Thread: About casting

Why can my code be compiled ? And why can't J_SuperClass be cast to J_SubClass ?
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The following is my code:

class J_SuperClass {
}
 
class J_SubClass extends J_SuperClass {
}
 
public class J_Test {
	public static void main(String[] args) {
		J_SuperClass a = new J_SuperClass();
		J_SubClass b = new J_SubClass();
		b = (J_SubClass)a;
	}
}

Search and read up on "java downcasting".

I have read up the concept of what you said. However, I want to know if 'a' can reference a J_SubClass or not. And why ? That's the key.

If you mean, "Can a use methods of the subclass?" the answer is "No."

Originally Posted by CheukKwan

I have read up the concept of what you said. However, I want to know if 'a' can reference a J_SubClass or not. And why ? That's the key.

Of course 'a' can reference a J_SubClass which is a subtype of J_SuperClass which is declared type of 'a'.