Name: skT45625 Date: 06/06/2000


java version "1.2.2"
Classic VM (build JDK-1.2.2-W, green threads, sunwjit)


This happens, if working around lack of genericity by applying the
gof.strategy_pattern: The ``generic'' [Strategy-]class, of which
[ConcreteStrategy-]subclasses /*Integer, Float, ... */ are used, is
Number.

My example [Context-]class, which is something like a generic vectorspace,
offers its services via parameters of type Number. The client can use this
vectorspace-like class by providing actual parameters of compatible classes
like Integer, Float, ... . Of course, my vectorspace-like class has to
perform some calculations, e.g. additions.

The problem now is, that I cannot construct the result object, which should
be an object of the same subclass of Number, which my client provided as input
parameters. This is because the numerical value has to be given to the result
object at time of its construction. No means to change this value after
construction are available.

For assigning a simple constant ``0'', the following code fragment worked:


      Class
        coordinate_class = coordinate(0).getClass()
    ; Number
        coordinate = (Number)coordinate(0).getClass().newInstance()
    ; Constructor
        coordinate_constructor
        = coordinate_class.getConstructor(new Class[] {String.class})
    ; // neutral element for coordinates:
      Number
        neutral_coordinate
        = (Number)coordinate_constructor.newInstance(new Object[]{"0"})
    ;
    ; set_coordinate(index, neutral_coordinate)


, because here [occasionally] construction from a string object had the same
semantics for every subclass of Number. This doesn't work for arbitrary values.
And it is inadequately complex.


Proposal1:
  extend the signature of Number to offer the usual arithmetic operations, like
  we know it from BigDecimal. Although, if you think on, you end up in a
  rather complete model of algebraic structures like groups, rings and fields.

or

Proposal2:
  extend Java with a genericity concept.
 
or

worse Proposal:
  make Number-s mutable, i.e. give them methods to change their value after
  construction.
(Review ID: 105782)
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