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Question

I need help with a Circular Doubly Linked List in Java.

This is my code (originally coded by "sanfoundry"; it uses interfaces):

LinkedList.java:

public class LinkedList<T extends Comparable<T>> implements
        ILinkedList<T> {

    private ILinkedListNode<T> head;
    private ILinkedListNode<T> end;
    private int size;

    public LinkedList() {
        head = null;
        end = null;
        head = null;
        size = 0;
    }


    @Override
    public void append(T element) { 
        ILinkedListNode<T> tempNode = new LinkedListNode(element, null, null);

        if (head == null) {
            head = tempNode;
            end = head;


        } else {    
            tempNode.setPrev(end);
            tempNode.setNext(tempNode);
            end = tempNode;
        }
        size++;

    }
// should return element at position "index"
    @Override
    public T get(int index) {
        return null;
    }

    @Override
    public int size() {
        return size;
    }

    @Override
    public ILinkedListNode<T> getHead() {
        return head;
    }


}

Now I need help to get it working. Did I do something wrong and what do I have to code in method "public T get (int index)"? Sorry, but I'm a Java noob :(

EDIT: Is this a possible solution?

public T get(int index) {

T element = null;

if (index == 0) {
    element = head.getElement();

} else if (index == size()-1) {
    element = head.getPrev().getElement(); // end.getElement() also possible

} else {
    ILinkedListNode<T> temp = head;
    for (int i = 0; i < index; i++) {
        temp = temp.getNext();
    }

    element = temp.getElement();

}
return element;

}

Answer 1

You should traverse the LinkedList, keeping track of your current position as you go. When your current position is equal to the index passed in, then you can return the T from that node.

Read about traversing a linked list here .

Answer 2

Try making some test cases. Ideally you'll want to use a real test framework but using a normal main method could work. For example:

public static void main(String[] args) {
    ILinkedList<String> a = new LinkedList<String>();

    System.out.println(a.size()); // 0
    System.out.println(a.getHead()); // null

    a.append("foo");
    System.out.println(a.size()); // 1
    System.out.println(a.get(0)); // "foo"
    System.out.println(a.get(1)); // decide yourself what this should result in

    a.append("bar");
    System.out.println(a.size()); // 2
    System.out.println(a.get(0)); // "foo"
    System.out.println(a.get(1)); // "bar"

    a.append("baz");
    System.out.println(a.size()); // 3
    System.out.println(a.get(0)); // "foo"
    System.out.println(a.get(1)); // "bar"
    System.out.println(a.get(2)); // "baz"

}

Expand the test as necessary. See if the code returns what you expect it to, or if the code never returns, or throws an exception, etc.... The easiest way to check whether your code is running properly is, after all, to actually run it.

Hint: the code, as of this writing, has some errors.

Also, if the code can run as expected, consider:

• Traversing the nodes backward if it's faster than forward.
• Using a recursion instead of iteration.