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Re: Using main from abstract class in concrete class

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On 8/13/2017 4:47 AM, Cecil Westerhof wrote:
> I have an abstract class. There can be several concrete classes that
> extend this class. Is it possible to write a main in the abstract
> class which will be used by the concrete classes and that in the main
> a instance of the concrete class is created?

Well, your question is gibberish, but I'm assuming what you're really 
asking is "can I have a static method that is invoked by dynamic 
dispatch on the receiver class?" The answer to that question is "not 
really."

So static methods are inherited in scope, such that calling 'Sub.foo' 
looks for a static method in Sub first and then looks for it in Super if 
it couldn't find it. But static methods have no concept of a "this" 
pointer like instance methods do. Thus, there is nothing that could 
possibly be dispatched on.

You can work around this flaw by using Class objects and using 
reflection to search for the method. Of course, the superclass method 
still doesn't have a notion of the current class object being invoked, 
so you'd have to pass the class parameter in. And you face the issue 
that this kind of reflection is not first-class in Java.

An alternative? Well, factories! Constructors are essentially a kind of 
static method [1], and it's very desirable to be able to have some sort 
of generic construction mechanism. The standard design pattern is to 
have a factory class, which has a method that can actually construct the 
object in question. Now you can add on to this factory class other 
methods that can dispatch on the factory class instance--which is 
effectively the same as class object itself.


[1] Okay, internally they're a special nonvirtual method. But given that 
you need to know the type of the receiver to execute the new opcode, 
they're more like a static method than an instance method in terms of use.


-- 
Beware of bugs in the above code; I have only proved it correct, not 
tried it. -- Donald E. Knuth

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Using main from abstract class in concrete class Cecil Westerhof <Cecil@decebal.nl> - 2017-08-13 11:47 +0200
  Re: Using main from abstract class in concrete class Andreas Leitgeb <avl@logic.at> - 2017-08-13 10:59 +0000
    Re: Using main from abstract class in concrete class Andreas Leitgeb <avl@logic.at> - 2017-08-13 11:38 +0000
  Re: Using main from abstract class in concrete class Cecil Westerhof <Cecil@decebal.nl> - 2017-08-13 15:43 +0200
  Re: Using main from abstract class in concrete class Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-08-13 10:32 -0400
    Re: Using main from abstract class in concrete class Eric Sosman <esosman@comcast-dot-net.invalid> - 2017-08-13 10:37 -0400
    Re: Using main from abstract class in concrete class Cecil Westerhof <Cecil@decebal.nl> - 2017-08-13 17:17 +0200
      Re: Using main from abstract class in concrete class Andreas Leitgeb <avl@logic.at> - 2017-08-17 18:56 +0000
        Re: Using main from abstract class in concrete class Cecil Westerhof <Cecil@decebal.nl> - 2017-08-18 06:37 +0200
          Re: Using main from abstract class in concrete class Andreas Leitgeb <avl@logic.at> - 2017-08-18 19:48 +0000
            Re: Using main from abstract class in concrete class Cecil Westerhof <Cecil@decebal.nl> - 2017-08-18 23:37 +0200
              Re: Using main from abstract class in concrete class Cecil Westerhof <Cecil@decebal.nl> - 2017-08-19 01:32 +0200
  Re: Using main from abstract class in concrete class Joshua Cranmer 🐧 <Pidgeot18@verizon.invalid> - 2017-08-13 12:03 -0500

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