— Scott McNealy, Sun Microsystems Chairman, President and ceo

ArrayList
animals) {
animals.add(new Cat());
}
Yikes!! We just stuck a Cat in what 
might be a Dogs-only ArrayList.
So that’s the problem. There’s certainly nothing wrong with adding a
Cat to an ArrayList, and that’s the whole point of having an 
ArrayList of a supertype like Animal—so that you can put all types of 
animals in a single Animal ArrayList. 
But if you passed a Dog ArrayList—one meant to hold ONLY Dogs—
to this method that takes an Animal ArrayList, then suddenly you’d 
end up with a Cat in the Dog list. The compiler knows that if it lets 
you pass a Dog ArrayList into the method like that, someone could, 
at runtime, add a Cat to your Dog list. So instead, the compiler just 
won’t let you take the risk. 
If you declare a method to take ArrayList it can take ONLY an 
ArrayList, not ArrayList or ArrayList.

---

572
 
chapter 16
Wait a minute... if this is why they won’t let 
you pass a Dog ArrayList into a method that 
takes an Animal ArrayList—to stop you from 
possibly putting a Cat in what was actually a Dog list, 
then why does it work with 
arrays? Don’t you have 
the same problem with arrays? Can’t you still add 
a Cat object to a Dog[] ?
Array types are checked again at 
runtime, but collection type checks 
happen only when you compile
Let’s say you do add a Cat to an array declared as Dog[] (an array that 
was passed into a method argument declared as Animal[], which is a 
perfectly legal assignment for arrays).
public void go() {
Dog[] dogs = {new Dog(), new Dog(), new Dog()};
takeAnimals(dogs);
}
public void takeAnimals(Animal[] animals) {