How to lock/unlock a resource? - language-agnostic

Question

First 2 parts of this query has been addressed in link and further would like to understand more about third part before implementation.
Part III (3 points)
Implement a "lockable" doubly-linked list ADT: a list in which any node can be "locked." A locked node can never be removed from its list. Any attempt to remove a locked node has no effect (not even an error message). Your locked list classes should be in the list package alongside DList and DListNode.
First, define a LockDListNode class that extends DListNode and carries information about whether it has been locked. LockDListNodes are not locked when they are first created. Your LockDListNode constructor(s) should call a DListNode constructor to avoid code duplication.
Second, define a LockDList class that extends DList and includes an additional method
public void lockNode(DListNode node) { ... }
that permanently locks "node".
Your LockDList class should override just enough methods to ensure that
(1) LockDListNodes are always used in LockDLists (instead of DListNodes), and
(2) locked nodes cannot be removed from a list.
WARNING: To override a method, you must write a new method in the subclass with EXACTLY the same prototype. You can’t change a parameter’s type to a subclass. Overriding won’t work if you do that.
Your overriding methods should include calls to the overridden superclass methods whenever it makes sense to do so. Unnecessary code duplication will be penalized.
It is important to understand locking and unlocking a DListNode without using any existing Java package. With an approach of, boolean flag; being member of LockDListNode to represent lock/unlock status, setting of this flag is non-atomic. Because, if i set flag = true; this set operation could be a multi-line instruction at byte code level.
Once this clarification that became a bottleneck to implement lock/unlock is resolved, Inheritance and override stuff could be taken care easily after this.
My question:
Can you suggest an approach to implement lock/unlock feature on DListNode?
Note: This query has nothing to do with knowledge of Java.

Answer 1

The new class LockDListNode will extend DListNode. Now you need to understand inheritance. DListNode should have an delete operation method you need to override that method not not do anything. See the code template below:
public class DListNode {
public DListNode{
}
public void delete(){
// TODO some delete code here
}
}
public class LockDListNode extends DListNode {
public LockDListNode{
super();
}
#override
public void delete(){
// DO nothing here
}
}
Now if you do
DListNode unableToDeleteNode = new LockDListNode();
unableToDeleteNode.delete();
You created an object of LockDListNode and call delete method it will call the delete for the LockDListNode and not DListNode.