Implementing Circular Linked List in java

Question

I'm having a bit of an issue with implementing my circularly linked list. I'm working on a problem that requires you to implement any ADT yourself. I seem to be okay with adding nodes to the list, however I'm in unfamiliar territory when it comes to removing. I included the first two remove methods to give you an idea of where my head is at, how would I go about removing the last node in the list?

public class LinkedList {
    private Node head;
    private Node tail;
    private int size = 0;
    LinkedList() {
        head = null;
        current = null;
        previous = null;
        tail = null;
        size = 0;
    }

    //checks if list is empty
    public boolean isEmpty() {
        return head == null;
    }
    //add new node to front of circularly linked list
    public void addToFront(E x) {
        if (head == null) {
            head = new Node(x);
        } else {
            Node n = new Node(x);
            x.next() = head;
            head = x;
        }
    }

    public void addtoMiddle(E x) {
        x.next = current.next();
        current.next = x;
        size = size + 1;
    }

    public void addToEnd(E x) {
        x.next = null;
        tail.next() = x;
        tail = x;
        size = size + 1;
    }

    public void removeFirst(E x) {
        if (head = null) {
            System.out.println("Error! List is empty!");
        } else {
            head = current.next();
            size = size + 1;
        }
    }

    public void removeMiddle(E x) {
        previous.next() = current.next();
        current.next() = null;
        size = size + 1;
    }

Answer 1

I know this is not directly an answer to your question but I feel like Thomas has given the needed explanation here.

Since you have a lot of syntax or incompleteness errors in the example I would recommend to comment out all functions so it has no more errors. Then comment them in again one by one correcting every error.

Some advices:

You use class members which are not defined e.g. current, previous.

Decide if next should be a member or function.

You need to define a class for Node (containing its members and functions), like you did for LinkedList.

Answer 2

In a circular linked list the last node's next points to the head, so you loop through your nodes until node.next.equals( head ). Note that next must never be null and if you have only one node then you have head.next = head.

In a circular doubly linked list you also have a previous node, i.e. you can iterate backwards. In that case your last node is just head.previous.

A small ascii picture:

head -next---> node -next---> node -next---> last
 | ^  <---prev-      <---prev-      <---prev- | ^
 | |                                          | |
 | |_____________________________________next_| |
 |_prev_________________________________________|