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Re: Self-updating relationship

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  Re: Self-updating relationship James Kuyper <jameskuyper@verizon.net> - 2015-09-02 17:14 -0400
    Re: Self-updating relationship Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-09-02 17:46 -0400
      Re: Self-updating relationship Arne Vajhøj <arne@vajhoej.dk> - 2015-09-03 21:06 -0400
        Re: Self-updating relationship e.d.programmer@gmail.com - 2015-09-04 04:46 -0700
          Re: Self-updating relationship Arne Vajhøj <arne@vajhoej.dk> - 2015-09-04 19:43 -0400
      Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-04 11:49 -0400
        Re: Self-updating relationship Arne Vajhøj <arne@vajhoej.dk> - 2015-09-04 19:45 -0400
          Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-04 22:10 -0400
            Re: Self-updating relationship Arne Vajhøj <arne@vajhoej.dk> - 2015-09-04 22:31 -0400
              Re: Self-updating relationship Arne Vajhøj <arne@vajhoej.dk> - 2015-09-04 22:38 -0400
              Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-04 22:51 -0400
    Re: Self-updating relationship James Kuyper <jameskuyper@verizon.net> - 2015-09-02 18:49 -0400
      Re: Self-updating relationship James Kuyper <jameskuyper@verizon.net> - 2015-09-02 19:28 -0400
        Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-02 23:52 -0400
          Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-03 10:32 -0400
            Re: Self-updating relationship James Kuyper <jameskuyper@verizon.net> - 2015-09-03 10:53 -0400
              Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-03 11:31 -0400
                Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-03 11:35 -0400
                Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-03 11:49 -0400
                Re: Self-updating relationship James Kuyper <jameskuyper@verizon.net> - 2015-09-03 12:16 -0400
                  Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-03 13:03 -0400
            Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-03 10:55 -0400
        Re: Self-updating relationship James Kuyper <jameskuyper@verizon.net> - 2015-09-03 00:06 -0400
      Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-02 23:38 -0400
        Re: Self-updating relationship Jeff Higgins <jeff@invalid.invalid> - 2015-09-03 01:22 -0400

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#32499

On 09/03/2015 12:16 PM, James Kuyper wrote:

> OK - I thought I was beginning to understand  what was going on, but now
> I'm thoroughly confused. You're saying that this behavior (Specifically,
> the behavior of re-calculating the subtraction each time
> spaces.getValue() is used) is a property of the
> javafx.beans.property.Property<T> interface, rather than a property of
> the javafx.beans.binding.Binding<T> interface, despite the fact that it
> happens when he uses javafx.beans.binding.NumberBinding, and doesn't
> happen when he uses javafx.beans.property.SimpleIntegerProperty?
>
> How does that work? Does javafx.beans.binding.NumberBinding use
> javafx.beans.property.Property<T> while
> javafx.beans.property.SimpleIntegerProperty uses
> javafx.beans.binding.Binding<T>? That's not at all what I would have
> expected, based solely upon the names.
>
>
Perhaps our discussion is going beside itself it the threads.
I would like to use the example code that I posted elsewhere
and neglect the javafx.beans.binding package, in particular
the NumberBinding interface if that is acceptable.

If so I will reproduce the example here.
You'll need access to the JavaFX documentation, located here:
With frames
<https://docs.oracle.com/javase/8/javafx/api/index.html?overview-summary.html>
No frames
<https://docs.oracle.com/javase/8/javafx/api/overview-summary.html>


import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;

public class Main
{ public static void main(String[] args)
   {

     final IntegerProperty height
     = new SimpleIntegerProperty();

     final IntegerProperty number
     = new SimpleIntegerProperty();

     final IntegerProperty spaces
     = new SimpleIntegerProperty();

     spaces.bind(height.subtract(number));

     height.set(10);
     number.set(2);
     System.out.print(spaces.get()); // 8

     spaces.unbind();
     height.set(20);
     System.out.print(spaces.get()); // 8

} }

Here spaces is a SimpleIntegerProperty, a subclass of
IntegerPropertyBase which is an implementation of the
Property<T> interface. IntegerPropertyBase is where the
Property.bind(ObservableValue<? extends T> observable) and
Property.unbind() are implemented.

The expression height.subtract(number) creates an
ObservableValue<Number>, through NumberExpression ( so much
for neglecting the javafx.beans.binding package) which
IntegerPropertyBase uses to bind height and number to spaces
as specified in his documentation.

As I stated elsewhere, the plethora of mixin interfaces
makes it difficult (for me) to discuss, but writing code
that uses it is simple.

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#32491

On 09/03/2015 10:32 AM, Jeff Higgins wrote:
> On 09/02/2015 11:52 PM, Jeff Higgins wrote:
>> On 09/02/2015 07:28 PM, James Kuyper wrote:
>
>>> Your earlier version had:
>>>>      final javafx.beans.binding.NumberBinding spaces =
>>>>      height.subtract( number );
>>>
>>> which causes spaces.getValue() to behave like the equivalent of a C++
>>> member function whose return value is calculated using the specified
>>> formula freshly each time "spaces" is referred to. Is this a
>>> characteristic of binding.NumberBinding, or some more general Java
>>> feature?
>>
>> It's a characteristic of the
>> javafx.beans.binding.Binding<T> interface.
>> <https://docs.oracle.com/javase/8/javafx/api/javafx/beans/binding/Binding.html>
>>
>>
>
> Sorry, this is incorrect.
> It's a characteristic of the
> javafx.beans.property.Property<T> interface.
> <https://docs.oracle.com/javase/8/javafx/api/javafx/beans/property/Property.html>
>

Actually, javafx.beans.property.IntegerPropertyBase
provides the bind method and associated machinery
in the given case.

final javafx.beans.property.IntegerProperty height
= new javafx.beans.property.SimpleIntegerProperty( 10 );

final javafx.beans.property.IntegerProperty number
= new javafx.beans.property.SimpleIntegerProperty(  2 );

final javafx.beans.binding.NumberBinding spaces =
height.subtract( number );

java.lang.System.out.println( spaces.getValue() );

height.set( 20 );

java.lang.System.out.println( spaces.getValue() ); /* prints:

8
18                                                         */

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#32483

On 09/02/2015 07:43 PM, Stefan Ram wrote:
> James Kuyper <jameskuyper@verizon.net> writes:
>> formula freshly each time "spaces" is referred to. Is this a
>> characteristic of binding.NumberBinding, or some more general Java feature?
> 
>   It is a characteristics of a certain standard Java library
>   �JavaFX� that contains classes for "binding" properties to
>   other properties. Binding can establish a spreadsheet-like
>   relationship, so that one property can get updated when
>   another property is updated. But this is a feature of the
>   standard library, not of the language proper.
> 
>> In C++, the equivalent of the first form above would be simpler than the
>> equivalent of the second form, so it feels odd that the second form is
>> simpler in Java. I'll try to keep that in mind should I ever need to
>> program in Java again. It sounds like I've got a lot to learn before I
>> even get started.
> 
>   I am not aware of a similar means of binding in the standard
>   library of C++. But surely, one can implement it, if needed.

When I was talking about C++ equivalents, I was probably thinking of
something much simpler than what you're referring to as "binding". I
assumed that your original version worked something like the following:

struct Updating
{
    int height;
    int number;
    Updating(int _height, int _number) : height(_height),
	number(_number) {};
    int spaces(void) const { return height - number; }
};

And what I was wondering about is which part of your code made it behave
that way, rather than the way this struct would:

struct OneTime
{
    int height;
    int number;
    int spaces;
    OneTime(int _height, int _number) : height(_height),
        number(_number), spaces(_height - _number) {};
};

I'm not saying that OneTime is preferable to Updating, I was just
unclear about which features of your Java code caused it to behave more
like Updating than like OneTime. As I now understand it,
javafx.beans.binding.NumberBinding is that feature.
-- 
James Kuyper

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#32481

On 09/02/2015 06:49 PM, James Kuyper wrote:
> On 09/02/2015 05:25 PM, Stefan Ram wrote:
>> James Kuyper <jameskuyper@verizon.net> writes:
>>>> final javafx.beans.property.IntegerProperty height
>>>> = new javafx.beans.property.SimpleIntegerProperty( 10 );
>>>> final javafx.beans.property.IntegerProperty number
>>>> = new javafx.beans.property.SimpleIntegerProperty(  2 );
>>>> final javafx.beans.binding.NumberBinding spaces =
>>>> height.subtract( number );
>>>> java.lang.System.out.println( spaces.getValue() );
>>>> height.set( 20 );
>>>> java.lang.System.out.println( spaces.getValue() ); }}
>>> What's the java feature that makes "spaces" above update automatically?
>>> To put it another way - if I didn't want such updates, what should I
>>> write differently?
>>
>>    This depends on the details of the behavior you want instead.
>
> I read a bit about Java many years ago, and reached the conclusion that
> it had some syntactic similarities C++, but had a very different way of
> implementing objects. I played with it long enough to get something
> working, but concluded that it wasn't what I needed for any purpose I
> was trying to achieve at the time. I'm otherwise very unfamiliar with java.
>
> I get the impression from what you're saying, that the code you've shown
> results in each call to spaces.getValue() performing a fresh calculation
> of height-number, using the values of those things at the time
> getValue() is called. I'm asking about what you would have to do
> differently to have spaces set to the value of height - number at one
> particular time, and to have spaces.getValue() continue to report that
> same value, even if the values of height and number later change. It
> seems to me that this should be easier than arranging for automatic
> updates - in fact, I'd expect it to be trivial.
>
>>    For one example, you can simply omit »height.set( 20 )«, and
>>    »spaces« will not be updated. Or, you could just write:
>
> Well, I'm asking how to ensure that "spaces" will not be updated, even
> if "height" is, so that doesn't qualify.
>
>> public final class Main
>> { public static void main( final java.lang.String[] args )
>>    { final javafx.beans.property.IntegerProperty spaces
>>      = new javafx.beans.property.SimpleIntegerProperty( 8 );
>>      java.lang.System.out.println( spaces.getValue() ); }}
>>
>>    Which just will print »8« and not update spaces, or just
>>
>> public final class Main
>> { public static void main( final java.lang.String[] args )
>>    { int const spaces = 8; java.lang.System.out.println( spaces ); }}
>
> Those both set spaces to 8, rather than height - number. If
> height-number is currently 8, that happens to be the right value, but
> isn't what I'm looking for.
>
>>    or just,
>>
>> public final class Main
>> { public static void main( final java.lang.String[] args )
>>    { java.lang.System.out.println( 8 ); }}
>>
>>    to get the same behavior.
>
> That seems to print out 8 without even storing it anywhere, and without
> any connection to "height" or "number".
>

You may use the unbind method of the
javafx.beans.property.Property<T> interface.
<https://docs.oracle.com/javase/8/javafx/api/javafx/beans/property/Property.html>

import javafx.beans.property.IntegerProperty;
import javafx.beans.property.SimpleIntegerProperty;

public class Main
{ public static void main(String[] args)
   {

     final IntegerProperty height
     = new SimpleIntegerProperty();

     final IntegerProperty number
     = new SimpleIntegerProperty();

     final IntegerProperty spaces
     = new SimpleIntegerProperty();

     spaces.bind(height.subtract(number));

     height.set(10);
     number.set(2);
     System.out.print(spaces.get()); // 8

     spaces.unbind();
     height.set(20);
     System.out.print(spaces.get()); // 8

} }

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#32486

On 09/02/2015 11:38 PM, Jeff Higgins wrote:

> import javafx.beans.property.IntegerProperty;
> import javafx.beans.property.SimpleIntegerProperty;
>
> public class Main
> { public static void main(String[] args)
>    {
>
>      final IntegerProperty height
>      = new SimpleIntegerProperty();
>
>      final IntegerProperty number
>      = new SimpleIntegerProperty();
>
>      final IntegerProperty spaces
>      = new SimpleIntegerProperty();
>
>      spaces.bind(height.subtract(number));
>
>      height.set(10);
>      number.set(2);
>      System.out.print(spaces.get()); // 8
>
>      spaces.unbind();
>      height.set(20);
>      System.out.print(spaces.get()); // 8
>
> } }

So, if I take spaces to be a spreadsheet cell with an Observable value
it's a pretty simple matter to bind it to any of my CellFormula(s) which
are also Observable values of a type compatible with the spaces cell.

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