Java - 函数式编程（映射、过滤器）

Question

I am just getting started with functional programming in Java. I'd like some help with a simple exercise to get up to speed.

Suppose one has the following two interfaces:

     interface P {
       boolean filter(int v);
     }

     interface F {
      int apply(int v);
    }

and one is required to create a map function that takes a function f as an argument and returns a Node that applies f to all elements. Secondly, one is required to create a filter function that returns a Node with all elements that match a predicate p within the Following class:

    public class Node {
         private int item;
         private Node next;

        public Node(int item, Node next){
               this.item = item;
               this.next = next;
        }
       /* Create a new Node that applies function f to all elements */
          public Node map(F f){

         }
     /* Creates a new Node with all elements that match predicate p */
         public Node filter(P p){

        }
      }

Answer 1

        public Node map(F f){
            Node start = new Node(f.apply(item), null);
            Node current = start;

            for(Node originalNode = this.next; originalNode != null; originalNode = originalNode.next) {

                Node copyOfNextNode =new Node(f.apply(originalNode.item), null);
                current.next = copyOfNextNode;
                current = current.next;
            }
            return start;
        }

        /* Creates a new Node with all elements that match predicate p */
        public Node filter(P p){
            Node start = null;
            Node current = null;

            for(Node originalNode = this; originalNode != null; originalNode = originalNode.next) {

                if(p.filter(originalNode.item)) {
                    Node copyOfNextNode =new Node(originalNode.item, null);
                    if(current == null) {
                        current = copyOfNextNode;
                        start = current;
                    } else {
                        current.next = copyOfNextNode;
                        current = current.next;
                    }
                }
            }
            return start;
        }