how to delete/search double linked list

[Thomaat1]

Hello everyone!

I'm trying to create a double linked list with functions like delete last, delete first, add first, add last.

However the search and delete any does not work. I've tried a few different methods which are posted below in the code (as comments).

Any help would be greatly appreciated! :)

public class DoublyLinkedList {
  private static Node first; 

  private static Node last; 
  

  public DoublyLinkedList() {
    first = null; 
    last = null;
  }

  public boolean isEmpty(){
    return first == null;
  }

  public void addToFront(String dd){
    Node newNode = new Node(dd); 

    if (isEmpty()) 
      last = newNode; 
    else
      first.previous = newNode; 
    newNode.next = first; 
    first = newNode; 
    
  }

  public void addToEnd(String dd){
    Node newNode = new Node(dd); 
    if (isEmpty()) 
      first = newNode; 
    else {
      last.next = newNode; 
      newNode.previous = last; 
    }
    last = newNode; 
   
  }

  public Node deleteFirst(){ 
    Node temp = first;
    if (first.next == null) 
      last = null; 
    else
      first.next.previous = null;
    first = first.next; 
    
    return temp;
    
  }

  public Node deleteLast(){ 
    Node temp = last;
    if (first.next == null)
      first = null; 
    else
      last.previous.next = null;
    last = last.previous; 
    
    return temp;
  
  }
  

 /* public static Node search (Node first, String item){
        for(int i= 0; i<10;i++){
            if (first.next.dData=="26"){
                System.out.println("blah");
               return first;}
               first=first.next;
            }
            return null;
        }
  /*public static Node delete(Node first, String data) {
       if (first.dData == data)
            return first.next;
            
       Node previous = null;
       Node current = first;
       System.out.println("1");
       
       while (current != null) {
            if (current.dData == "26") {
                    previous.next = current.next;
                     System.out.println("1");
                    return first;
                }
            else {
                    previous = current;
                    current = current.next;
                }
            }
             
            return first;
        }*/
  
  /*public Node deleteKey(String key){
    Node current = first; 
    while (current.dData != key)
    {
      current = current.next;
      if (current == null)
        return null; // cannot find it
    }
    if (current == first) // found it; first item?
      first = current.next; 
    else
      current.previous.next = current.next;

    if (current == last) // last item?
      last = current.previous; 
    else
      // not last
      current.next.previous = current.previous;
    return current; // return value
  }*/

  public void print() {
    System.out.print("List (first to last): ");
    Node current = first; // start at beginning
    while (current != null) // until end of list,
    {
      current.displayNode();
      current = current.next; // move to next Node
    }
    System.out.println("");
  }

  public void printFromLast() 
  {
    System.out.print("List : ");
    Node current = last;
    while (current != null){
      current.displayNode();
      current = current.previous;
    }
    System.out.println("");
  }

  public static int count(Node x)
  {   // count number of elements in list
        // when we start counting at Node x
        if (x==null) return 0;
        return 1 + count(x.next);
  }
 
  
  
  public static void main(String[] args) {
    DoublyLinkedList theList = new DoublyLinkedList();

    int b = 22;
    int c = 44;
    for(int i = 0; i < 6; i++)
    {
    b = i+22;
    c = i+44;
    String bString=Integer.toString(b);
    String cString=Integer.toString(c);
    theList.addToFront(bString);
    theList.addToEnd(cString);
}
    theList.print();
    theList.printFromLast();

    System.out.println("The length of the list before deleting = " + count(first));
    
    theList.deleteFirst();
    theList.deleteLast();
  //  theList.deleteKey("25");

   //delete(first, "26");
    
    Node found = search(first, "23");
    
   // System.out.println(found.dData);
    
    theList.print();

    theList.print();
   
   System.out.println("The length of the list = " + count(first));
   
  }

}

class Node {
  public String dData; // data item

  public Node next; // next Node in list

  public Node previous; // previous Node in list

  public Node(String d)
  {
    dData = d;
  }

  public void displayNode(){
    System.out.print(dData + " ");
  }

}

[Petr]

Sorry But where do you store a data from this classes?

[j2me64]

Sorry But where do you store a data from this classes?

inside the class Node is a field named dData where you can store a string.

@Toomat1: use first.next.dData.equals("26") inside the search method and not ==

[Petr]

No I mean other. I mean follows. Topic starter say that he want to create some classes, which implement a linked list.
The Linked list this list, where previous node links on next node. This structure does not allow get some node for index.
for example
3 -> 5 -> 0 -> 1
The same way you need create class Node, which contain value and links on previous and next node and class, which store whole elements.
So I don't understand where store whole elements.
Can anyone explain where store whole elements in post above code?

[Thomaat1]

Thanks alot for the replies! with the comment of jw2me64 and just slowly checking every line I figured out how to solve delete + search methods.

Petr, you mean I can't for example search for a location nr '1' for example or? I cannot for example delete nr '50' in the list of 1 to 100 right? I probably need to add a second variable to the Node class which keeps track of its position in the list. I think that's not too hard, but in this case not required.

[Petr]

Not I mean other, how you know Java has some collections, which implement interface List. I mean about ArrayList and LinkedList.
Let's imagine follows situation, you need add a node in center a list.
If you use ArrayList you need create new ArrayList for it. but if you use LinkedList you just point that needs node will links on other node it all.