Java - Delete the last node of the Doubly Linked List

In this method, the last node of the doubly linked list is deleted. For example - if the given list is 10->20->30->40 and the last node is deleted, the list becomes 10->20->30.

Deleting the last node of the Doubly Linked List involves checking the head for empty. If it is not empty, then check the head next for empty. If the head next is empty, then release the head, else traverse to the second last node of the list. Then, link the next of second last node to NULL and delete the last node.

The function pop_back is created for this purpose. It is a 3-step process.

void pop_back() {
  if(this.head != null) {
    
    //1. if head in not null and next of head
    //   is null, release the head
    if(this.head.next == null) {
      this.head = null;
    } else {
      
      //2. Else, traverse to the second last 
      //   element of the list
      Node temp = new Node();
      temp = this.head;
      while(temp.next.next != null)
        temp = temp.next;
      
      //3. Change the next of the second 
      //   last node to null and delete the
      //   last node
      Node lastNode = temp.next;
      temp.next = null; 
      lastNode = null;
    }
  }
}

The below is a complete program that uses above discussed concept of deleting the last node of the doubly linked list.

//node structure
class Node {
    int data;
    Node next;
    Node prev;
};

class LinkedList {
  Node head;

  LinkedList(){
    head = null;
  }

  //Add new element at the end of the list
  void push_back(int newElement) {
    Node newNode = new Node();
    newNode.data = newElement;
    newNode.next = null;
    newNode.prev = null; 
    if(head == null) {
      head = newNode;
    } else {
      Node temp = new Node();
      temp = head;
      while(temp.next != null)
        temp = temp.next;
      temp.next = newNode;
      newNode.prev = temp;
    }    
  }

  //Delete last node of the list
  void pop_back() {
    if(this.head != null) {
      if(this.head.next == null) {
        this.head = null;
      } else {
        Node temp = new Node();
        temp = this.head;
        while(temp.next.next != null)
          temp = temp.next;
        Node lastNode = temp.next;
        temp.next = null; 
        lastNode = null;
      }
    }
  }

  //display the content of the list
  void PrintList() {
    Node temp = new Node();
    temp = this.head;
    if(temp != null) {
      System.out.print("The list contains: ");
      while(temp != null) {
        System.out.print(temp.data + " ");
        temp = temp.next;
      }
      System.out.println();
    } else {
      System.out.println("The list is empty.");
    }
  }    
};

// test the code 
public class Implementation {
  public static void main(String[] args) {
    LinkedList MyList = new LinkedList();

    //Add four elements in the list.
    MyList.push_back(10);
    MyList.push_back(20);
    MyList.push_back(30);
    MyList.push_back(40);
    MyList.PrintList(); 

    //Delete the last node
    MyList.pop_back();
    MyList.PrintList();  
  }
}

The above code will give the following output:

The list contains: 10 20 30 40
The list contains: 10 20 30