inheritance implementation?

[Yang <te...@gmail.com>]

I understand that avro does not have inheritance now, so I am wondering what
is the best way to achieve the following goal:

I define Apple, Orange, and Fruit. Apple and Orange should ideally derive
from Fruit, but since there is no built-in mechanism,
we create an internal member for aboth Apple and Orange, encapsulating the
contents of Orangle

Apple :{
Fruit: fruit_member

string: pattern_on_skin
}

Orange : {

Fruit: fruit_member

string: skin_thickness
}


Fruit: {
int : size,
string: color
int: weight
}



say I want to pass objects of both Apple and Orange to some scale to measure
the total weight,
I can pass them just as Objects,


int findTotalWeight(List<Object> l ) {

    int result=0;
    for(Object o : l ) {
       result += ???????  <============ somehow get access to the
fruit_member var ??
   }
}


so what is the best way to fill in the line above with "<====" ? doing a lot
of instanceof  is kind of cumbersome


Thanks
Yang

Re: inheritance implementation?

[Yang <te...@gmail.com>]

Thanks guys,
the inverted idiom is indeed easier

I'll try that

Yang

On Tue, May 31, 2011 at 1:36 PM, Doug Cutting <cu...@apache.org> wrote:

> You might invert the encapsulation, so that each Fruit instance contains
> a 'flavor' field that's a union of types like Apple and Orange?
>
> Doug
>
> On 05/31/2011 08:33 PM, Yang wrote:
> > I understand that avro does not have inheritance now, so I am wondering
> > what is the best way to achieve the following goal:
> >
> > I define Apple, Orange, and Fruit. Apple and Orange should ideally
> > derive from Fruit, but since there is no built-in mechanism,
> > we create an internal member for aboth Apple and Orange, encapsulating
> > the contents of Orangle
> >
> > Apple :{
> > Fruit: fruit_member
> >
> > string: pattern_on_skin
> > }
> >
> > Orange : {
> >
> > Fruit: fruit_member
> >
> > string: skin_thickness
> > }
> >
> >
> > Fruit: {
> > int : size,
> > string: color
> > int: weight
> > }
> >
> >
> >
> > say I want to pass objects of both Apple and Orange to some scale to
> > measure the total weight,
> > I can pass them just as Objects,
> >
> >
> > int findTotalWeight(List<Object> l ) {
> >
> >     int result=0;
> >     for(Object o : l ) {
> >        result += ???????  <============ somehow get access to the
> > fruit_member var ??
> >    }
> > }
> >
> >
> > so what is the best way to fill in the line above with "<====" ? doing a
> > lot of instanceof  is kind of cumbersome
> >
> >
> > Thanks
> > Yang
> >
>

Re: inheritance implementation?

[Doug Cutting <cu...@apache.org>]

You might invert the encapsulation, so that each Fruit instance contains
a 'flavor' field that's a union of types like Apple and Orange?

Doug

On 05/31/2011 08:33 PM, Yang wrote:
> I understand that avro does not have inheritance now, so I am wondering
> what is the best way to achieve the following goal:
> 
> I define Apple, Orange, and Fruit. Apple and Orange should ideally
> derive from Fruit, but since there is no built-in mechanism,
> we create an internal member for aboth Apple and Orange, encapsulating
> the contents of Orangle
> 
> Apple :{
> Fruit: fruit_member
> 
> string: pattern_on_skin
> }
> 
> Orange : {
> 
> Fruit: fruit_member
> 
> string: skin_thickness
> }
> 
> 
> Fruit: {
> int : size,
> string: color
> int: weight
> }
> 
> 
> 
> say I want to pass objects of both Apple and Orange to some scale to
> measure the total weight,
> I can pass them just as Objects,
> 
> 
> int findTotalWeight(List<Object> l ) {
> 
>     int result=0;
>     for(Object o : l ) {
>        result += ???????  <============ somehow get access to the
> fruit_member var ??
>    }
> }
> 
> 
> so what is the best way to fill in the line above with "<====" ? doing a
> lot of instanceof  is kind of cumbersome
> 
> 
> Thanks
> Yang
>      

Re: inheritance implementation?

[Scott Carey <sc...@richrelevance.com>]

You can do this a few ways.  The composition you list will work, the member variable should be of type Fruit.

Or you can put the type object inside the fruit:

record Fruit {
int size;
string color;
int weight;
union { Apple, Orange } type;
}

record Orange {
string skin_thickness;
}

record Apple {
string skin_pattern;
}

However, Avro's IDL language and the Specific compiler in Java will not compile this into a class hierarchy.  You can use a wrapper class in Java to do that.  A factory method to create a specific Fruit subclass by inspecting a Fruit would use instanceof to determine the union type and create the corresponding object.

One way or the other, you do need to do some instanceof / casting depending on what you are accessing.  I have used the pattern above, with the 'type' inside the outer general object.

On 5/31/11 11:33 AM, "Yang" <te...@gmail.com>> wrote:

I understand that avro does not have inheritance now, so I am wondering what is the best way to achieve the following goal:

I define Apple, Orange, and Fruit. Apple and Orange should ideally derive from Fruit, but since there is no built-in mechanism,
we create an internal member for aboth Apple and Orange, encapsulating the contents of Orangle

Apple :{
Fruit: fruit_member

string: pattern_on_skin
}

Orange : {

Fruit: fruit_member

string: skin_thickness
}


Fruit: {
int : size,
string: color
int: weight
}



say I want to pass objects of both Apple and Orange to some scale to measure the total weight,
I can pass them just as Objects,


int findTotalWeight(List<Object> l ) {

    int result=0;
    for(Object o : l ) {
       result += ???????  <============ somehow get access to the fruit_member var ??
   }
}


so what is the best way to fill in the line above with "<====" ? doing a lot of instanceof  is kind of cumbersome


Thanks
Yang