Linked List search method - linked-list

Question

I am practicing build a doubly linked list contains string value.
In find method, I have a NullPointer Exception
here is my code.
package LinkedList;
package LinkedList;
public class LinkedList {
// 노드 클래스
class Node {
String value;
Node prev;
Node next;
Node(String v, Node p, Node s) {
value = v;
prev = p;
next = s;
}
public String getValue() {
return value;
}
public Node getPrev() {
return prev;
}
public Node getNext() {
return next;
}
public void setPrev(Node p) {
prev = p;
}
public void setNext(Node n) {
next = n;
}
}
Node head;
Node tail;
int size = 0;
public LinkedList() {
head = new Node(null, null, null);
tail = new Node(null, head, null);
head.setNext(tail);
}
public int size() {
return size;
}
public boolean isEmpty() {
return size == 0;
}
public String first() {
if (isEmpty()) {
return null;
}
return head.getNext().getValue();
}
public String last() {
if (isEmpty()) {
return null;
}
return tail.getPrev().getValue();
}
public void addFirst(String value) {
addBetween(value, head, head.getNext());
}
public void addLast(String value) {
addBetween(value, tail.getPrev(), tail);
}
public void addBetween(String v, Node p, Node s) {
Node newNode = new Node(v, p, s);
p.setNext(newNode);
s.setPrev(newNode);
size++;
}
public String remove(Node node) {
Node p = node.getPrev();
Node s = node.getNext();
p.setNext(s);
s.setPrev(p);
size--;
return node.getValue();
}
public String removeFirst() {
if (isEmpty()) {
return null;
}
return remove(head.getNext());
}
public String removeLast() {
if (isEmpty()) {
return null;
}
return remove(tail.getPrev());
}
public void insert(String value) {
Node current = head;
// first
if (isEmpty()) {
addFirst(value);
} else {
// check
while (current.getNext() != tail || current.getValue().compareTo(value) > 0) {
current = current.getNext();
}
// last
if (current.getNext() == tail) {
addLast(value);
} else // between
{
addBetween(value, current.getNext(), current);
}
}
}
/* !!!!!!!!!!!!!! ERORR !!!!!!!!!!!! */
public void find(String value) {
Node current = head.getNext();
while ((current != null) || !(current.getValue().equals(value)))
current = current.getNext();
if (current.getValue().equals(value)) {
System.out.println("found " + value);
} else {
System.out.println("Not found " + value);
}
}
// • Traverse the list forwards and print
// 순회
public void fowardTraverse() {
Node current = head.getNext();
System.out.print(current.getValue());
while (current.getNext() != tail) {
current = current.getNext();
System.out.print(" -> " + current.getValue());
}
}
// • Traverse the list backwards and print
// 뒤로 순회
public void backwardTraverse() {
Node current = tail.getPrev();
System.out.print(current.getValue());
while (current.getPrev() != head) {
current = current.getPrev();
System.out.print(" -> " + current.getValue());
}
}
// • Delete a node from the list
// 지우기
public String delete(String value) {
return value;
}
// • Delete/destroy the list
// 파괴하기
public void destroy() {
}
public static void main(String[] args) {
// TODO Auto-generated method stubs
LinkedList a = new LinkedList();
a.insert("a");
a.insert("b");
a.insert("c");
a.insert("d");
a.insert("e");
a.insert("f");
a.insert("g");
a.insert("h");
a.insert("i");
// a.fowardTraverse();
a.find("a");
}
I don't understand why I get a nullpointException at the line,
It suppose to put a node contains a.

Answer 1

Make sure you check for Non-NULL before dereferencing:
Node current = head.getNext();
and
if (current.getValue().equals(value)) {
to be replaced by
Node current;
if(head != NULL) current = head.getNext();
and
if (current != NULL && current.getValue().equals(value)) {

Answer 2

Because your Head is empty...(No pun intended)
before Addfirst calling..your structure:
head=[null,null,tail],tail=[null,head,null];
you are sending ("a",head,tail)
and your storing it in new node making it a structure like:
head=[null,null,newNode]==>newNode["a",head,tail]==>tail[null,newNode,null]
So search will compare null to a (in find) giving you the error .....
---Edit 1:
#JanghyupLee, My Bad, Didn't do a closer look on find method...however , I found that in condition of "if" you are using condition
current != null || ......
After first line that is ( current=head.next)..current becomes not null..
which is causing the condition in while to ignore the right part of '||' (short circuit) until current has a value of null...
Once the current becomes null then it goes for the next statement to check for value..causing null pointer exception