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Thread: Is my understanding of explicit casting correct?

  1. #1
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    Question Is my understanding of explicit casting correct?

    For the below code:
    public interface Printable {
        void print();
    }
     
     class ShoppingItem {
        public void description() {
            System.out.println("Shopping Item");
        }
    }
     
     class Book extends ShoppingItem implements Printable {
        public void description() {
            System.out.println("Book");
        }
     
        public void print() {
            System.out.println("Printing book");
        }
    }
    }
    Why is it that this fails:
    class MyClass {
        public static void main(String args[]) {
            Printable printable = new Book();//Implicit cast
            printable.description(); //This fails
     
        }
    And this succeeds?:
    class MyClass {
        public static void main(String args[]) {
            Printable printable = new Book();//Implicit cast
            ((Book)printable).description(); //Explicit cast - This works
     
        }

    I had initially understood that the reference could only access members of the object that it shared. Implicit casting is what makes this possible by cause I can assign a sub-class to a superclass reference .
    Explicit casting (in this example that I'm reading) seems to indicate that explicit casting gives a base-class reference type access to the finer implementation of the subclassed object.
    Is this correct? It seems that implicit and explicit casting are complimentary and don't overlap at all.


  2. #2
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    Default Re: Is my understanding of explicit casting correct?

    first printable.descirption() fails, as compiler will complain that it does not know any method called description() in Printable interface.

    In second case as you have casted to Book interface, so there is no issue with compiler. and as real object too is book, so no issue at run time as well.

  3. #3
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    Default Re: Is my understanding of explicit casting correct?

    Thanks Deepak, I understand now.

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