Set tail=null when head=null in singly linked list in java?

Question

In java singly LinkedList how do I write code that nulls the tail(tail=null) when I null the head(head=null)? I am a beginner and hence find it difficult. I have included if(head==null)in every method but also want to implement code that sets tail=null when head=null so as to avoid a bug. Here is my code

public class SinglyLinkedList{
  public Node head;
  public Node tail;
  public int size;


  public Node createLL(int num){
    Node node=new Node();
    node.value=num;
    node.next=null;
    head=node;
    tail=node;

    size=1;
    return head;
  }

  public void insertNode(int num,int location){
    Node node=new Node();
    node.value=num;
    
    if(head==null){
      createLL(num);
      return;
    }

    if(location==0){
      node.next=head;
      head=node;
    }

    else if(location>=size){
      node.next=null;
      tail.next=node;
      tail=node;
    }

    else{
      Node tempNode=head;
      int index=0;

      while(index<location-1){
        tempNode=tempNode.next;
        index++;
      }
     node.next=tempNode.next;
     tempNode.next=node;
    }
    size++;
  }

  public void traverse(){
    if(head==null){
      System.out.println("The linked list is empty");
    }
    Node tempNode=head;
    for(int i=0;i<size;i++){
      System.out.print(tempNode.value);
      if(i!=size-1){
        System.out.print("->");
      }
      tempNode=tempNode.next;
    }
    System.out.println();
  }

  public boolean searchElement(int num){
    Node tempNode=head;
    for(int i=0;i<size;i++){
      if(tempNode.value==num){
        System.out.println("The value is present at index:"+i);
        return true;
      }
      tempNode=tempNode.next;
    }
    System.out.println("The value is not present");
    return false;
  }

  public void deleteNode(int location){
    if(head==null){
      System.out.println("The linked list is not present");
      return;
    }

    else if(location==0){
      head=head.next;
      size--;
      if(size==0){
        tail=null;
      }
    }

    else if(location>=size){
      Node tempNode=head;
      for(int i=0;i<size-1;i++){
        tempNode=tempNode.next;
      }
      if(head==null){
        tail=null;
        size--;
        return;
      }
      tempNode.next=null;
      tail=tempNode;
      size--;
      
      head=tail=null;
    }

    else{
      Node tempNode=head;
      int index=0;

      while(index<location-1){
        tempNode=tempNode.next;
        index++;
      }
      tempNode.next=tempNode.next.next;
      size--;
    }
  }

  public void deleteSinglyLinkedList(){
    if(head==null){
      System.out.println("The linkedlist is absent");
    }
    head=tail=null;
    System.out.println("The entire linked list has been deleted");
  }
}

if(head == null) tail = null; or(equvalent): if(head == null) tail = head;;) — xerx593
– xerx593, Commented Jan 9, 2022 at 21:42
Where should I place this code in my program in order for it work on all operations like insertion, deletion ,traversal and search? — 2019079925
– 2019079925, Commented Jan 9, 2022 at 21:47
Please add more details to your question! Please also have a look at minimal reproducible example and how-to-ask — xerx593
– xerx593, Commented Jan 9, 2022 at 21:48
Like I wrote in my answer to your previous question, you should better add a method to your class that takes care of the clearing, and not let the main code change the value of head directly. — trincot
– trincot, Commented Jan 10, 2022 at 7:09

Wadite RK · Accepted Answer · 2022-01-09 22:39:43Z

After a few comments, edits and going over your code, I would like you to look into your deleteNode() method. You might have encountered problems since you check for head==null before the actual removal of a node, I commented on problematic lines:

public void deleteNode(int location){
    if(head==null){
      System.out.println("The linked list is not present");
      return;
    }
    else if(location==0){
      head=head.next;
      size--;
      if(size==0){
        tail=null;
      }
    }
    else if(location>=size){
      Node tempNode=head;
      for(int i=0;i<size-1;i++){
        tempNode=tempNode.next;
      }
      if(head==null){ // This is a good check, however -
        tail=null;    // it should be written after the removal line.
        size--;       // Also, this check shouldn't reduce size any further.
        return;       // <- this return also stops the method from actually removing a node.
      }
      tempNode.next=null; // this is the actual removal line.
      tail=tempNode;
      size--;
      
      head=tail=null;    // potentialy problematic line - 
    }                    // seems to set both head and tail to null, why?

    else{
      Node tempNode=head;
      int index=0;

      while(index<location-1){
        tempNode=tempNode.next;
        index++;
      }
      tempNode.next=tempNode.next.next;
      size--;
    }
  }

If you would move the if (head.. to work after ```tempNode.next = null``, you shouldn't encounter times where the head is null and the tail is not.

Here is how the if (location>=size) should look after changes:

    else if(location>=size){
      Node tempNode=head;
      for(int i=0;i<size-1;i++){
        tempNode=tempNode.next;
      }

      tempNode.next=null; 
      tail=tempNode;
      size--;

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

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