Why does this compile?

Question

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?

Answer 1

Because conceivably getList() could be returning a subclass of HashMap which also implements List. Unlikely, yes, but possible, and therefore compilable.

Answer 2

For one thing List is an interface. There is no reason why there couldn't exist a subclass of HashMap which also implements the List interface. In this situation it would be perfectly valid.