casting

[Charlie Swanson]

Could anyone explain why the below code results in an error:
File: junktest.java
class junktest {
public static void main(String args []) {
junk j1 = new junk();
junksub js;
System.out.print("inside junktest");
js = (junksub)j1;
}
}
File: junk.java
class junk {
public static void main(String args []) {
System.out.println("This is junk");
}
}
File: junksub.java
class junksub extends junk {
public static void main(String args []) {
System.out.println("This is junksub");
}
}
Thank you in advance!

[Kathy Lynch]

For starters, main is not a method that you put into all classes you create. I would remove it from class junk and junksub altogether.
Then, you can't make class junk a junksub. All instances of junksub are junk, but not all instances of junk are junksub. Does that make sense?

[ryan burgdorfer]

That's right, you can upcast no problem, but not downcast (class-wise). Also, I beleive that whatever class you decide to keep your one main() method in has to be declared as public.
Someone correct me if I am wrong.
------------------

• Ryan Burgdorfer
  
• Java Acolyte in
  
• Columbus, OH USA
  

[Mike Curwen]

They can't be protected or private, but they don't need to be explicitly public either.

class Test {
 public static void main(String args[]) {
   System.out.println("Hello World");
 }
}

This works, so the default (or package) access is allowed as well as public.

[Charlie Swanson]

I still do not understand why am I getting an error when I do an explicit cast down.
Eventhough I have the main declared in each class it is not causing the casting error.
Thank you,

[Jyotsna Clarkin]

Charlie:
You cannot force an object of junk()to be an object of junksub() even with an explicit cast.
For example:
Create a class Animal
Create class Dog extends Animal
You cannot forcibly make any Animal a Dog by explicit casting
But a Dog is naturally an Animal (defined in the class hierarchy)
I rewrote your code using the example

// File: Animal.java
class Animal {
public static void main(String args []) {
System.out.println("This is an Animal");
}
}

/File: Dog.java
class Dog extends Animal {
public static void main(String args []) {
System.out.println("This is a Dog");
}
}

//File: AnimalTest.java
class AnimalTest {
public static void main(String args []) {
Animal someUnknownAnimal = new Animal(); // This is an Animal
Animal poodle = new Dog(); // This is a Dog
Dog dog;
Dog dog1;
System.out.print("inside AnimalTest");
//dog = (Dog) someUnknownAnimal; Cannot force Animal to be a Dog
dog1 = (Dog) poodle; // This Animal is really a Dog
}
}

Please use code tags when you post code http://www.javaranch.com/ubb/ubbcode.html
regards,
Jyotsna

[Charlie Swanson]

Hello,
I am confused on this.
From my books, I know that an explicit cast is required when going from a superclass to a subclass. Can anyone explain how or when this can be done?

[ryan burgdorfer]

It can be done only when the reference you are casting actually IS an object of the subclass type. No other case will work.
------------------

• Ryan Burgdorfer
  
• Java Acolyte in
  
• Columbus, OH USA
  

[Kathy Lynch]

I suggest that you read "How my dog learned Polymorphism" in the Campfire section of the JavaRanch. It discusses these concepts really well (at least it really helped me).

[This message has been edited by Kathy Lynch (edited April 06, 2001).]

[Charlie Swanson]

Thank you very much for the explanations.