[Advanced-java] OOP's Question

[Don Bate]

Bob,

	There are a few statements in your posting that are 
inaccurate. In your second sentence the answer is that the methodA() 
in C will be invoked. The fourth sentence is wrong. In both cases the 
object is of class A. The implementer of A chose to use some of the 
implementation of B and/or C to implement the complete behavior of A. 
That is inheritance. The runtime does have a dispatch table that is 
built when the constructor is invoked.

	The use of super is not all that common except for 
constructors. The main other real usage that I see is when the 
implementation of methodA in A might invoke super.methodA because 
it's augmenting what the parent classes' methods are doing in their 
implementations. So generally, the runtime default of using the 
most-derived method is correct and is only overridden when an 
implementer needs more explicit control. Java is actually a more 
simplified solution than the C++ usage of virtual and non-virtual 
methods.

	Inheritance vs. polymorphism. These are two distinct concepts 
that are often confused. Of the two, polymorphism is actually the 
more valuable concept. Implementation inheritance is actually a form 
of coupling and can cause maintenance issues. The complete 
implementation of an object A is actually defined in classes A, B, 
and C.  A change in the implementation of B or C is also a change in 
the complete implementation of an object of class A. The use of super 
is actually an increase in coupling and a dependency of the 
implementation of A on C. The original example is a suspicious usage 
of super in that it is B that is making the decision to use C's 
implementtion and not A. Polymorphism is what the classes outside of 
A, B, or C can see. They can treat it as a C regardless of whether it 
is actually an A, B, or C instance. Polymorphism reduces coupling.

	The last question is simple. The JVM uses the most-derived 
implementation of the object instance to determine the method to 
invoke. super overrides that default to one above the current 
derivation. Note that this is determined by the instance:

	B obj = new A();

will use A's methodA() if obj.methodA () is invoked, while

	B obj = new B();

will use C's methodA() if obj.methodA() is invoked.

At 2:36 PM +0530 4/30/07, Bob V wrote:
>Let us for a moment remove the overrided version of methodA() in A. 
>Then I think the methodA() in C will be invoked or a runtime 
>exception. Subsequently if we "revive" the methodA() implementation 
>in A, I think the methodA() in A will be invoked. Although the 
>invoking object in both the runs was object of B. More than the 
>Polymorphic behaviour in this case of overriding, I think the JVM 
>has some kind data structure that indicates what method to invoke.
>
>And also as a corollary, in this case the methodA() of C will be 
>called only if super.methodA() is used in B else the methodA of A 
>will be invoked. Isn't there loss of transparency in this behaviour. 
>The runtime ought to decide by itself which object to invoke.
>
>What if there are multiple levels of hierarchy and in every layer 
>the methodA is overrided. How does the JVM decide what method to 
>invoke?
>
>-B
>
>----------------------------------------------------------------------------------------------------------------
>
>The answer is simple and lies in the very definition of "polymorphism".
>Choosing the right behavior of an object.
>
>Keep aside the classes for a moment and look at what "object" is the
>function being called upon? Whether it is through super's method or its
>own, the object on which function is being called would choose the "right"
>function.
>
>When you call super.somefunc() and it calls methodA(), the method call is
>"still" going to an object of class A, which would eventually choose
>overridden method defined in class A. Think in term of objects at
>"runtime".
>
>Prashant Parashar
>
>
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-- 
Don Bate               | Specializing in Consulting and Mentoring in
Bate Consulting, Inc   | Object-Oriented Technologies,
                        | Software Architecture, and Software Process
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