如果我们检查 head==null 为什么不在 Java 链表中检查 tail==null 呢？

Question

I am a beginner in Java and currently completing a Udemy course on DSA. I am learning linked lists and am working on methods to insert and delete nodes to and from linked lists respectively.

From what I have learnt so far I know that we use condition head==null to check if the linked list is empty or not.

If the condition head==null is true then LinkedList is empty else it is not empty.

However, shouldn't we check whether tail==null as well because the tail will always refer to the last node in the LinkedList even after we make head==null ?

Here is my code:

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

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

    size=1;
    return head;
  }
//Insert Node
  public void insertNode(int num,int location){
    Node node=new Node();
    node.value=num;
    
    if(head==null){//Used to check if linked list is empty or not?
      createLL(num);
      return;
    }

    else 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++;
  }
//Delete Node
  public void deleteNode(int location){
    if(head==null){//Used to check if linked list is empty or not?
      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--;
    }

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

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

Answer 1

Empty list:

head -> null
tail -> null

Single node list:

head -> node1
node1.next -> null
tail -> node1

Multi node list:

head -> node1
node1.next -> node2
node2.next -> null
tail -> node2

Where -> means "reference points to". So there's no need to check for both head/tail for nullness. If either of them is null, it means the list has no nodes and is empty.

Answer 2

Nodes in a Linked-List exist as a value and memory address (for the next node) pair as you have implemented in the program above. For the last node, the end of the list is represented as NULL.

We check if the head is NULL since if the memory address of the next node in the linked list is NULL, then that means the end of the list has been reached. The tail of the linked list will always be NULL since it would represent the end of the list, and there would be no next node to point to.