ClassCastError vs. Compilation Error

[Terence Gronowski]

When is there a ClassCastError or a CompilationError? Are there some rules? Are there some more cases as I list below?

If I have two sibling objects (objects with the same parent) as in #1 there is a compilation error, this means, that the compiler can foresee that something is wrong.

If I have a polymorphic declaration Parent p=new Child and I do not cast in the IS-A direction (correct would be: Double result=(Double)cce;) a ClassCastException is thrown at runtime.

It is somehow clear for me, that you cannot cast a Double into a String, but what is the cue behind this?

public class SiblingTest{
 public static void main(String [] args){
   Derived1 d1=new Derived1();
   Derived2 d2=new Derived2();
   //d1=(Derived1)d2;    //CompilerError: inconvertible Types #1 
   Double result=(Double)cce; //correct
   
   Object cce=new Double(1.0);
   //String s=(String)cce; //ClassCastException

   }
}

class Base{}
class Derived1 extends Base{}
class Derived2 extends Base{}

[Jia Tan]

I think because String and Object are in the same inheritance tree, so comiple with no error. If you change

Derived2 d2=new Derived2(); 

to

Object d2=new Derived2(); 

and uncomment the cast statement, it will compile but raise ClassCastException at runtime.

[Vishal Gupta]

Terence Gronowski wrote:When is there a ClassCastError or a CompilationError? Are there some rules? Are there some more cases as I list below?

Hi Terence, please refer to this thread for casting rules: - webpage

If I have two sibling objects (objects with the same parent) as in #1 there is a compilation error, this means, that the compiler can foresee that something is wrong.

if you apply referencing rule here. ("A reference of type X can point to objects of class X, or to objects of any of X's subtypes. "
then this can be true

    
Base x1 = d1; or
Base x2 = d2;

now downcasting is required when you assign "reference of type B" (e.g B b) to a "reference that is supertype of B" (e.g a)"
i.e.

	
 d1 = (Derived1)x1;
 d2 = (Derived2)x1;

and in your case d2 is not a supertype of d1!

It is somehow clear for me, that you cannot cast a Double into a String, but what is the cue behind this?

That is again referencing rule - "if the object pointed to by the reference on right hand side(e.g a) is not assignment compatible with the reference type you are assigning to (e.g b) according to the referencing rule, a ClassCastException will be issued at run time"

and the complete (compiled code) is

  
public class SiblingTest{
	 public static void main(String [] args){
	   Derived1 d1=new Derived1();
	   Derived2 d2=new Derived2();
	   //d1=(Derived1)d2;    //CompilerError: inconvertible Types #1 
	   
	   Base x1 = d1;
	   Base x2 = d2;

	   d1 = (Derived1)x1;
	   d2 = (Derived2)x1;
	   
	   Object cce=new Double(1.0);
	   String s=(String)cce; //ClassCastException
	   Double result=(Double)cce; //correct
	   }
	}

	class Base{}
	class Derived1 extends Base{}
	class Derived2 extends Base{}
  

I hope this would help..

[Terence Gronowski]

Many thanks Jia and Vishal

I remembered a much simpler rule:

First: Does it compile? Is the inheritance tree ok?

class Base{}
class Derived1 extends Base{}
class Derived2 extends Base{}

public class CompilerTest{
 public static void main(String [] args){
   Derived1 d1=new Derived1();
   Derived2 d2=new Derived2();
   d1=(Derived1)d2;    //CompilerError: inconvertible Types
                       //Inheritance Tree not ok
   }
}

Here the inheritance tree is not ok, no compile!

----------------


Second: If I have a downcast without a foregoing upcast there will be a ClassCastException

class Base{}
class Derived1 extends Base{}
class Derived2 extends Derived1{}  //changed!

public class CompilerTest{
 public static void main(String [] args){
   Derived1 d1=new Derived1();
   Derived2 d2=new Derived2();
   //d1=d2; //runs ok is upcast
   d2=(Derived2)d1;  //Downcast whithout foregoing Upcast
                     //results in a ClassCast exception
   }
}

In the above example, the inheritance tree is ok. But if d1=d2, the upcast, is whiped out, there is the class cast exception.

What do I with d1=d2? I put the broader d2 reference in the narrower type, so that I have all the information of d2.

I am just studiing your checking rules with mine....what are the differences?

[Terence Gronowski]

Applying the rule "no Downcast without prior Upcast" in a trap, the scjp-trappers would have ready for us:

interface Iface{}
class Base implements Iface {}
class Derived1 extends Base{}
class Derived2 extends Derived1{}

public class TypicalCCETest{
 public static void main(String [] args){
   Derived1 d1=new Derived1();
   Derived2 d2=new Derived2();
   //which statements result in a ClassCastException
   //insert here
   //a)
   //Iface i=d2;   =>Upcast compiles and runs

   //b)
   //Base b=(Derived2)d2; => Upcast compiles and runs, (Derived2) not necessary

   //c)Iface i=new Base();
   //d2=(Derived2)i; => Downcast! Attention! ClassCastException, Constructor Derived2() has not run
   

   //d)
   //Iface i=new Derived2();
   //d2=(Derived2)i; //=> Downcast with foregoing Upcast, ok, i has the contents of Constructor Derived2()
   
   //e
   Base b=(Base)(Iface)d2;//=> Upcast, runs ok

   }
}

Strategy:

1. Is the ineritance tree ok, if yes proceed, the code compiles, if no=compilation error
2. All upcasts will run ok (Parent=Child), also nonsense casts as in e), a) b) and e) will run
3. Are there any downcasts (Child=Parent)? if yes beware, in c) and d) there are
4. Is there any upcast before the downcast? Has the Child-Constructor ever run before on the parent-reference? If yes ok, if no =>ClassCastException
in c) the foregoing upcast lacks!

Is this a save procedure not not beeing predated?

[Terence Gronowski]

...it is not. There are some wired possibilities, as composing types and references in a "wild" way:

in f) there ist a compiler error
g) runs ok

class Base implements Iface {}
class Derived1 extends Base{}
class Derived2 extends Derived1{}

public class TypicalCCETest{
 public static void main(String [] args){
   Derived1 d1=new Derived1();
   Derived2 d2=new Derived2();
   //which statements result in a ClassCastException
   //insert here
   //f)
   //Iface i=(Derived2)d1;//compiler error, Derived1 cannot be cast to Derived2
   
   //g
   //Iface i=(Base)d1; //Upcast, runs ok
   }
}

It seems not so easy to derive a fool proof strategy for this sort of problems!

[Ankit Garg]

Terence this code is not giving any compilation error (as it should)

interface Iface{}
class Base implements Iface {}
class Derived1 extends Base{}
class Derived2 extends Derived1{}

public class TypicalCCETest{
 public static void main(String [] args){
   Derived1 d1=new Derived1();
   Derived2 d2=new Derived2();
   Iface i=(Derived2)d1;//no compiler error
   
   }
}

I don't understand what you are confused with. Can you please explain more clearly your confusion...

[Terence Gronowski]

oh sorry, it is a ClassCastError as it should be. It is not a compilation error.

I want to have some reliable rules to for the questions "does it compile" or "there is a ClassCastError".

My example should be a recipe how to do that. Here is it again:

a) is the inheritance sequence ok, if yes proceed, if no => compilation error
b) is there any downcasting? no => it should run, yes => proceed
c) is there an upcast on the reference variable which is involved in the downcast? => yes => runs
d) If ther is no preceeding upcast => ClassCastError

Does that recipe work?