B[] type

I agree,
it was simple, my mind was just overwhelmed by the studies ;)

Thanks for helping me mate.

[Prasad Kharkar]

you are welcome !!
another thing I wanted to tell
if you find anything wrong with my post
you can tell me directly
don't ignore if you did not find the answer
tell me directly if I am wrong
wish you good luck for the exam

[Prithvi Sehgal]

Hello,

Waiting for your result Eman, just dropped a note to your gmail. Will be back to ranch day after as my holidays are starting.

Anxiously waiting for Eman Result,