Interface specification in Ada

Interface specification in Ada

[Dennis Hoppe]

Hi,

my preferred programming language is Java and I'm used to declare in an 
interface some common methods, that all subclasses have to implement. 
For example:

interface Bicycle {
   void changeCadence(int newValue);
   void changeGear(int newValue);
   void speedUp(int increment);
   void applyBrakes(int decrement);
}

class RacingBicycle implements Bicycle {
    // remainder of this class implemented as before
}

If I accidentially implement RacingBicycle without providing the four 
methods mentioned above, the compiler would draw attention to this 
situation.

In Ada, i found the keyword "interface" only in the context of multiple 
inheritance. The usage would be:

package Bicycle is
    type Object is interface;

    procedure changeCadence (newValue : in Integer) is abstract;
    procedure changeGear (newValue : in Integer) is abstract;
    procedure speedUp (increment : in Integer) is abstract;
    procedure applyBrakes (decrement : in Integer) is abstract;
end Bicycle;


with Bicycle;

package RacingBicycle is

    type Object is new Bicycle.Object with null record;

    -- omitted needed procedures to raise an error
end RacingBicycle;


Unfortunately, the build process runs through without any errors. In 
this case, I want the compiler to notify me about missing procedure 
implementations, because they are declared abstract in the interface.

I think, I missed something in the hierarchy, but I don't see it.


Thank you in advance,
   Dennis Hoppe

Re: Interface specification in Ada

[christoph.grein]

You missed the "this" parameter, which is implicit in Java, but
explicit in Ada:

package Bicycle is
  type Object is interface;

  procedure changeCadence (Ob: in out Object; newValue : in Integer)
is abstract;

etc.

Re: Interface specification in Ada

[Georg Bauhaus]

christoph.grein@eurocopter.com schrieb:
> You missed the "this" parameter, which is implicit in Java, but
> explicit in Ada:
> 
> package Bicycle is
>   type Object is interface;
> 
>   procedure changeCadence (Ob: in out Object; newValue : in Integer)
> is abstract;

And in Ada 2005 the "this" parameter can be suitably
named, and when used, it can be written just like in
Java,

   Peugeot: Bicyle.Object;

   Peugeot.changeCadence(27);

Incidentally, I'd suggest you consider defining distict integer
types that reflect gears etc. By doing so you can have the
compiler check that a cadence number is not inadvertently
used as a gear number. E.g.,

   type Gear is range 1 .. 21;
...
   procedure changeGear
      (The_Bike: in out Object; newValue : in Gear) is abstract;

or

   type Gear
     (Front_Wheel: Wheel_Number;
      Rear_Wheel: Wheel_Number)
   is private;

and so on.

Re: Interface specification in Ada

[Robert A Duff]

christoph.grein@eurocopter.com writes:

> You missed the "this" parameter, which is implicit in Java, but
> explicit in Ada:
>
> package Bicycle is
> � type Object is interface;
>
> � procedure changeCadence (Ob: in out Object; newValue : in Integer)
> is abstract;

Also, it's a good idea to use the "overriding" keyword.

- Bob

Re: Interface specification in Ada

[Dennis Hoppe]

Hello Christoph, Georg and Robert,

thank you for your advices. Indeed Java relieves me of state "this".


christoph.grein@eurocopter.com wrote:
> You missed the "this" parameter, which is implicit in Java, but
> explicit in Ada:
> 
> package Bicycle is
>   type Object is interface;
> 
>   procedure changeCadence (Ob: in out Object; newValue : in Integer)
> is abstract;
> 
> etc.

Re: Interface specification in Ada

[Georg Bauhaus]

Dennis Hoppe wrote:

> Indeed Java relieves me of state "this".

When I started writing Java many years ago, I too thought that
like in the Java books, when I want to name an object's
component it is not neccessary to refer to the object.
So don't do that, leave out "this".
But now I find myself writing "this.a" and "this.b" all the
time, to make sure I can easily see what "a" actually stands
for: does it name a component or does it name a method parameter
or local variable? With "this" there is no question. There is
almost never a question in Ada because you always name the
object in some way.