Question

I am trying to implement the deletion function in

public static boolean delete(Object d, ListElement head){
ListElement p=find(d,head);
if(p.data==null){return false;}
else {
    System.out.println("Delete Successfully!");
    if(head.data==d){head=head.next;}
    else{
    while(head.next.data!=d){
        head=head.next;
    }
    head.next=head.next.next;}
    return true;}

}

This function basically check if the element d is in the list,
-if not->return false;

-else check whether the element is the first element of the list, if true, change the head to its next,

-else traverse to the list element in front of it.

The problem is case the element to delete is the first element, such as boolean s=ListElement.delete(1,d); I cannot use “head=head.next;” to assign new value to head. But java is passed by reference, why cannot I change that?

//actually I found my question is whether we can change the reference passed to the function inside the function
like:

    void fun(dog a){
    a=new dog("Po");
}
main()   
{dog b=new dog("Jo");fun(b);}

//so will b be changed?

Answer 1

The reference to the first list element is either held by the list object itself or by an “invisible” root element (in case of single linked list).

So you either have to pass the entire list to the method or, if you have that invisible root, pass the root as head.

public static boolean delete(Object d, MyLinkedList<ListElement> list) {

  ListElement head = list.getHead();
  if (head.data.equals(d)) {   // <- ALWAYS use equals, never == to compare objects!!
    list.setHead(head.next);
  } else {
    ListElement element = head.next;

    // ... the rest is similiar to your algorithm

  } 
}