Java - Functional Programming (Map, Filter)

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){

        }
      }

What have you tried? Have you learnt about loops? That's what you need to use here. Or recursion. — JB Nizet
– JB Nizet, Commented Dec 29, 2019 at 13:59
You're on the right track. But you only applied the function to the item of the current node. You also need to create copies for all the other nodes, and apply the function to their item.. Your loop condition is wrong, too. It's really all about loops If you feel confused by the apply method, just start by creating copies of the nodes, without changing their item. Then modify the loop to apply the function to each item. — JB Nizet
– JB Nizet, Commented Dec 29, 2019 at 14:12
What exactly are you asking for feedback about? This is unclear to me as you have worded things. — JGFMK
– JGFMK, Commented Dec 29, 2019 at 14:18

BHAWANI SINGH · Accepted Answer · 2019-12-30 14:46:32Z

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;
        }

edited Dec 30, 2019 at 14:46

answered Dec 29, 2019 at 14:24

BHAWANI SINGH

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2 Comments

Mickybo Yakari Over a year ago

Thanks a lot for your help. Apparently, there is a mistake in the code for the filter method. I'll have a look again tomorrow.

Mickybo Yakari Over a year ago

I just went over your code again. The one mistake is that the starting point in the for loop should be 'Node originalNode = this' instead of 'originalNode = this.next'. Thanks for your help!

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