What is a first class citizen function?

Question

What is a first class citizen function?

Does Java supports first class citizen function?

Edit:
As mention on Wikepedia

F

Answer 1

A first class function can be passed around. A typical example is the map function. Here is an example in Scala that squares the elements of a list:

val square = (x:Int) => x*x

val squaredList = List(1,2,3,4).map(square _)
//--> List(1,4,9,16)

The square function is here an argument to the map method, which applies it to every element. If you want to do something like this in Java, you have to use a method wrapped in a class, something like this:

interface F{ B apply(A a); }

static  List map(List list, F f) {
  List result = new ArrayList();
  for(A a:list) result.add(f.apply(a));
  return result;   
}

//we have to "wrap" the squaring operation in a class in order to make it a function
F square = new F(){ 
  Integer apply(Integer a) { return a*a; }
}

List ints = Arrays.asList(1,2,3,4);
List squares = map(ints, square);

Looking at this you can see that you can get the same task somehow done in Java, but with more overhead, and without "native" support by the language, but by using a workaround (wrapper classes). So Java doesn't support first class functions, but can "simulate" them.

Hopefully Java 8 will support first class functions. If you want to have some support for this now, look at http://functionaljava.org/ or http://functionalj.sourceforge.net/ , or have a look at the Scala language.