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As you learned in the previous module, 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.
An object of the class C
Finally, the code in Listing 6
| Listing 6 . A new object of the class named C. |
|---|
Object varC = new C();
String v3 = varC.toString();System.out.println(v3); |
What will the output look like?
By now, you should know what to expect in the way of text appearing on the computer screen. The code in Listing 6 causes the following text to be displayed:
toString in class C
Overridden version of toString was called
This confirms what you should already have known by now. In particular, even though the reference to the object of the class C is stored in a reference variable of type Object , the overridden version of the toString method defined in the class named C was executed. Again, this is runtime polymorphic behavior based on class inheritance and method overriding.
No downcasting was required
It is also very important to note that no downcasting was required in order to call the toString method in any of the cases shown above.
Because a default version of the toString method is defined in the Object class, the toString method can be called without a requirement for downcasting on a reference to any object stored in a variable of type Object . This holds true for any of the eleven methods defined in the class named Object (although some of those methods are declared final and therefore may not be overridden) .
Polymorphism manifests itself in Java in the form of multiple methods having the same name.
From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:
In this module, I have continued my discussion of the implementation of polymorphism using method overriding through class inheritance, and have concentrated on a special case in that category.
More specifically, in this module, I have discussed the use of the Object class as a completely generic type for storing references to objects of subclass types, and have explained how that results in a very useful form of runtime polymorphism. .
I briefly mentioned the default version of the eleven methods defined in the Object class, and explained that in some cases, those default versions are meant to be overridden.
I provided a sample program that illustrates the overriding of the toString method, which is one of the eleven methods defined in the Object class.
In the next module, I will embark on an explanation of runtime polymorphic behavior based on the Java interface and method overriding.
In my opinion, this is one of the most important concepts in Java OOP, and the one that seems to give students the greatest amount of difficulty.
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 7 below.
| Listing 7 . Complete program listing. |
|---|
/*File Poly04.java
Copyright 2002, R.G.BaldwinThis program illustrates polymorphic
behaviorProgram output is:A@111f71
toString in class BtoString in class C
**************************************/class A extends Object{
//This class is empty}//end class A
//===================================//class B extends A{
public String toString(){return "toString in class B";
}//end overridden toString()}//end class B
//===================================//class C extends B{
public String toString(){return "toString in class C";
}//end overridden toString()}//end class B
//===================================//public class Poly04{
public static void main(String[]args){
Object varA = new A();String v1 = varA.toString();
System.out.println(v1);Object varB = new B();String v2 = varB.toString();
System.out.println(v2);Object varC = new C();String v3 = varC.toString();
System.out.println(v3);}//end main
}//end class Poly04 |
-end-
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