— Scott McNealy, Sun Microsystems Chairman, President and ceo

generic 
types
Here’s how it works with regular 
arrays:

---

collections 
with 
generics
you are here
4
569
Using polymorphic arguments and generics
So we saw how the whole thing worked with arrays, but will it work 
the same way when we switch from an array to an ArrayList? Sounds 
reasonable, doesn’t it?
First, let’s try it with only the Animal ArrayList. We made just a few 
changes to the go() method:
public void go() {
ArrayList animals = new ArrayList();
animals.add(new Dog());
animals.add(new Cat());
animals.add(new Dog());
takeAnimals(animals);
}
public void 
takeAnimals(ArrayList animals)
{
for(Animal a: animals) {
a.eat();
}
}
A simple change from Animal[] to 
ArrayList. 
We have to add one at a time since there’s no 
shortcut syntax like there is for array creation.
This is the same code, except now the “animals” 
variable refers to an ArrayList instead of array.
The method now takes an ArrayList 
instead of an array, but everything else is 
the same. Remember, that for loop syntax 
works for both arrays and collections.
Passing in just ArrayList
%java TestGenerics2
animal eating
animal eating
animal eating
animal eating
animal eating
animal eating
File Edit Window Help CatFoodIsBetter
Compiles and runs just fine

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570
 
chapter 16
But will it work with ArrayList ?
Because of polymorphism, the compiler let us pass a Dog array 
to a method with an Animal array argument. No problem. And 
an ArrayList can be passed to a method with an 
ArrayList argument. So the big question is, will the 
ArrayList argument accept an ArrayList? If it works 
with arrays, shouldn’t it work here too?
public void go() {
ArrayList animals = new ArrayList()
;
animals.add(new Dog());
animals.add(new Cat());
animals.add(new Dog());
takeAnimals(animals);