multiple casting

[John King]

Multiple down casting

I have a base class called BaseClass and a base interface BaseInterface.

class MyClass expends BaseClass implements BaseInterface

After I create an instance of MyClass called myClass, can I do this
casting:

BaseClass baseClass = (BaseClass) ((BaseInterface) myClass));

[marc weber]

Have you tried it?

[John King]

Originally posted by marc weber:
Have you tried it?

I do not have a computer with me.

[marc weber]

This works fine...

interface BaseInterface {}
 
class BaseClass {}
 
class MyClass extends BaseClass implements BaseInterface {}
 
class Test {
  public static void main(String[] args) {
    MyClass myClass = new MyClass();
    BaseClass baseClass = (BaseClass)((BaseInterface)myClass);
  }
}

[Ernest Friedman-Hill]

Originally posted by John King:

I do not have a computer with me.

And you posted this using...?

[marc weber]

Originally posted by Ernest Friedman-Hill:
... And you posted this using...?

Doh! That never even occurred to me.

[John King]

Originally posted by Ernest Friedman-Hill:


And you posted this using...?

I was using a terminal that only allows me to browse the Internet in a public place. Thanks guys.

I tested it. As Marc said, it compiles fine. It was an interview question.
I do not understand why Java allows this type of casting. It seems not very useful.

[Paul Clapham]

There are hundreds of ways to write useless and pointless code in Java. But trying to write a language definition that makes code like "i = i++" illegal because it doesn't do anything and does it in a confusing way, for example, would be extremely difficult. It's better to just write a simple language and let programmers do what they will.

[marc weber]

Originally posted by Paul Clapham:
... It's better to just write a simple language and let programmers do what they will.

I agree with the first part.

[John King]

Originally posted by Paul Clapham:
But trying to write a language definition that makes code like "i = i++" because it doesn't do anything .

You brought an interesting point in Java. I like to summarize this:

int i=10;

i=i++; // i is still 10
i=i--; // i = 10.

i += i++; // it gives i=20

i=10;
i += ++i; // it gives i=21

i=10;
i -= i++; // it gives i=0

i=10;
i -= --i; // it gives i=1

i=10;
i += i--; // it gives i=20