Re: Template method pattern in Java ???

From: Avner Ben (avner_at_skilldesign.com)
Date: 02/23/04

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    Date: Mon, 23 Feb 2004 10:04:07 +0100

    Steven Wurster wrote:

    > [snip]
    > From the client perspective, all classes are pure interfaces. The
    > only caveats being that contracts may refer to other routines and it
    > is possible to figure out how some routines are coded just from the
    > contract; and the fact that abstract classes cannot be instantiated.
    > But, in general, the client does not and should not care whether or
    > not a class has implementations for any of its routines. Just code to
    > the interface.
    >
    > From the descendant programmer perspective, this is a different issue.
    > It matters which routines are abstract because those will have to be
    > implemented should the programmer wish that specific descendant to be
    > instantiated. It is an orthogonal issue. The catch here is with
    > inheritance and polymorphism:

    Perhaps we need two kinds of interface: usage interface and extension
    interface.

    >
    > Java, Eiffel, and I'm guessing C#, treat every routine as potentially
    > polymorphic. C++ of course does not, and you have to ask for dynamic
    > binding, and have the foresight that it will be needed.

    In C#, as in C++, a method is non-virtual by default. I am not happy
    with that decision.

    But Eiffel
    > also allows for the inheritance of non-purely abstract classes. What
    > this means is that the descendant programmer has freedom to add
    > polymorphism to any class, and at the same time inherit from any other
    > class. Level of abstraction is irrelevant. This is different in Java
    > and possibly C#, where polymorphism can be added to any class, but
    > inheritance from another class at the same time is limited to purely
    > abstract ones (interfaces in the Java world). This severely limits
    > the concept of inheritance. That is my entire argument here. Java is
    > flawed in this notion.

    In C++, you can inherit from any class, abstract or otherwise and to my
    best knowledge, in Java, too. Java distinguishes between extends (base
    class) and implements (interface). C# does not. In C++ and C# you can
    use objects polymorphically only through messages predefined to be virtual.

    > [snip]
    > I'm not sure what you mean by repeating of the conditions? You mean
    > he repeates the pre and postconditions? This is nothing new. Eiffel
    > does this for you, automatically no less. Eiffel also automatically
    > takes care of joining multiply inherited routines when one is abstract
    > and the other isn't. For the cases where multiply inherited routines
    > are both implemented, you have to select which one is used "up the
    > tree", if you will, while you have full choice over how to define the
    > new routine in the new class.
    >

    We speak about documentation here - there is no formal DBC support in
    C++. Although Stepanov never mentions either Eiffel or B. Meyer, they
    seem to be at least influenced from the same source.

            Avner.

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