Has-A relation

[Ash Gill]

hi everyone, i have a small doubt. Given the following code:

class Animal {}
class Dog extends Animal {}
class TestIt {
public static void main (String [] args){
Animal a = new Dog();
}
}

can we say that class TestIt has-a Animal and class TestIt has-a Dog?

thanks

[Boris Mechkov]

Ash Gill wrote:hi everyone, i have a small doubt. Given the following code:

class Animal {}
class Dog extends Animal {}
class TestIt {
public static void main (String [] args){
Animal a = new Dog();
}
}

can we say that class TestIt has-a Animal and class TestIt has-a Dog?

thanks

No. You cannot say neither. Look closely at the code.

Animal a = new Dog() part is declared and initialized in the method "main()". This is a local reference.
HAS-A is correct when you are dealing with instance reference. Look at this code:

class Animal {}
class Dog extends Animal {}
class TestIt {
Animal a = new Dog(); //now class TestIt HAS-A animal reference which is a Dog
public static void main (String [] args){}
}

Let me know if you have more questions.

[Hama Kamal]

Ash Gill wrote:hi everyone, i have a small doubt. Given the following code:

class Animal {}
class Dog extends Animal {}
class TestIt {
public static void main (String [] args){
Animal a = new Dog();
}
}

can we say that class TestIt has-a Animal and class TestIt has-a Dog?

thanks

Hi Ash,
i think we could say that if it was instance variable , since TestIt has a reference to class Animal so TestIt has-a Animal and since Dog is-a Animal so TestIt has-a Dog as well....please correct me if you think i'm wrong!

[Boris Mechkov]

Hama Kamal wrote:

Ash Gill wrote:hi everyone, i have a small doubt. Given the following code:

class Animal {}
class Dog extends Animal {}
class TestIt {
public static void main (String [] args){
Animal a = new Dog();
}
}

can we say that class TestIt has-a Animal and class TestIt has-a Dog?

thanks

Hi Ash,
i thinak we can say that , since TestIt has a reference to class Animal so TestIt has-a Animal and since Dog is-a Animal so TestIt has-a Dog as well....please correct me if you think i'm wrong!

Hama,
you cannot say that TestIt HAS-A animal, nor a Dog. The reference is declared in the main() method, so HAS-A is not valid here. HAS-A is related to instance variables only (variables declared outside a method declarations, but within the class declaration)

[Ash Gill]

Hi Boris, thanks for the explanation. one more doubt, Does the Has-a relation work in case of static references, like in line ##1 below?

class Animal {}
class Dog extends Animal {}
class TestIt {
static Animal a = new Dog();        ##1 can we still say TestIt has-a Animal.
public static void main (String [] args){}
}

[Hama Kamal]

Boris Mechkov wrote:

Hama Kamal wrote:

Ash Gill wrote:hi everyone, i have a small doubt. Given the following code:

class Animal {}
class Dog extends Animal {}
class TestIt {
public static void main (String [] args){
Animal a = new Dog();
}
}

can we say that class TestIt has-a Animal and class TestIt has-a Dog?

thanks

Hi Ash,
i thinak we can say that , since TestIt has a reference to class Animal so TestIt has-a Animal and since Dog is-a Animal so TestIt has-a Dog as well....please correct me if you think i'm wrong!

Hama,
you cannot say that TestIt HAS-A animal, nor a Dog. The reference is declared in the main() method, so HAS-A is not valid here. HAS-A is related to instance variables only (variables declared outside a method declarations, but within the class declaration)

Hi Boris

you are right i've already corrected my statement in the above post.

[Boris Mechkov]

Hama Kamal wrote:

Boris Mechkov wrote:

Hama Kamal wrote:

Ash Gill wrote:hi everyone, i have a small doubt. Given the following code:

class Animal {}
class Dog extends Animal {}
class TestIt {
public static void main (String [] args){
Animal a = new Dog();
}
}

can we say that class TestIt has-a Animal and class TestIt has-a Dog?

thanks

Hi Ash,
i thinak we can say that , since TestIt has a reference to class Animal so TestIt has-a Animal and since Dog is-a Animal so TestIt has-a Dog as well....please correct me if you think i'm wrong!

Hama,
you cannot say that TestIt HAS-A animal, nor a Dog. The reference is declared in the main() method, so HAS-A is not valid here. HAS-A is related to instance variables only (variables declared outside a method declarations, but within the class declaration)

Hi Boris

you are right i've already corrected my statement in the above post.

No problem friend. ;-)

[Boris Mechkov]

Ash Gill wrote:Hi Boris, thanks for the explanation. one more doubt, Does the Has-a relation work in case of static references, like in line ##1 below?

class Animal {}
class Dog extends Animal {}
class TestIt {
static Animal a = new Dog();        ##1 can we still say TestIt has-a Animal.
public static void main (String [] args){}
}

Yes, it does work as well.

[Ash Gill]

Hama Kamal wrote:

Ash Gill wrote:hi everyone, i have a small doubt. Given the following code:

class Animal {}
class Dog extends Animal {}
class TestIt {
public static void main (String [] args){
Animal a = new Dog();
}
}

can we say that class TestIt has-a Animal and class TestIt has-a Dog?

thanks

Hi Ash,
i think we could say that if it was instance variable , since TestIt has a reference to class Animal so TestIt has-a Animal and since Dog is-a Animal so TestIt has-a Dog as well....please correct me if you think i'm wrong!

hi Hama, thanks. i now understand that if 'a' was an instance variable then TestIt definately Has-A Animal but i m still doubtful about TestIt having a dog.

[Boris Mechkov]

Ash Gill wrote:

Hama Kamal wrote:

Ash Gill wrote:hi everyone, i have a small doubt. Given the following code:

class Animal {}
class Dog extends Animal {}
class TestIt {
public static void main (String [] args){
Animal a = new Dog();
}
}

can we say that class TestIt has-a Animal and class TestIt has-a Dog?

thanks

Hi Ash,
i think we could say that if it was instance variable , since TestIt has a reference to class Animal so TestIt has-a Animal and since Dog is-a Animal so TestIt has-a Dog as well....please correct me if you think i'm wrong!

hi Hama, thanks. i now understand that if 'a' was an instance variable then TestIt definately Has-A Animal but i m still doubtful about TestIt having a dog.

The reference variable type determines what a certain class HAS. So class TestIt HAS-A Animal. Remember, object creation happens at runtime. What if we have an overloaded constructor that assigns a CAT to the animal reference (providing a CAT IS-A Animal)? Then would you say the TestIt class has a CAT as well? Bottom line is TestIt HAS-A Animal reference.

[Hama Kamal]

so do you guys mean that
TestIt has-a Animal true
TestIt has-a Dog false ?

[Boris Mechkov]

Hama Kamal wrote:so do you guys mean that
TestIt has-a Animal true
TestIt has-a Dog false ?

Yes.
TestIt class has only an Animal. This is determined by the reference variable. If we had this:

class test {

Dog g;
Cat c;
Car b = new Porsche();

}

...then we can say that Test HAS-A Dog, a Cat and a Car.

[Hama Kamal]

but if it was like

class Animal {}   
class Dog extends Animal {}   
class TestIt {   
{   
   Dog g = new Dog();  
}   
}  

we could say that

TestIt has-a Dog and TestIt has-a Animal >>> since we can access all the members of Animal class through object reference variable g .... am i right?

[Boris Mechkov]

Hama Kamal wrote:but if it was like

class Animal {}   
class Dog extends Animal {}   
class TestIt {   
{   
   Dog g = new Dog();  
}   
}  

we could say that

TestIt has-a Dog and TestIt has-a Animal >>> since we can access all the members of Animal class through object reference variable g .... am i right?

My understanding is that when we say HAS-A means whether a class HAS an instance reference to another class. So, to answer your question, i would say that testIt HAS-A Dog, but TestIt does not have an Animal. Dog itself IS-A Animal, but the reference that TestIt has is a DOG reference, so TestIt HAS-A Dog.

[Hama Kamal]

Boris Mechkov wrote:

Hama Kamal wrote:but if it was like

class Animal {}   
class Dog extends Animal {}   
class TestIt {   
{   
   Dog g = new Dog();  
}   
}  

we could say that

TestIt has-a Dog and TestIt has-a Animal >>> since we can access all the members of Animal class through object reference variable g .... am i right?

My understanding is that when we say HAS-A means whether a class HAS an instance reference to another class. So, to answer your question, i would say that testIt HAS-A Dog, but TestIt does not have an Animal. Dog itself IS-A Animal, but the reference that TestIt has is a DOG reference, so TestIt HAS-A Dog.

well i don't know really know, let's take someone else's idea about this issue,,,,, but one more thing
are you sure that in the below code

class Animal {}   
class Dog extends Animal {}   
class TestIt {   
public static void main (String [] args){   
Animal a = new Dog();   
}   
}  

TestIt has-a Animal is wrong?! because as i searched some documents i couldn't find that !! it only says that if a class has a rference to another class then it has it ,, it does not say that it shoud be declared outside of the method body,,, my point is are you absolutely sure about that? becasue im not,,, thanks

[Boris Mechkov]

Hama Kamal wrote:

Boris Mechkov wrote:

Hama Kamal wrote:but if it was like

class Animal {}   
class Dog extends Animal {}   
class TestIt {   
{   
   Dog g = new Dog();  
}   
}  

we could say that

TestIt has-a Dog and TestIt has-a Animal >>> since we can access all the members of Animal class through object reference variable g .... am i right?

My understanding is that when we say HAS-A means whether a class HAS an instance reference to another class. So, to answer your question, i would say that testIt HAS-A Dog, but TestIt does not have an Animal. Dog itself IS-A Animal, but the reference that TestIt has is a DOG reference, so TestIt HAS-A Dog.

well i don't know really know, let's take someone else's idea about this issue,,,,, but one more thing
are you sure that in the below code

class Animal {}   
class Dog extends Animal {}   
class TestIt {   
public static void main (String [] args){   
Animal a = new Dog();   
}   
}  

TestIt has-a Animal is wrong?! because as i searched some documents i couldn't find that !! it only says that if a class has a rference to another class then it has it ,, it does not say that it shoud be declared outside of the method body,,, my point is are you absolutely sure about that? becasue im not,,, thanks

In the code above "Animal a" is local variable/reference. After we are done/exit the method "main()" this reference stops existing. The object might still exists, but not the reference. So, we cant really say that TestIt HAS-A Animal in this case.

This is beside the fact that HAS-A concerns only instance variables, NOT local variable. If "Animal a" was instance variable (one declared outside a method declaration but within a class declaration) then class testIt would be correct to say HAS-A Animal.
What book are you using to study? Kathy and Bert's book on SCJP6 is the best out there.

[Hama Kamal]

What book are you using to study? Kathy and Bert's book on SCJP6 is the best out there.

you are right ,, but i didn't see that point in the book,,, Kathy and Bert's book

[Ash Gill]

thanks Boris and Hama, so
Has-A relationship only applies to static or instance variables of a class &
Has-A has to do with the reference type of the object and not the actual object type.

[Hama Kamal]

Ash Gill wrote:thanks Boris and Hama, so
Has-A relationship only applies to static or instance variables of a class &
Has-A has to do with the reference type of the object and not the actual object type.

Well I do agree with Boris's explanations they were all logical but yet i'm not sure about those two points i think we need to confirm them through a java text book,, which states those two points explicitly !!!

Hey folks we need a little help over here ......

[Hemakanta Sethi]

Boris Mechkov wrote:

Hama Kamal wrote:

Ash Gill wrote:hi everyone, i have a small doubt. Given the following code:

class Animal {}
class Dog extends Animal {}
class TestIt {
public static void main (String [] args){
Animal a = new Dog();
}
}

can we say that class TestIt has-a Animal and class TestIt has-a Dog?

thanks

Hi Ash,
i thinak we can say that , since TestIt has a reference to class Animal so TestIt has-a Animal and since Dog is-a Animal so TestIt has-a Dog as well....please correct me if you think i'm wrong!

Hama,
you cannot say that TestIt HAS-A animal, nor a Dog. The reference is declared in the main() method, so HAS-A is not valid here. HAS-A is related to instance variables only (variables declared outside a method declarations, but within the class declaration)

Hemakanta,
In this case Dependency Relation is valid. Because instance variable created inside member of class TestIt.
Dependency Relation can be find in two cases-
1. when instance variable created inside member of a class.
2. When object is passing as a parameter to a method or a constructor.

[Ash Gill]

hi sethi,

In one of the practice exams of ocpjp ( k&b practice exam book) i found the explanation that: for Has-a relationship, the instance of another class has to be declared as a member of the class so that it can be part of the state of the class. so in the question i asked, i can confirm, Has-A doesn't exist.