利用链表的方法不起作用

Question

The display method is my program is not displaying anything at all.

The program has to have someone entire names into a circular linked list, then backup the linked list into another circular linked list.

Then it user must delete names until 1 is left, and the display the winner along with the list of original names, using the backup in the order that they were entered

import java.io.*;
import java.util.*;
import java.text.*;

public class Linkedlist
{
    static public class Node
    {
        Node prev, next;
        String data;
    }


    public static void delete (Node tail) throws IOException
    {

        BufferedReader stdin = new BufferedReader (new InputStreamReader (System.in));
        System.out.println ("Please input a name to be deleted");
        String tobedeleted = stdin.readLine ();
        Node delete = tail;
        while (delete.prev != null)
        {
            if (delete.data.equals (tobedeleted))
            {
                String temp = delete.data;
                delete.data = tail.data;
                tail.data = temp;
                tail = tail.prev;
            }
            delete = delete.prev;
        }
    }


    public static String findvictor (Node tail) throws IOException
    {
        int size = 0;
        for (Node n = tail ; n.prev != null ; n = n.prev)
        {
            size++;
        }
        if (size == 1)
        {
            return tail.data;
        }
        else
        {
            delete (tail);
            return findvictor (tail.prev);
        }
    }


    public static void backup (Node tail, Node backuptail)
    {
        Node tobebackuped = tail;
        Node backuphead = null;
        Node backup = new Node ();
        backuptail = backup;
        while (tobebackuped.prev != null)
        {
            backup.data = tobebackuped.data;
            backuphead = backup;
            backup = new Node ();
            backup.next = backuphead;
            backuphead.prev = backup;
            tobebackuped = tobebackuped.prev;
        }
    }


    public static void display (Node tail, Node backuptail) throws IOException
    {
        System.out.println ("CONGRATULATIONS, " + findvictor (tail) + ", YOU ARE THE WINNER!");
        System.out.println ("");
        System.out.println ("This is a list of all the contestants:");
        Node current = backuptail;
        while (current.prev != null)
        {
            System.out.println (current.data);
            current = current.prev;
        }
    }


    public static void main (String[] args) throws IOException
    {
        BufferedReader stdin = new BufferedReader (new InputStreamReader (System.in));
        Node head = null;
        Node node = new Node ();
        Node tail = node;
        while (true)
        {
            String str = stdin.readLine ();
            if (!str.equals ("fin"))
            {
                node.data = str;
                head = node;
                node = new Node ();
                node.next = head;
                head.prev = node;
            }
            else
            {
                break;
            }
        }
        Node backuptail = null;
        backup (tail, backuptail);
        display (tail, backuptail);
    }
}

Answer 1

You've fallen into a simple trap in Java. You've passed a value that you want to update into a function, and expected the value to be updated in the calling method. Right here:

Node backuptail = null;
backup (tail, backuptail);
display (tail, backuptail);

Here's what's happening: Java is passing your pointer by value. That means it's creating a copy of your pointer (backuptail) for use within the method backup(). That means your local variable in main() never gets updated.

The fix is simple. Change your backup method to return the value instead:

public static Node backup (Node tail)
{
    Node tobebackuped = tail;
    Node backuphead = null;
    Node backup = new Node ();
    Node backuptail = backup;
    while (tobebackuped.prev != null)
    {
        backup.data = tobebackuped.data;
        backuphead = backup;
        backup = new Node ();
        backup.next = backuphead;
        backuphead.prev = backup;
        tobebackuped = tobebackuped.prev;
    }

    return backuptail;
}

Then change your method calls apropriately:

Node backuptail = backup (tail);
display (tail, backuptail);

Now the resulting backup pointer is stored locally within your main, and can be passed to display().