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Thread: About casting

  1. #1
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    Default About casting

    Why can my code be compiled ? And why can't J_SuperClass be cast to J_SubClass ?
    result.jpg
    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;
    	}
    }


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    Default Re: About casting

    Search and read up on "java downcasting".

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    CheukKwan (October 18th, 2013)

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    Default Re: About casting

    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.

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    Default Re: About casting

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

  6. #5

    Default Re: About casting

    Quote Originally Posted by CheukKwan View Post
    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'.

  7. #6
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    Default Re: About casting

    Because inheritance goes one way.
    J_SubClass references J_SuperClass because J_SubClass extends J_SuperClass. However, J_SuperClass is not even aware that J_SubClass exists to begin with.
    A super class does not know, or care, what classes extend from it.
    All J_SubClasses can be casted into J_SuperClasses, but it doesn't go the other way. J_SuperClass cannot be casted into a J_SubClass.
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