Class Casting.

[pjerald]

What is the restrictions in castng...
Inthe following prog if i uncomment c = p.
It throws,

Casting.java:28 : incompatible types
found : test.Parent
required: test.Child

is it possible to cast both up and down the hierarchy ?

package test;

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

class Child extends Parent {
	public static void print()
	{
		System.out.print("Child");
	}
}

class GrandChild extends Child {
	public static void print()
	{
		System.out.print("GrandChild");
	}
}
public class Casting {
	public static void main(String args[]){
		Parent p = new Parent();
		Child c = new Child();
		GrandChild g = new GrandChild();
		p = c;
//		c = p;
		System.out.println("Compiles Good");		
	}
}

Thanks,
P.Jerald

[r035198x]

What is the restrictions in castng...
Inthe following prog if i uncomment c = p.
It throws,

Casting.java:28 : incompatible types
found : test.Parent
required: test.Child

is it possible to cast both up and down the hierarchy ?

package test;

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

class Child extends Parent {
	public static void print()
	{
		System.out.print("Child");
	}
}

class GrandChild extends Child {
	public static void print()
	{
		System.out.print("GrandChild");
	}
}
public class Casting {
	public static void main(String args[]){
		Parent p = new Parent();
		Child c = new Child();
		GrandChild g = new GrandChild();
		p = c;
//		c = p;
		System.out.println("Compiles Good");		
	}
}

Thanks,
P.Jerald

No you cannot cast both up and down the inheritance hierarchy.

I'm sure you can play around on the compiler to find out which casts are allowed. Also take note of the difference between casts that fail at compile time and those that fail at run time.

[JosAH]

No you cannot cast both up and down the inheritance hierarchy.

You can always cast upwards, towards the root which is the Object class. You
can only cast downwards if the object at hand actually *is an* instance of the
target class you want to cast to or a subclass thereof. A Parent most certainly
is not a Child in the example, hence the cast failure.

kind regards,

Jos

[r035198x]

You can always cast upwards, towards the root which is the Object class. You
can only cast downwards if the object at hand actually *is an* instance of the
target class you want to cast to or a subclass thereof. A Parent most certainly
is not a Child in the example, hence the cast failure.

kind regards,

Jos

Yep, I suppose my post ended up all wrong. I wanted to convey that it's not correct to say that "it's always possible to cast both ways" and that the OP is best left to discover those that work and those that do not work ...

[pjerald]

A Parent most certainly is not a Child in the example, hence the cast failure.

kind regards,

Jos

Thanks, jos and r035198x.

Regards,
P.Jerald.

[pjerald]

You can always cast upwards, towards the root which is the Object class. You
can only cast downwards if the object at hand actually *is an* instance of the
target class you want to cast to or a subclass thereof. A Parent most certainly
is not a Child in the example, hence the cast failure.


kind regards,

Jos

So, We cannot cast a parent class to its child. Am i wright ?

[JosAH]

So, We cannot cast a parent class to its child. Am i wright ?

Yep, true; here's an example:

[code=java]
public class MeansOfTranspor tation { ... }
public class CompactCar extends MeansOfTranspor tation { ... }
public class Submarine extends MeansOfTranspor tation { ... }
...
Submarine yellow= new Submarine();
MeansOfTranspor tation mot= (MeansOfTranspo rtation)yellow; // valid up-cast
CompactCar c= (CompactCar)mot ; // invalid down-cast
[/code]

As you can see a submarine clearly isn't a compact car.

kind regards,

Jos

[heat84]

You need an explicit cast from parent class to child class but no need for explicit class from child to parent to child so if you do this there will be no error

#
public static void main(String args[]){
#
Parent p = new Parent();
#
Child c = new Child();
#
GrandChild g = new GrandChild();
#
p = c;
#
c = (Child)p;
#
System.out.prin tln("Compiles Good");
#
}