The Java implementation for generics is radically different from the C# equivalent; I won't reiterate issues that have been thoroughly discussed before, but suffice to say that Java generics are implemented as a backwards-compatible compiler extension that works on unmodified VMs.

The implications of this are considerable, and I'd like to present one of them. Lets fast forward a bit and consider a relatively new language feature in Java (introduced, I believe, with J2SE 5.0): autoboxing. A thoroughly overdue language feature, autoboxing allows the seamless transition from regular value types (e.g. the ubiquitous int) to object references (Integer); before autoboxing you couldn't simply add a value to an untyped ArrayList, you had to box (wrap) it in a reference type:

ArrayList list = new ArrayList();
list.add( 3 );			// Compile-time error
list.add( new Integer( 3 ) );	// OK

Eventually Java caught up with C# (which introduced autoboxing in 2002), and with a modern compiler the above code would be valid.

With the introductions out of the way, here's a pop-quiz: what does the following code print?

HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
map.put( 4, 2 );
short n = 4;
System.out.println( Integer.toString( map.get( n ) ) );

As a long-time C# programmer I was completely befuddled when the code resulted in a NullPointerException. Huh? Exception? What? Why?

It took me a while to figure it out: because Java generics are compile-time constructs and are not directly supported by the VM, what actually happens is that the underlying container class accepts regular Object instances (reference types); the compile-time check merely asserts that n can be promoted from short to int, whereas the actual object passed to the container class (via autoboxing) is a Short! Since the container doesn't doesn't actually have a runtime generic type per se, the collection merely looks up the reference object in the map, fails to find it (I guess the Object.hashCode implementation for value types simply returns the reference value as the hash code as in C#) and returns null. Doh! *slaps forehead*