— Scott McNealy, Sun Microsystems Chairman, President and ceo

ArrayList dogs = new ArrayList();
dogs.add(new Dog());
dogs.add(new Dog());
 takeAnimals(dogs); 
}
public void 
takeAnimals(ArrayList animals)
{
for(Animal a: animals) {
a.eat();
}
}
We know this line worked fine.
Make a Dog ArrayList and put a couple dogs in.
Passing in just ArrayList
%java TestGenerics3
TestGenerics3.java:21: takeAnimals(java.util.
ArrayList) in TestGenerics3 cannot be applied to 
(java.util.ArrayList)
takeAnimals(dogs);
^
1 error
File Edit Window Help CatsAreSmarter
When we compile it:
It looked so right, 
but went so wrong...
Will this work now that we changed 
from an array to an ArrayList?
polymorphism and 
generics

---

collections 
with 
generics
you are here
4
571
And I’m supposed to be OK with this? That 
totally screws my animal simulation where the 
veterinary program takes a list of any type of 
animal, so that a dog kennel can send a list of dogs, 
and a cat kennel can send a list of cats... now 
you’re saying I can’t do that if I use collections 
instead of arrays?
What could happen if it 
were allowed...
Imagine the compiler let you get away with that. It let you pass an 
ArrayList to a method declared as:
public void takeAnimals(
ArrayList
animals) {
for(Animal a: animals) {
a.eat();
}
}
There’s nothing in that method that looks harmful, right? After all, 
the whole point of polymorphism is that anything an Animal can 
do (in this case, the eat() method), a Dog can do as well. So what’s 
the problem with having the method call eat() on each of the Dog 
references? 
Nothing. Nothing at all.
There’s nothing wrong with that code. But imagine this code instead:
public void takeAnimals(