can you explain me classCastException ?

[naved momin]

 c = (clrek) e;

this line throws an class cast exception
i know that class cast exception occurs when you try to cast an object that is not an instance of that class
but here clark is an instance of Employee class

and the big thing is it cast an employee to manager in the test() with no class cast exception so , why it cannot do the same in main ()

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package instanceoddemo;

/**
 *
 * @author Admin
 */
class Employee
{
    void show()
    {
        System.out.println("hi emp");
    }
}
class Manager extends Employee
{
    void get()
    {
        System.out.println("hi manager");
    }
}
class clark extends Employee
{
    
}
public class InstanceOdDemo {

    /**
     * @param args the command line arguments
     */
    void test(Employee e)
    {
        if(e instanceof Manager)
        {
        
            Manager m = (Manager) e;
            m.get();
        }else if(e instanceOf clark)
        {
             System.out.println("hi clrak ");
         }else
        if(e instanceof Employee)
        {
            e.show();
        }
        
    }
    public static void main(String[] args) {
        // TODO code application logic here
       Manager m = new Manager();
       
       Employee e = new Employee();
     
       clark c = new clark();
       
       c = (clark) e;
       
       
     
    }
}

[Jeanne Boyarsky]

Employee e = new Employee();  
       
       clark c = new clark();  
         
       c = (clark) e;  //invalid
  

Employee is a superclass of clark. This means you cannot cast from employee to clark. For example I can't cast from Animal to Dog unless my variable happens to be a Dog. If it is an Animal directly, this doesn't makes sense.

Also, it is a convention for class name to being with a capital. So it would be Clerk. (also fixing the typo for the vowel.)

[naved momin]

Jeanne Boyarsky wrote:

Employee e = new Employee();  
       
       clark c = new clark();  
         
       c = (clark) e;  //invalid
  

Employee is a superclass of clark. This means you cannot cast from employee to clark. For example I can't cast from Animal to Dog unless my variable happens to be a Dog. If it is an Animal directly, this doesn't makes sense.

Also, it is a convention for class name to being with a capital. So it would be Clerk. (also fixing the typo for the vowel.)

thanks for the reply , but if this cant be done then i suppose this
should also throw the classCastException

Object o = null ;
         e =(Employee) o;
         c = (clrek) e;

but this code doesnt throw any classCastException , now what you wanna say ?

[Matthew Brown]

There's no problem assigning null to a Clerk reference. null always satisfies "IS-A".

I suppose one way of looking at it is to say that while it's not true to say that an Animal IS-A Dog, a hole where an Animal could be is also a hole where a Dog could be.

[naved momin]

Matthew Brown wrote:There's no problem assigning null to a Clerk reference. null always satisfies "IS-A".

I suppose one way of looking at it is to say that while it's not true to say that an Animal IS-A Dog, a hole where an Animal could be is also a hole where a Dog could be.

object ref is null not clarks

if so , then why we dont get classcastException in test()

i just want to ask why it doesnt throw class cast exception in test()

void test(Employee e)
        {
            if(e instanceof Manager)
            {

                Manager m = (Manager) e;  //why not castCastException 
                m.get();
            }else if(e instanceof clrek)
            {
                clrek c = (clrek) e;          //why not castCastException
                System.err.println(" hi clrak ");
            }else
            if(e instanceof Employee)
            {
                e.show();
            }


        }

and throws classCastException when i do same thing which i have done in the first if statement under test() and that is

 m = (Manager) e;

can any one explain me why this both things are considered different ?

[Matthew Brown]

You don't get a ClassCastException if the object is actually of the type you're trying to cast it to. In your code there, you are checking whether the Employee referenced by e is actually a Manager (using instanceof). If that's true, then the cast is safe. If it's not true, then the cast isn't even attempted.

Same with Clerk - because you're using instanceof, you're checking that the cast is safe before attempting it. If it's not safe, you skip it. Using instanceof like this is a way to avoid ClassCastExceptions.

The difference in your original main method is that you're not carrying out this safety check...and then you create a new Employee(), which can't be cast to a Manager.

[naved momin]

Matthew Brown wrote:You don't get a ClassCastException if the object is actually of the type you're trying to cast it to. In your code there, you are checking whether the Employee referenced by e is actually a Manager (using instanceof). If that's true, then the cast is safe. If it's not true, then the cast isn't even attempted.

Same with Clerk - because you're using instanceof, you're checking that the cast is safe before attempting it. If it's not safe, you skip it. Using instanceof like this is a way to avoid ClassCastExceptions.

The difference in your original main method is that you're not carrying out this safety check...and then you create a new Employee(), which can't be cast to a Manager.

but at the end of the day ..can i cast a base class to its super class ?
like

manager m ;
employee e;
m = (manager)e;

[Matthew Brown]

naved momin wrote:but at the end of the day ..can i cast a base class to its super class ?

Careful with your terminology - "base class" and "super class" are the same thing.

To clarify: you can cast a reference to a class to a reference to its superclass - but it's not necessary, as those casts are always safe and so can be done implicitly:

Manager m = new Manager();
Employee e1 = (Employee)m; // is OK, but the cast is unnecessary
Employee e2 = m; // is also OK

You can cast a reference to a class to a reference to its sub class, but whether it works or not (i.e. whether you get a ClassCastException or not) depends on the actual object:

Employee e1 = new Manager();
Employee e2 = new Employee();
Manager m1 = (Manager)e1; // this will work
Manager m2 = (Manager)e2; // this will compile, but will throw an exception

[naved momin]

Matthew Brown wrote:

naved momin wrote:but at the end of the day ..can i cast a base class to its super class ?

Careful with your terminology - "base class" and "super class" are the same thing.

To clarify: you can cast a reference to a class to a reference to its superclass - but it's not necessary, as those casts are always safe and so can be done implicitly:

Manager m = new Manager();
Employee e1 = (Employee)m; // is OK, but the cast is unnecessary
Employee e2 = m; // is also OK

You can cast a reference to a class to a reference to its sub class, but whether it works or not (i.e. whether you get a ClassCastException or not) depends on the actual object:

Employee e1 = new Manager();
Employee e2 = new Employee();
Manager m1 = (Manager)e1; // this will work
Manager m2 = (Manager)e2; // this will compile, but will throw an exception

thanks for this
but i wanted to ask you , it works fine for the reference
this works fine

Employee e1 = new Manager();

           Employee e = new Employee();

//           clrek c = new clrek();
//
//           new InstanceOdDemo().test(c);

           Manager m =  (Manager) e1;
           

but why this isnt working i wanted to know the reason ,
what i know is that employee is a super class and manager is a derived class , so it should be safe to cast a derived class to a super class ? dont you think ? ya vice versa is not possible i know that

public static void main(String[] args) {
            // TODO code application logic here
           Manager m = new Manager();

           Employee e = new Employee();

//           clrek c = new clrek();
//
//           new InstanceOdDemo().test(c);

           m =  (Manager) e;
           
           
             

        }

[Matthew Brown]

naved momin wrote:what i know is that employee is a super class and manager is a derived class , so it should be safe to cast a derived class to a super class ? dont you think ?

But that's not what you're doing!

naved momin wrote:

public static void main(String[] args) {
    Manager m = new Manager();
    Employee e = new Employee(); // this is the object you're casting - an Employee
    m =  (Manager) e;   // and here it's being cast to Manager - a subclass
}

That is casting a superclass (Employee) to its derived class (Manager) - the opposite of what you're saying you're doing.

[naved momin]

Matthew Brown wrote:

naved momin wrote:what i know is that employee is a super class and manager is a derived class , so it should be safe to cast a derived class to a super class ? dont you think ?

But that's not what you're doing!

naved momin wrote:

public static void main(String[] args) {
    Manager m = new Manager();
    Employee e = new Employee(); // this is the object you're casting - an Employee
    m =  (Manager) e;   // and here it's being cast to Manager - a subclass
}

That is casting a superclass (Employee) to its derived class (Manager) - the opposite of what you're saying you're doing.

oh i see thanks dude for making me correct ...thanks once again