Question

My problem: My delete node method works fine for deleting any specified node from a user created list except for the first element. How do I get this method to be able to delete the front of a list?

public void deleteNode(node spot, node front) {
    node current = spot, previous = front;

    while(previous.next != current) {
        previous = previous.next;
    }
    previous.next = current.next;
}

This is the full program code.

import java.io.*;

public class LinkedList {
public int num;
public node front;

//set front to null
public void init() {
    front = null;
}

//make a new node
public node makeNode(int num) {
    node newNode = new node();
    newNode.data = num;
    newNode.next = null;
    return newNode;
}

//find the end of a list
public node findTail(node front) {
    node current = front;

    while(current.next != null) {
        current = current.next;
    }
    return current;
}

//find a specified node
public node findSpot(node front, int num) {
    node current = front;
    boolean searching = true, found = false;

    while((searching)&&(!found)) {
        if(current == null) {
            searching = false;
        }
        else if(current.data == num) {
            found = true;
        }
        else {
            current = current.next;
        }
    }
    return current;
}

//delete a specified node
public void deleteNode(node spot, node front) {
    node current = spot, previous = front;

    while(previous.next != current) {
        previous = previous.next;
    }
    previous.next = current.next;
}

//add nodes to the end of a list
public void add2Back(node front, int num) {
    node tail;

    if (front == null) {
        front = makeNode(num);
    }
    else {
        tail = findTail(front);
        tail.next = makeNode(num);
    }
}

//add nodes after a specified node
public void addAfter(int num, node spot) {
    node newNode;
    newNode = makeNode(num);
    newNode.next = spot.next;
    spot.next = newNode;
}

//print out a list
public void showList(node front) {
    node current = front;

    while(current != null){
        System.out.println(current.data);
        current = current.next;
    }
}

public static void main(String [] args) throws IOException{
    //make a new list and node
    LinkedList newList = new LinkedList();
    node newNode = new node();
    //add data to the nodes in the list
    for(int j = 1; j < 10; j++){
        newList.add2Back(newNode, j);
    }
    //print out the list of nodes
    System.out.println("Auto-generated node list");
    newList.showList(newNode);

    //ask the user how many nodes to make, make those nodes, and show them
    System.out.println("Please enter how many nodes you would like made.");
    BufferedReader inputReader = new BufferedReader(new InputStreamReader(System.in)) ;
    String inputData = inputReader.readLine();
    int listLength = Integer.parseInt(inputData);
    LinkedList userList = new LinkedList();
    node userNode = new node();
    for(int j = 1; j < listLength; j++) {
        userList.add2Back(userNode, j);
    }
    userList.showList(userNode);

    //ask the user to add a new node to the list after a specified node
    System.out.println("Please enter a number for a node and then choose a spot from the list to add after.");
    BufferedReader inputReader2 = new BufferedReader(new InputStreamReader(System.in)) ;
    String inputData2 = inputReader2.readLine();
    BufferedReader inputReader3 = new BufferedReader(new InputStreamReader(System.in)) ;
    String inputData3 = inputReader3.readLine();
    int newNodeValue = Integer.parseInt(inputData2);
    int nodeInList = Integer.parseInt(inputData3);
    userList.addAfter(newNodeValue, userList.findSpot(userNode, nodeInList));
    userList.showList(userNode);

    //ask the user to delete a specified node
    System.out.println("Please enter a node to delete.");
    BufferedReader inputReader4 = new BufferedReader(new InputStreamReader(System.in)) ;
    String inputData4 = inputReader4.readLine();
    int nodeToDelete = Integer.parseInt(inputData4);
    userList.deleteNode(userList.findSpot(userNode, nodeToDelete), userNode);
    userList.showList(userNode);
}
}

Answer 1

The problem is that your deleteNode does not modify the front member variable of your list, because the front variable inside deleteNode is a method parameter, not the instance variable front.

Here is what you need to do:

• Exposing front as a public member of the LinkedList is a violation of encapsulation. Make front a private variable.
• Remove parameter front from all methods that take it; use the private member front instead.
• Add a check in deleteNode to see if the spot to be deleted is the front. If it is, perform a special operation that assigns front a new value, and exit; otherwise, do the while loop that you already have.