Why does this compile?

I was taken aback earlier today when debugging some code to find that something like the following does not throw a compile-time exception:

 public Test () { 
     HashMap map = (HashMap) getList(); 
 }

 private List getList(){
     return new ArrayList();
 }

As you can imagine, a ClassCastException is thrown at runtime, but can someone explain why the casting of a List to a HashMap is considered legal at compile time?