Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.java.programmer > #27262 > unrolled thread

question about casting and inheritance

Back to article view | Back to comp.lang.java.programmer

Contents

  question about casting and inheritance marc.at.compass@gmail.com - 2014-01-09 10:48 -0800
    Re: question about casting and inheritance leif.roar@invalid.com (Leif Roar Moldskred) - 2014-01-09 13:03 -0600
    Re: question about casting and inheritance Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-01-09 14:07 -0500
    Re: question about casting and inheritance markspace <markspace@nospam.nospam> - 2014-01-09 13:00 -0800
    Re: question about casting and inheritance Ricardo Palomaes <rpmdisguise-java@nowhere.com> - 2014-01-09 23:24 +0100
      Re: question about casting and inheritance Ricardo Palomares <rpmdisguise-java@nowhere.com> - 2014-01-12 12:20 +0100
    Re: question about casting and inheritance Patricia Shanahan <pats@acm.org> - 2014-01-10 03:04 -0800
      Re: question about casting and inheritance Arved Sandstrom <asandstrom2@eastlink.ca> - 2014-01-11 09:38 -0400
    Re: question about casting and inheritance Roedy Green <see_website@mindprod.com.invalid> - 2014-01-10 16:36 -0800
    Re: question about casting and inheritance marc.at.compass@gmail.com - 2014-01-11 08:01 -0800

#27262 — question about casting and inheritance

Hello,
I'm preparing for Java 7 OCA exam. I have a q about casting..
I learned you can cast a variable of a class to another object only if there's a hierarchical relation between the variable's class _and_ the object's class you cast to. And also, the varibla should point to an object og the class you're casting to or you get a ClassCastException during runtime.
So I made up this example:

class A {
	String uniqueMethodForA(){
		return ("uniqueMethodForA");
	}
}

class B extends A { 
	String uniqueMethodForB(){
		return ("uniqueMethodForB");
	}
}

public class LearnJava{
    public static void main(String[] args) throws Exception {
		LearnJava LJ = new LearnJava();
		A a = new A();
		B b = new B();
		System.out.println(a.uniqueMethodForA());
		System.out.println(b.uniqueMethodForB());
		a=b;
		b=(B)a;
		System.out.println("=============");
		System.out.println(a.uniqueMethodForA());
		System.out.println(b.uniqueMethodForB());
   }
}
outputs:
uniqueMethodForA
uniqueMethodForB
=============
uniqueMethodForA
uniqueMethodForB

As you see variable a points to an instance of A and not to (an instance of) B. I effectively say "treat a as if it is b". I can even call a method on b that is unique for B. But a didn't point to an instance of B in the first place at all!
Why is it the line b=(B)a; doesn't give me a ClassCastException during runtime?

[toc] | [next] | [standalone]

#27263

marc.at.compass@gmail.com wrote:
> 
> As you see variable a points to an instance of A and not to (an instance of) 
> B. I effectively say "treat a as if it is b". I can even call a method on b 
> that is unique for B. But a didn't point to an instance of B in the first 
> place at all!
>
> Why is it the line b=(B)a; doesn't give me a ClassCastException during runtime?

Because on the line before you do the cast, you set /a/ to point to the same
instance of /B/ as /b/ points to: 
  a = b;

You no longer have an object of class /A/ in /a/, but one of class /B/ and
_that_ can safely be cast to /B/ without error.

-- 
Leif Roar Moldskred

[toc] | [prev] | [next] | [standalone]

#27264

On 1/9/2014 1:48 PM, marc.at.compass@gmail.com wrote:
> Hello,
> I'm preparing for Java 7 OCA exam. I have a q about casting..
> I learned you can cast a variable of a class to another object only if there's a hierarchical relation between the variable's class _and_ the object's class you cast to. And also, the varibla should point to an object og the class you're casting to or you get a ClassCastException during runtime.
> So I made up this example:
>
> class A {
> 	String uniqueMethodForA(){
> 		return ("uniqueMethodForA");
> 	}
> }
>
> class B extends A {
> 	String uniqueMethodForB(){
> 		return ("uniqueMethodForB");
> 	}
> }
>
> public class LearnJava{
>      public static void main(String[] args) throws Exception {
> 		LearnJava LJ = new LearnJava();
> 		A a = new A();
> 		B b = new B();
> 		System.out.println(a.uniqueMethodForA());
> 		System.out.println(b.uniqueMethodForB());
> 		a=b;
> 		b=(B)a;
> 		System.out.println("=============");
> 		System.out.println(a.uniqueMethodForA());
> 		System.out.println(b.uniqueMethodForB());
>     }
> }
> outputs:
> uniqueMethodForA
> uniqueMethodForB
> =============
> uniqueMethodForA
> uniqueMethodForB
>
> As you see variable a points to an instance of A and not to (an instance of) B. I effectively say "treat a as if it is b". I can even call a method on b that is unique for B. But a didn't point to an instance of B in the first place at all!
> Why is it the line b=(B)a; doesn't give me a ClassCastException during runtime?

     Because "a" refers to the object originally pointed to by "b",
that is, to the object created by "new B()".  That object is a B
object, so converting its A-type reference "a" to a B-type reference
is no problem.   For contrast, try

	A a = new A();
	B b = new B();
	...
	A a2 = b;
	B b2 = (B) a;  // ClassCastException here

-- 
Eric Sosman
esosman@comcast-dot-net.invalid

[toc] | [prev] | [next] | [standalone]

#27267

On 1/9/2014 10:48 AM, marc.at.compass@gmail.com wrote:
> 		a=b;
> 		b=(B)a;

> Why is it the line b=(B)a; doesn't give me a ClassCastException during runtime?

Because the line above assigns 'b' to 'a'.  It's actually the only 
interesting thing inside the program.  The second line with the cast is 
a tautology: it casts 'b' to a 'B'.

A more explicit naming scheme for your variables would help a lot to 
understand this.

[toc] | [prev] | [next] | [standalone]

#27268

El 09/01/14 19:48, marc.at.compass@gmail.com escribió:
> As you see variable a points to an instance of A and not to (an
> instance of) B.


In fact, variable "a" is a reference to an object of class A. Since
class B extends A, an object of class B is also an object of class A
(it is "just a more specific type of A"). Thus, when you do:

a = b;

a is referencing the same object than b, ie., an object of class B.
While accessing that object from "a", it only can behave as a class A
object, and you couldn't do a.uniqueMethodForB(). However, as soon as
you access the object from b again, you can access again the methods
exclusive of a class B object. Note that, during this sequence:

a = b;
b = (B) a;

the object itself has not been touched nor modified at all; you only
have modified the references "a" and "b" (and "b" has been modified
only to leave exactly as it was).


> I effectively say "treat a as if it is b". I can
> even call a method on b that is unique for B. But a didn't point to
> an instance of B in the first place at all!


Yes, it did. At least, since you did a = b. :-)



> Why is it the line b=(B)a; doesn't give me a ClassCastException
> during runtime?


Because a holds a reference to a class A object that happens also to
be a class B object, so it is not wrong to reference as such.

HTH

[toc] | [prev] | [next] | [standalone]

#27279

El 11/01/14 18:40, Stefan Ram escribió:
> Ricardo Palomaes <rpmdisguise-java@nowhere.com> writes:
>> class B extends A, an object of class B is also an object of class A
> 
>   I try to say »/instance/ of class A« in such a case.


I was about to change it before sending it, indeed, :-) but I thought
it would be clearer to the OP to see "object".

[toc] | [prev] | [next] | [standalone]

#27269

On 1/9/2014 10:48 AM, marc.at.compass@gmail.com wrote:
> Hello,
> I'm preparing for Java 7 OCA exam. I have a q about casting..
> I learned you can cast a variable of a class to another object only if there's a hierarchical relation between the variable's class _and_ the object's class you cast to. And also, the varibla should point to an object og the class you're casting to or you get a ClassCastException during runtime.
> So I made up this example:
>
> class A {
> 	String uniqueMethodForA(){
> 		return ("uniqueMethodForA");
> 	}
> }
>
> class B extends A {
> 	String uniqueMethodForB(){
> 		return ("uniqueMethodForB");
> 	}
> }
>
> public class LearnJava{
>      public static void main(String[] args) throws Exception {
> 		LearnJava LJ = new LearnJava();
> 		A a = new A();
> 		B b = new B();
> 		System.out.println(a.uniqueMethodForA());
> 		System.out.println(b.uniqueMethodForB());
> 		a=b;
> 		b=(B)a;
> 		System.out.println("=============");
> 		System.out.println(a.uniqueMethodForA());
> 		System.out.println(b.uniqueMethodForB());
>     }
> }
> outputs:
> uniqueMethodForA
> uniqueMethodForB
> =============
> uniqueMethodForA
> uniqueMethodForB
>
> As you see variable a points to an instance of A and not to (an instance of) B. I effectively say "treat a as if it is b". I can even call a method on b that is unique for B. But a didn't point to an instance of B in the first place at all!
> Why is it the line b=(B)a; doesn't give me a ClassCastException during runtime?
>

There are two ways of thinking about Java that, in combination, make
this type of issue very simple and clear:

1. Distinguish between the type of an expression and the class of an object.

2. Every non-null reference expression is a pointer to an object whose
class is matches the expression's type, or extends it, or implements it.

Every expression has a type that is determined at compile time. For a
variable or parameter the type is directly declared. The type of
variable a is A, and the type of variable b is B. Other expressions have
types that can be determined from the nature of the expression and the
types of its operands. The type of "(B)a" is B.

Each object has a class that is determined by how the object is created,
through clone() or new. The class of the object created by "new B()" is
B, and remains B regardless of what variables or expressions point to it.

Applying this thinking to the code above, "a=b" assigns to variable a
the value of b, which is a pointer to an object of class B. Because B
extends A, the compiler knows that variable b either is null or is a
pointer to an object that variable a is allowed to point to, so no cast
is needed.

"b=a" would be rejected at compile time because variable a can point to
objects that variable b cannot point to.

"b=(B)a" is accepted by the compiler because variable a can point to an
object that variable b can also point to. It would cause a
ClassCastException at run time if, in fact, variable a pointed to an
object variable b cannot. It points to an object that was created by
"new B()", an object whose class is B, so the cast is valid.

Patricia

[toc] | [prev] | [next] | [standalone]

#27276

On 01/10/2014 07:04 AM, Patricia Shanahan wrote:
[SNIP]>
> There are two ways of thinking about Java that, in combination, make
> this type of issue very simple and clear:
>
> 1. Distinguish between the type of an expression and the class of an
> object.
>
> 2. Every non-null reference expression is a pointer to an object whose
> class is matches the expression's type, or extends it, or implements it.
>
> Every expression has a type that is determined at compile time. For a
> variable or parameter the type is directly declared. The type of
> variable a is A, and the type of variable b is B. Other expressions have
> types that can be determined from the nature of the expression and the
> types of its operands. The type of "(B)a" is B.
>
> Each object has a class that is determined by how the object is created,
> through clone() or new. The class of the object created by "new B()" is
> B, and remains B regardless of what variables or expressions point to it.
>
> Applying this thinking to the code above, "a=b" assigns to variable a
> the value of b, which is a pointer to an object of class B. Because B
> extends A, the compiler knows that variable b either is null or is a
> pointer to an object that variable a is allowed to point to, so no cast
> is needed.
>
> "b=a" would be rejected at compile time because variable a can point to
> objects that variable b cannot point to.
>
> "b=(B)a" is accepted by the compiler because variable a can point to an
> object that variable b can also point to. It would cause a
> ClassCastException at run time if, in fact, variable a pointed to an
> object variable b cannot. It points to an object that was created by
> "new B()", an object whose class is B, so the cast is valid.
>
> Patricia

"Simple and clear" to a person that knows OO quite well already, 
Patricia. :-)

AHS
-- 
When a true genius appears, you can know him by this sign:
that all the dunces are in a confederacy against him.
-- Jonathan Swift

[toc] | [prev] | [next] | [standalone]

#27273

On Thu, 9 Jan 2014 10:48:34 -0800 (PST), marc.at.compass@gmail.com
wrote, quoted or indirectly quoted someone who said :

>I'm preparing for Java 7 OCA exam. I have a q about casting..

see http://mindprod.com/jgloss/cast.html
for material to help you understand casting in general.
Others will point out what you did wrong in this case.
-- 
Roedy Green Canadian Mind Products http://mindprod.com
Unlike many machines, computers require no water once they are
manufactured.

[toc] | [prev] | [next] | [standalone]

#27277

Thanks all for the replies! Very informative. I'm a further on the path of casting and definitely will follow up on the link about casting given.

thanks again!

Marc 

[toc] | [prev] | [standalone]

Back to top | Article view | comp.lang.java.programmer

csiph-web