linked list get first &last element?

Question

my program is about doubly linked list and I want to add two method one to get first element and another one to get last element ,these are my codes but when I add any code two this method it give me an error

package doublelinkeddd;

import java.util.NoSuchElementException;

public class DoublyLinkedListImpl<E> {

private Node head;

private Node tail;

private int size;

public DoublyLinkedListImpl() {

size = 0;

}

private class Node {

E element;

Node next;

Node prev;

public Node(E element, Node next, Node prev) {

this.element = element;

this.next = next;

this.prev = prev;

}

public boolean isEmpty() { return size == 0; }

public E first(){

}

public E last(){

}

public void addFirst(E element) {

Node tmp = new Node(element, head, null);

if(head != null ) {head.prev = tmp;}

head = tmp;

if(tail == null) { tail = tmp;}

size++;

System.out.println("adding: "+element);

}

public static void main(String a[]){

DoublyLinkedListImpl<Integer> dll = new DoublyLinkedListImpl<Integer>();

dll.addFirst(10);

dll.addFirst(34);

}

husoski · Answer

That compiles just fine, except for the missing return statements in the first() and last() methods.

The only trick in coding those two is that you need to return a reference to an E type object.  If the list is empty, you'll either return null or throw an exception.  The first() method might look like:

E first() {
.... if (head==null) return null;
.... return head.element;
}

Change the "return null;" to an appropriate throw statement if you need an exception instead of a null result.  The last() method is nearly the same.

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linked list get first &last element?

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