Generics

[Anit Nair]

Following is the piece of code from K&B 1.5 pg 587

 
class AnimalsDoctorGeneric
{
public void checkAnimals(ArrayList<Animal> animals)
{
}
 
public static void main(String [] args)
{
List<Dog> dogs =  new ArrayList<Dog>();
AnimalsDoctorGeneric doc = new AnimalsDoctorGeneric();
doc.checkAnimals(dogs);//send a List Dog
}
}

Besideds above code theres a parent class Animal and Dog class extends Animal.
Now compiling above code gives error

    
 
checkAnimals(java.util.ArrayList<Animal> :wink:  in AnimalsDoctorGeneric
ics cannot be applied to (java.util.List<Animal> :wink: 
                doc.checkAnimals(dog);

The explanation given for error states that ArrayList of subtype cannot be assigned to ArrayList of supertype, rightly so. But I feel that even if the
checkAll method was modified as below

    
public void checkAnimals(ArrayList<Dog> animals)
{
}

It would still give compilation error just because we cannot pass ArrayList reference to a List reference regardless of the generic type used. Please correct me if i am wrong.

[Deepak Bala]

Did you try to compile it ?

[Anit Nair]

yes i did, try out a similar example.I think regardless of the generic type passing a List reference to Arraylist reference would give compilation errors. Let me know if you any other thoughts on this.

[Ram Manoj]

There is an explanation following the example, of why it throws errors in the K&B book.

After reading it, you will find

public void checkAnimals(ArrayList<Animal> animals)

should be used as

public void checkAnimals(ArrayList<? extends Animal> animals)