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Except for type boolean , values of primitive types can be assigned to any variable whose type represents a range that is as wide or wider than the range of the value's type. (Values of type boolean can only be assigned to variables of type boolean .)
With respect to reference types, the reference to an object instantiated from a given class can be assigned to any of the following without the use of a cast:
Assignments of references, other than those listed above, require the use of a cast to change the type of the reference.
It is not always possible to perform a successful cast to convert the type of a reference. Whether or not a cast can be successfully performed depends on the relationship of the classes involved in the class hierarchy.
A reference to any object can be assigned to a reference variable of the type Object , because the Object class is a superclass of every other class.
When we cast a reference along the class hierarchy in a direction from the root class Object toward the leaves, we often refer to it as a downcast .
Whether or not a method can be called on a reference to an object depends on the current type of the reference and the location in the class hierarchy where the method is defined. In order to use a reference of a class type to call a method, the method must be defined at or above that class in the class hierarchy.
A sample program is provided that illustrates much of the detail involved in type conversion, method invocation, and casting with respect to reference types.
I will begin the discussion of runtime polymorphism through method overriding and inheritance in the next module.
I will demonstrate that for runtime polymorphism, the selection of a method for execution is based on the actual type of object whose reference is stored in a reference variable, and not on the type of the reference variable on which the method is called.
This section contains a variety of miscellaneous information.
Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.
I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.
In the past, unknown individuals have misappropriated copies of my modules from cnx.org, converted them to Kindle books, andplaced them for sale on Amazon.com showing me as the author. I receive no compensation for those sales and don't know who doesreceive compensation. If you purchase such a book, please be aware that it is a bootleg copy of a module that is freelyavailable on cnx.org.
Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.
A complete listing of the program is shown in Listing 11 below.
| Listing 11 . Complete program listing. |
|---|
/*File Poly02.java
Copyright 2002, R.G.BaldwinThis program illustrates downcasting
Program output is:m in class B**************************************/
class A extends Object{//this class is empty
}//end class A//===================================//
class B extends A{public void m(){
System.out.println("m in class B");}//end method m()
}//end class B//===================================//
class C extends Object{//this class is empty
}//end class C//===================================//
public class Poly02{public static void main(String[] args){Object var = new B();
//Following will not compile//var.m();
//Following will not compile//((A)var).m();
//Following will compile and run((B)var).m();//Following will compile and run
B v1 = (B)var;//Following will not execute
//C v2 = (C)var;//Following will not compile
//C v3 = (B)var;}//end main
}//end class Poly02 |
-end-
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