Help :: Illegal Casting Question

[hatem odeh]

Dear All , I faced the attached question in Exam Lab, and I still cant figure it out why there are two illegal casts , the reference variable is of the same type we are casting the object to , so it should compile ... please explain

[Balraj Momi]

A[] aa1 = (A[]) r1 //Illegal

I think above is Illegal because r1 is just an Interface reference type,not of type array.

ra1 = (Runnable[]) ca; //Illegal

In this case also same situation holds good. If you want to make this work change ca to array type and make C a non final class. OR typecast ca to just Runnable reference by making C a non-final class.

[hatem odeh]

will you please tell me the rule so i can memorize for the exam to solve such questions

[Ankit Garg]

Hatem, you just need to understand that you cannot type cast between a single element type and an array type, like

int[] arr = ...;
int i = arr;//error

This is applicable for both primitive and objects. The only exception is Object type. Since it is the super type of all the types, you can assign an array to it. So any cast between a reference type will not work.

Now a cast between two non-related classes is not allowed. So if you try

String s = ...;
Integer i = (Integer)s;  //error

Here String and Integer classes are in different hierarchy so the cast is not allowed. But a cast is allowed between an interface type and another interface type or class.

interface I1 {}
interface I2 {}
class C1 {}
class C2
:
.....
    I1 i1 = ....;
    I2 i2 = (i2)i1;   //allowed
    C1 c1 = ....;
    C2 c2 = (C2) c1; //error
    c1 = (C1) i1;   //allowed

The basic idea behind this is that at runtime, the cast might be successful as there might be a type, which is sub-type of both the interfaces or one interface or class. The cast between C1 and C2 is not allowed because under no circumstances, can there be an object which is compatible with both (as a class cannot extend multiple classes). Suppose I create a class like this

class C3 extends C1 implements I1, I2 {}

Now if we review the original type casts, then they will all work

    I1 i1 = new C3();
    I2 i2 = (i2)i1;   //ok at runtime
    C1 c1 = ....;
    C2 c2 = (C2) c1; //error
    c1 = (C1) i1;   //ok at runtime

But the cast is not allowed between a non-related class and interface, if the class is final

interface I1 {}
final class C1 {}
......
    I1 i1 = ....;
    C1 c1 = (C1) i1;   //error

In this case there cannot be a class, which is a sub-class of both I1 and C1 (as C1 is final). This is why the cast will generate a compilation error.

HTH...

[Ankit Garg]

Balraj Momi wrote:ra1 = (Runnable[]) ca; //Illegal

In this case also same situation holds good. If you want to make this work change ca to array type and make C a non final class.

ca is already of an array type. It is just the matter of C being final...

[hatem odeh]

Ankit a very very very big thanks I am totally in a good shape for any casting questions cause of your explanation

[Ankit Garg]

hatem odeh wrote:Ankit a very very very big thanks.........

A very very big welcome

[Ghaith Rabadi]

Actulally it depends on the device it self, I think you have to change it.

Best regards