Re: [Jython-dev] casting

[Kevin B. <kb...@ca...>]

Woops!

I completely misread your post:

Marco wrote:
> how can i cast a java-instance to another type.
> ex.
> x = myJavaClass1()
> y = (myJavaClass2) x
> 
> where:
> myJavaClass2 extends myJavaClass1

So you have:

public class A
{
}

public class B extends A
{
}

and you want to do:

{
  A a = new A();
  B b = (B) a;
}

Right?

You can't do that -- you will get a ClassCastException, because you're trying to use an object as if it were something it _knows_ it is not.

You can only cast to what the instance _really_ _is_.

The usual pattern is:

{
  A a = new B();
  B b = (B) b;
}

Which works as I described:  So I've been using an instance of B as if it were an instance of A, but after the cast, I want to use it as what it really is.

The idiom, if you _really_ have an "A" and need to get a "B" is to do do a shallow copy/a delegation pattern something like this:

public B( A other ) // "copy constructor"
{
  // either a shallow copy, as you said, or retain reference to 'other' instance to delegate to
}

public static B makeB( A other )
{
  if ( other instanceof B )
  {
 	return (B) other;
  }
  else
  {
	return new B( other );
  }
}

This is not something you want to do often, and generally indicates you have a problem in the design.

kb