What does this statement with List and ArrayList mean?

Question

I am reading the Drools Planner examples and I came across code like this a lot:

List columnList = new ArrayList(n);

Answer 1

List is an interface and cannot be instantiated, so new List will trigger a compile-time error. You can do ArrayList columnList = new ArrayList(n);, but this way you can't easily switch between different List implementations(ArrayList is one of them).