在链表Java中复制节点

Question

just to start off, this is homework and thank you for your assistance ahead of time. I keep getting stuck on little problems so I am hoping you guys can help me with one. What I am trying to do is create a linked list that has multiples functions. The one I am having trouble with is sorting(I can do the other ones). Each node holds a string, an integer and a double. I need to be able to sort by each of these and by the order it was inputted, on the user's request. ***It is also important to mention that the variables in my object are private and my object is called list1. Basically, I have to make one linked list for the chronological order and one for each other order.

My plan is to insert the nodes in their correct order as the user inputs them. So as the user inputs a node, that node needs to go in the correct place in the chronological list and in the other lists. So, I need to copy the node to do this. However, I cannot simply just say

icopy(copy for integer) = newNode(node the user just inputted)

That only changes the address. When I went to my instructor he told me that I should say:

    icopy.data = newNode.data;

("data" being the shortcut way of mentioning that I need to get the individual data types within the node.) So I wrote:

    icopy.GetI() = newNode.GetI();  

When I do this I encounter this error: unexpected type required:variable, found:value. I am not sure what to do. Any assistance would be appreciated and I would be happy to clarify anything.

*GetI: method in my object that gives access to the integer value in each node.
*p: pointer for the Chronological
*pi: pointer for the integer.
*fi: front of the integer linked list

  public static void main(String args[])
  {
    String repeat = "y";
    boolean inserted = false;
    list1 fChr = null;
    list1 p = fChr;
    list1 icopy = null;
    list1 scopy = null;
    list1 dcopy = null;
    list1 fd = fChr;//front of the double list
    list1 fi = null;//front of the integer list
    list1 fStr = fChr;//front of the string list~
    while(repeat.equals("y"))//while the user agrees to adding a new node
    {
        if(fChr == null)// if the front is empty
        {
            fChr = new list1();//create a new node by calling object and sets it as the front
        }
        else
        {
            p = fChr;
            while(p.next != null)//finds the end of the Linked list
            {
                p = p.next;//moves the pointer p down the list
            }
            list1 newNode = new list1();
            icopy.GetI() = newNode.GetI();// make a copy of newNode
            p.next = nexNode;//put in chronological order
            while(p != null)
            {
                if(fi == null)
                {
                    fi = n;
                }
                else if(n.GetI() < fi.GetI)//check at beginning
                {
                    //put at beginning
                }                    

                else if(icopy.GetI() < p.next.GetI())//check in between nodes
                {
                    //put in between
                }
                //does it go at the end
            }
        }
        repeat = JOptionPane.showInputDialog("Would you like to add a node [y/n]");
    }
    PrintMenu(fChr, fi, fd, fStr);// sends the user to the menu screen
}

Answer 1

There are a few things here that you are not understanding. Firstly, in Java iCopy.getI() = ... makes no sense. When a method returns a value it needs to be assigned to a variable if you wish to change it. If you want to change the instance variable you need a separate method called something like iCopy.setI() .

It sounds as though you're not asking for help with the sorting so I'll restrict my answer to creating copies of the list.

What your professor is getting at is that the easiest way to ensure the data is consistent in your several linked lists is to separate the class storing the data from the nodes of the list. So I would expect your class structure to end up looking something like:

class Data {
    private final int intValue;
    private final String strValue;
    private final double doubleValue;
}

class Node {
    private final Data data;
    private Node next;

    public Node(Data data) {
        this.data = data;
        this.next = null;
    }
}

Now if you want to create a new linked list with the same data as the old one then you can add a constructor to Node that creates a reference to the original data:

class Node {
    public Node copy() {
        Node copy = new Node(data);
        if (next != null)
            copy.next = next.copy();
        return copy;
    }
}

Hopefully you can see what that does: it creates a new node referencing the same data as this one and then uses recursion to copy the rest of the list.

Now creating each of the sort orders could look like:

Node listByInt = list.copy();
/* code to sort listByInt according to data.intValue */

Add a comment if you want some hints on sorting as well but I suggest you get your code to the point of having equal copies of lists before attempting that.

As a final note, you don't necessarily need to have separate linked lists to solve this problem. An alternative would be to store the original insertion order in the node. You could then sort by any order (including original insertion order) before printing the list. Personally I'd prefer that as a solution unless there are performance issues (eg you need to use each sorted list many times).