Question

My Node class, representing a node of a linked list, is defined like this:

public class Node
{
    Node next;
    String data;

    public Node (String data)
    {
        this.data = data;
    }
}

and I’m using it like this:

Node node, head, tail;
String name; // name to be entered
int count = 0;

// initialize the head to null

head = null;

do
{
    System.out.print ("Enter a name. Type q to end.");
    name = stdin.readLine ();

    // create a new node if the user doesn't type q
    if (!name.equals ("q"))
    {
        node = new Node (name);
        node.next = head;
        count++;

        // update the head to point to the new front of the list
        head = node;
    }
}
while (!name.equals ("q"));  // loop continues until "quit" selected
node = head; 

Suppose I wanted to back the names up to a method in case I modify the original list. How can I do this? Without writing it to a file.

Name is the variable that gets stored in the linked list and after the user presses q I want to modify the list while keeping what the user stored as a back up in case he/she wants to backtrack or see the original list.

Answer 1

So it sounds as though you want to keep a history of the prior names for each element in the linked list. I would suggest that you store an array or linked list within each node of the linked list that shows the prior history of that item. For instance:

public class Node
    {
    Node next;
    String data;
    LinkedList<String> history;

    public Node (String data)
    {
        this.data = data;
    }
}

You could populate this in many ways, that all really depend upon your use case.

Also, why implement your own linked list? Java already comes with a linked list implementation (java.util.LinkedList). I’d suggest using this if you need an ordered list of the linked list variety. If you did this, then create a new data structure to be contained within it that has a name and history, then just maintain the history within that, such as:

public class DataItem
    {
    String data;
    LinkedList<String> history = new LinkedList<>();

    public DataItem (String data)
    {
        this.data = data;
    }

    public void setData (String data)
    {
        this.history.add(0, this.data);
        this.data = data;
    }
}

Ultimately, remember that strings are immutable in Java. So, a string cannot be modified. You only need to keep a reference to the prior string somewhere, you don’t need to copy the value.

To ultimately copy a tree of objects, you need to do what’s called a deep copy, basically going through the full structure and all collections, and cloning each object into a new object.