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Re: question about casting and inheritance

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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

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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

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