Abstract classes, Interfaces, and concrete classes

Question

I have another technical interview tomorrow and somehow I keep getting asked the same question and I feel my answer is not really up to par. The question is:
"What is the advantage of subclassing an abstract class versus concrete class?"
"What is the difference of using an interface versus an abstract class, which is better to use?"
For the first question, I usually answer performance is the advantage because you don't have to instantiate the class.
For the second question, I usually say that you can put implementation in an abstract class and you can't in an interface. I really can't answer the second part to this question.
Any ideas?

Answer 1

For the first question, I usually answer performance
is the advantage because you don't have to instantiate
the class. Try invoking the class B in the following somewhere in another class.
abstract class A{
   A(){
      System.out.println("abstract instantiated");
class B extends A{
  B(){super();}
}