— Scott McNealy, Sun Microsystems Chairman, President and ceo

generic 
types

---

collections 
with 
generics
you are here
4
541
1
Creating instances of generified 
classes (like ArrayList)
When you make an ArrayList, you have to tell it the type 
of objects you’ll allow in the list, just as you do with plain 
old arrays.
3
Declaring (and invoking) 
methods that take generic types
If you have a method that takes as a parameter, say, an 
ArrayList of Animal objects, what does that really mean? 
Can you also pass it an ArrayList of Dog objects? We’ll 
look at some subtle and tricky polymorphism issues that 
are very different from the way you write methods that 
take plain old arrays.
(This is actually the same point as #2, but that shows you 
how important we think it is.)
2
Declaring and assigning 
variables of generic types
How does polymorphism really work with generic 
types? If you have an ArrayList reference 
variable, can you assign an ArrayList to it? What 
about a List reference? Can you assign an 
ArrayList to it? You’ll see...
Learning generics
Of the dozens of things you could learn about generics, there are 
really only three that matter to most programmers:
new ArrayList
()
List
 
songList = 
new ArrayList
()
void foo(List
 
list)
x.foo(songList)
Q: 
But don’t I also need to learn how to create my OWN generic 
classes? What if I want to make a class type that lets people