Re: java class hierarchy


"Thomas" <arabel9@xxxxx> wrote in message
news:f7td3a$lcs$2@xxxxxxxxxxxxxxxxxx
Hello I 'm writing a simple ONP expression evaluator with functions in
java.
The constrains about the class hierarchy are like that :

abstract Symbol implements Evaluate (which contains
method evaluate)
/ \
/ \
/ \
abstract Function abstract Operand
/ | \ / |
\
Plus Minus Ln Literal Const Variable

The evaluation process should be : after parsing the input we put all
those
object into the queue. By reading the queue and using the stack to keep
the
temporary values (both of them I implement on my own) i have to do like
that
:
1) if it is an Operand put in on stack
2) if it is an function and
2)a we have enought arguments on stack we valuate the function
2)b we throw exepction
(i can distinguish beetwen this two types and here i don't need help,
because of the diffrent number of arguments they take and exceptions)

In the main loop I have to keep the refrence to the object taken from
FIFO,
and here is the problem : what type of refrence it should be ? Not a
symbol
because it is abstract, but it implements the Evaluate interface. I
confused. Any suggestions, please ?

There's no hard and fast rule, but as a rule of thumb, in this
situation you would use the least general type which is a supertype of all
the possible types you might get from your FIFO queue.

The issue is that you will probably want to do something with each
element you take out of the queue. Usually, to "do something" involves
invoking a method. Which means the method you want to invoke must be part
of the type of the reference (or you could use casting, but that gets
messy).

So say if you wanted to call the .doSomething() method on each
element, the type of the reference would need to be such that the
..doSomething() method is defined on that type. If the .doSomething()
method is defined in Symbol, then use Symbol as the type of the reference.
If the method is defined in Evaluate, then use Evaluate. If it's defined
in both, then it doesn't matter too much which one you use.

- Oliver


.

Relevant Pages

  • Re: simple static question
    ... reference to it's static queue, will the "lost" reference to class A I ... /do/ make use of classloaders. ...
    (comp.lang.java.programmer)
  • Re: simple static question
    ... reference to it's static queue, will the "lost" reference to class A I ... if someone is using custom classloaders. ...
    (comp.lang.java.programmer)
  • Re: GC SuppressFinalize and ReRegisterForFinalize
    ... SuppressFinalizer sets the "run my finalizer" bit to false, and that reference is removed from the queue, at which point the GC resets the bit to true. ... The finalization queue will keep on adding references to the same object ... >balance out several calls to Rereg with Suppress). ...
    (microsoft.public.dotnet.framework.clr)
  • Re: Overwriting / reassigning instance / garbage collection?
    ... >>reference count of zero and python will garbage collect it????? ... > Given that the only decent reason to use a Queue is to communicate ... messages that have to do with the commands I'm sending, ... I would think python would gc the first one? ...
    (comp.lang.python)
  • Re: How to cancel printing or to delete a print job that is stuck
    ... the print jobs aren't on the linux pc. ... same printer and those jobs don't show up in it's queue. ... Jobs get a reference count associated with them from the spooler and the ...
    (microsoft.public.windowsxp.print_fax)