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 actually an interface and therefore can not be instantiated. So in order to use list, you must create an object of its implementation, for example: ArrayList. Read http://docs.oracle.com/javase/1.4.2/docs/api/java/util/List.html for more information