inheritance doubt

[miguel lisboa]

i've class Contrato (abstract class) and classes ContratoParticular and ContratoConvencao that extends the former; can another class (eg class Person) have a reference to class Contrato, like:

 class Person {
//...
private Contrato c1;
//...
} 

thanks in advance

[Jayesh Lalwani]

Yes

You are welcome

[Nigel Browne]

You can not instantiate an instance of an abstract class, you must subclass it and then implement it's abstract methods in your sub class.

[miguel lisboa]

thanks to both for answering!

You can not instantiate an instance of an abstract class, you must subclass it and then implement it's abstract methods in your sub class.

my class diagram: (well, more or less)
______________________
Contrato
______________________
- tipo : String
...
______________________
+ recebido()
...
______________________

i dont want to instantiate Contrato (i know i cant), but was great if i could get a way to reference this abstract class in order to access (with a getter):
tipo
so i ask again: i can do it, right?
thanks again

[Stan James]

You can define a variable of the abstract type and make it refer to an instance of a concrete type.

public class Person
{
   private Contrato contrato = null;
   public void setContrato( Contrato a )
   {
      contrato = a;
   }
   public Contrato getContrato()
   {
      return contrato;
   }
}

All that compiles and runs when other classes call with an instance of ContratoParticular or ContratoConvencao. The Person class never knows or cares which one it is. Person can even call methods defined on Contrato without ever knowing what concrete class it has. The beauty and power of abstraction at work!

BTW: Translate Contrato for me. It has a very nice ring to it. I hope it's not something mundane, but this being a programming problem I don't have high hopes.

[miguel lisboa]

BTW: Translate Contrato for me. It has a very nice ring to it. I hope it's not something mundane, but this being a programming problem I don't have high hopes.

contrato is a portuguese word and means contract (i guess): something agreed between two entities.

All that compiles and runs when other classes call with an instance of ContratoParticular or ContratoConvencao. The Person class never knows or cares which one it is. Person can even call methods defined on Contrato without ever knowing what concrete class it has. The beauty and power of abstraction at work!

you mean this?

 
Person p = new Person();
String type = p.getContrato().getTipo(); 

since we are at this subject, by any chance do you know how to map inheritance with hibernate? because i'd have a Q...

[Marilyn de Queiroz]

Another set of examples:

List list1 = new ArrayList();
List list2 = new LinkedList();
Map map1 = new HashMap();
Map map2 = new TreeMap();

[miguel lisboa]

or

Set examples = New HashSet();  

[Layne Lund]

 Person p = new Person();
 String type = p.getContrato().getTipo(); 

I think he means that you can do something like this:

Contrato c = new ContratoParticular();
Person p = new Person();
p.setContracto(c);

In fact, the above code illustrates how you can declare a reference variable with the base class type and assign an instance of the subclass to it. This technique is actually very common. I often use code similar to the examples above using classes from the Collections framework.

Layne

Layne
[ February 16, 2005: Message edited by: Layne Lund ]

[miguel lisboa]

i've playing with all this for a while and i noticed that, when i query, i've to mention the right subclass. Is this allways like this?
(i'm using hibernate)
here Utente extends Pessoa

 Pessoa ut = new Utente();
ut.setApelido("da Silva");
Session s = HibernateUtil.getSession();
Transaction t = s.beginTransaction();
s.save(ut);

List pessoas = s.find("from Utente p");
System.out.println(pessoas.size() + " utente(s) encontrados:");
 
for (Iterator iter = pessoas.iterator(); iter.hasNext() :wink:  {
Utente u = (Utente) iter.next();
System.out.println(u.getApelido());// prints da Silva
}

t.commit();
s.close();
} 

[Layne Lund]

Originally posted by miguel lisboa:
i've playing with all this for a while and i noticed that, when i query, i've to mention the right subclass. Is this allways like this?
(i'm using hibernate)

I'm not sure what you mean by "I've to mention the right subclass". Which line of code in your example illustrates the idea you are trying to convey here?

Layne

[Marilyn de Queiroz]

If all Pessoas have a getApelido and setApelido, then you could say

    Pessoa pessoa = new Utente();
    pessoa.setApelido("da Silva");
    Session s = HibernateUtil.getSession();
    Transaction t = s.beginTransaction();
    s.save(pessoa);
    List pessoas = s.find("from Utente p");
    System.out.println( pessoas.size() + " pessoa(s) encontrados:" );
    for ( Iterator iter = pessoas.iterator(); iter.hasNext()   :wink:    {
        Utente u = (Utente) iter.next();
        System.out.println( u.getApelido() );// prints da Silva
    }
    t.commit();
    s.close();
} 

I never used Hibernate, but it looks like the question would be the same whether or not you use it.

It comes in handy when you have two different types of Pessoas that have the same method that returns different things.

For example, you may have two types of Animais, Gato and Cavalo. Gato and Cavalo extend Animal. Gato and Cavalo each have a method called fala() but they return different things. Try putting a Gato and a Cavalo into a List and then call
System.out.println( animal.fala());
[ February 16, 2005: Message edited by: Marilyn de Queiroz ]

[David Harkness]

Originally posted by miguel lisboa:
by any chance do you know how to map inheritance with hibernate?

For your specific case, check out this very similar thread.

[miguel lisboa]

I'm not sure what you mean by "I've to mention the right subclass". Which line of code in your example illustrates the idea you are trying to convey here?

i refer to my query - hql (hibernate query language queries objects), so i thought, maybe naively that i could query the superclass, but i get an error and have to query the concrete class:

List pessoas = s.find("from Utente p");  

Utente is a subclass of Pessoa

It comes in handy when you have two different types of Pessoas that have the same method that returns different things.

For example, you may have two types of Animais, Gato and Cavalo. Gato and Cavalo extend Animal. Gato and Cavalo each have a method called fala() but they return different things. Try putting a Gato and a Cavalo into a List and then call
System.out.println( animal.fala());

is quite agreable, for once, seeing someone using our language

i had two classes: like User and Technician (Utente and Tecnica) and i decided to put name, other name ,lastName, telephone and mobile phone in another class (Person). First i had a one to one relation between Utente or Tecnica and Person. After i tried inheritance, making Utente and Tecnica is-a Person. Now i believe this isnt quite useful, since i only inherit those accessors for name, otherName etc, but the other classes have their own (diferent) methods and so i cant use polymorphism in a complete way... so i got back to my original design.