Recursive sum function for a linked list in java

Question

I want to sum up all the values of a linked list recursively but it doesn't work. It says:

Cannot invoke "Element.sum()" because the return value of "Element.getNext()" is null

public class Element{
    private int value;
    private Element next;
}

public class MyList{
    private Element elements;
    public int sum(){
        if (elements == null) return 0;
            return elements.getValue() + elements.getNext().sum();
        }
    }
}

Answer 1

Here's a solution:

public class MyList{

    private Element elements;
    
    class Element{
        private int value;
        private Element next;
        Element(int value) {
            this.value = value;
        }
        public Element getNext() {
            return next;
        }
        public int getValue() {
            return value;
        }
        public void setNext(Element next) {
            this.next = next;
        }
        public int sum() {
            if (next == null) {
                return value;
            } else {            
                return value + next.sum();
            }
        }
    }

    public MyList(int data[]) {
        Element prev = null;
        for (int value : data) {
            Element e = new Element(value);
            if (prev == null) {
                elements = e;
            } else {
                prev.setNext(e);
            }
            prev = e;
        }
    }
    
    public int sum() {
        return elements == null ? 0 : elements.sum();
    }
    
    public static void main(String args[]) {
        MyList list = new MyList(new int[]{ 1, 2, 3});
        System.out.printf("sum = %d\n", list.sum());
    }
}