[3749] in java-interest
Re: Fundamental flaw in Java library distribution scheme
daemon@ATHENA.MIT.EDU (Jim Graham)
Sat Nov 25 07:01:10 1995
Date: Sat, 25 Nov 1995 02:36:53 -0800
From: flar@bendenweyr.Eng.Sun.COM (Jim Graham)
To: perry@cdt.luth.se
Cc: java-interest@java.Eng.Sun.COM
Hi Per (sorry to have gotten your name wrong the last time :-(),
> > > This is not wrong. Class L can't know that there now is a new
> > > method in class C. Think about it. If you add a method to a class
> > > C, wouldn't it be strange if classes using that class, suddenly
> > > started calling this new method, without even being recompiled!?
> >
> > The beauty of Java is that if you override a method, which involves
> > using the same arguments as the inherited method exactly, then the
> > new method *will* get called automatically at runtime *without*
> > recompilation!
>
> But it isn't really a new method, just an overridden one, i.e. the
> "old" method but with new contents. I think you said this yourself,
> that if a method is overloaded you get a completely new method, but
> if a method is overridden, it just gets new contents.
This is a key point to understand.
If you provide a method with even the slightest change in any of the
types of any of the arguments in a subclass as compared to an inherited
method you have absolutely not overridden the method, you have instead
overloaded it (as per the definition of these terms in Java). In the
example in question:
class Object {
public void equals(Object obj);
}
class C /* implicitly extends Object */ {
public void equals(C obj);
}
The C.equals method does not override the Object.equals method. It
overloads the method and it is essentially a distinguishable, or new,
method that must be specifically chosen at compile time. If the
argument had been declared as type Object, then it would override the
first method and would automatically be chosen whenever the original
equals(Object) method was invoked on an object of type C.
So, when you say '"old" method, but with new contents' above you are
defining something that is overloading and not overriding. It is a new
method (in the sense that overloading methods creates new methods that
happen to share a common name), and it is not "just an overridden one"
(which in Java would require it to have the *exact* same signature).
...jim
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