Comment by vbezhenar

Class is a set of functions. Closure is one function.

In old Java, it really was a class. In new Java, I'm not 100% sure anymore, but with verbose syntax it'll be an class. I made it as verbose as possible:

  Function<Integer, Integer> adder(int x) {
    class Adder implements Function<Integer, Integer> {
      int x1;
      @Override Integer apply(Integer y) {
        return x1 + y;
      }
    }

    Adder adder = new Adder();
    adder.x1 = x;
    return adder;
  }

and a bit less verbose with modern Java:

  Function<Integer, Integer> adder(int x) {
    return (y) -> x + y;
  }

So closure could be definitely be considered as a very simple class.