HAS-A problem

[Shiva Mohan]

    
public class Objective61 {
  public static void main(String[] args){
      Horse myHorse=new Horse();
      myHorse.priya();
  }  
   
}
 
class Horse extends Objective61{
   Cow myCow;
   void priya(){
       Cow myCow=new Horse();
      myCow.tie(4);
   }
  
}
 
class Cow extends Horse{
   
    int r;
  void tie(int r)  {//----------------method
      this.r=r;
  }
  
}

Horse has a Cow.because Horse declares an instance variable of type Cow.But,how can I call Cow class method in Horse class.Can you help me please? I tried my best but end up with error.

[Keith Lynn]

A Horse is not a Cow.

[Shiva Mohan]

Horse has a Cow.
class Horse{
Cow myCow;
}

[Keith Lynn]

Cow myCow=new Horse();

Take a look at this line though.

[Shiva Mohan]

Thanks for the reply.Did you mean that i need to take off that line.Could you please explain it more clear for me.

[Rusty Shackleford]

You are abusing the is-a and has-a relationships to the point where it becomes senseless:

class Cow extends Horse// this is saying Cow is a Horse if anything, both classes should extend Animal or FarmAnimal or the like

class Horse{
Cow myCow;
}

Why? A horse has legs, tail, ect. Can be ridden, eat, make noises, ect. But has a Cow? Is it freaking out or something?

class Horse extends Objective61 //why extend a class whose only member is main?

The problem Mr. Lynn pointed out is called derived class reference to a base class object. Look this up. You can't do this, because if you could it would have undefined behavior. You can do the converse and is a quite useful tool when used under a inheritance heirarchy that makes sense: Horse horse = new new Cow(); // not implying this makes sense
[ June 16, 2006: Message edited by: Rusty Shackleford ]

[Ken Blair]

A Horse is not a Cow. It does not extend Cow, it does not implement Cow, it is not in any way a subclass of Cow. You cannot create a Horse and assign it to a reference variable declared to be a Cow because they are not in any way the same thing.

I suggest looking at the Java Tutorial.

[Shiva Mohan]

Thanks for the reply.How about now?

    
public class Objective61 {
  public static void main(String[] args){
      Horse myHorse=new Horse();
      myHorse.priya();
  }  
   
}
 
class Horse extends Objective61{
   Cow myCow;
   void priya(){
      myCow.tie(4);
   }
  
}
 
class Cow {
   
    int r;
  void tie(int r)  {
      this.r=r;
  }
}

But it's giving NullPointerException.I already declared a number for tie() method.

[Rusty Shackleford]

Where is your reference to an instianted Cow object?

When myHorse is created, myCow is set to null by default, when not explicity created in a explicit constructor.

If a horse and a cow share attributes, they should be in their own class that implements some interface. That way Horse has a <whatever attribute that int is supposed to be>, and not a Cow which makes no sense.

Go back over the whole concept of inheritance and the reasons it is there.
[ June 16, 2006: Message edited by: Rusty Shackleford ]