An incomplete implementation of a linked list with a sentinel

Sentinel.java

public SLinkedList {
    // header always points to the sentinel node; header is NEVER null
    ListNode header;
    
    public SLinkedList() {
        // point to a dummy ListNode that serves as the sentinel node
        // its data doesn't matter
        header = new ListNode(-1); 
    }
  
    // finish this method
    public void add(int d) {
    
    }
  
    private class ListNode {
      int data;
      ListNode next;
      
      ListNode(int d) {
         data = d;
         next = null;
      }
    }
}