LinkedList Queue Implementation

Question

Hey guys/gals,
Just messing around here with a LinkedList Queue implementation. Look at the code. Theres the main class with the main method. Theres the Node class that holds info for my nodes. Theres the Queue class that sets up the Queue with a linkedlist. In my main method im having problems with accessing the 4th node. Its created and put into the queue(linkedList) but whenever it is processed with the toString() method the LinkedList is giving indexOutofBounds index 1 size 1. Not sure why. Any help is appreciated thank you in advance. Code is below.
import java.util.*;
public class DijAlg {
        static int source = 0;
        static Queue Queue = new Queue();
        static int NUMBEROFNODES = 4;
    public static void main(String args[]){
    Node NodeA = new Node(), NodeB = new Node(), NodeC = new Node(), NodeD = new Node();
    Node[] Nodes = {NodeA, NodeB, NodeC, NodeD};
    for (int i = 0; i< NUMBEROFNODES; i++){
     for (int v = 0; v< NUMBEROFNODES; v++){
         Nodes.dist[v] = 10000;
Nodes[i].previous[v] = 99;
Nodes[i].dist[source] = 0; // Distance from source to source
Queue.enqueue(NodeA);
Queue.enqueue(NodeB);
Queue.enqueue(NodeC);
Queue.enqueue(NodeD);
int i = 0;
while (!Queue.isEmpty()){
System.out.println("Node " + i);
System.out.println(((Node)Queue.peek()).toString());
Queue.dequeue();
i++;
static class Node {
int [] dist;
int [] previous;
Node(){
dist = new int [NUMBEROFNODES];
previous = new int [NUMBEROFNODES];
public String toString(){
String myString = "Dist ";
for(int i = 0; i < NUMBEROFNODES ; i++){
myString = myString.concat(String.valueOf(dist[i]) + ", ");
myString.concat("Prev ");
for(int i = 0; i < NUMBEROFNODES; i++){
myString = myString.concat(String.valueOf(previous[i]) + ", ");
return myString;
class Queue {
     LinkedList list;
     // Queue constructor
     Queue()
          // Create a new LinkedList.
          list = new LinkedList();
     boolean isEmpty()
     // Post: Returns true if the queue is empty. Otherwise, false.
          return (list.size() == 0);
     void enqueue(Object item)
     // Post: An item is added to the back of the queue.
          // Append the item to the end of our linked list.
          list.add(item);
     Object dequeue()
     // Pre: this.isEmpty() == false
     // Post: The item at the front of the queue is returned and
     // deleted from the queue. Returns null if precondition
     // not met.
          // Store a reference to the item at the front of the queue
          // so that it does not get garbage collected when we
          // remove it from the list.
          // Note: list.get(...) returns null if item not found at
          // specified index. See postcondition.
          Object item = list.get(1);
          // Remove the item from the list.
          // My implementation of the linked list is based on the
          // J2SE API reference. In both, elements start at 1,
          // unlike arrays which start at 0.
          list.remove(1);
          // Return the item
          return item;
     Object peek()
     // Pre: this.isEmpty() == false
     // Post: The item at the front of the queue is returned and
     // deleted from the queue. Returns null if precondition
     // not met.
          // This method is very similar to dequeue().
          // See Queue.dequeue() for comments.
          return list.get(1);
Edited by: venture on Dec 17, 2008 4:51 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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         

Answer 1

Thanks for the help Encephalopathic. I came to the same conclusion but it just didn't add up when I looked through the code. The problem was that whoever wrote the code for the queue class(I borrowed the code) had used get(1) instead of getFirst(), and remove(1) instead of removeFirst(). So it was totally skipping the first index A.K.A ----> 0. New code below. Also added a name field that helped me to debug, Node "A" was not being printed which told me it was skipping its index.
import java.util.*;
public class DijAlg {
        static int source = 0;
        static Queue Queue = new Queue();
        static int NUMBEROFNODES = 4;
    public static void main(String args[]){
    Node NodeA = new Node("A"), NodeB = new Node("B"), NodeC = new Node("C"), NodeD = new Node("D");
    Node[] Nodes = {NodeA, NodeB, NodeC, NodeD};
    for (int i = 0; i< NUMBEROFNODES; i++){
     for (int v = 0; v< NUMBEROFNODES; v++){
         Nodes.dist[v] = 10000;
Nodes[i].previous[v] = 99;
Nodes[i].dist[source] = 0; // Distance from source to source
Queue.enqueue(NodeA);
Queue.enqueue(NodeB);
Queue.enqueue(NodeC);
Queue.enqueue(NodeD);
int i = 0;
while (!Queue.isEmpty()){
System.out.println(((Node)Queue.peek()).toString());
Queue.dequeue();
i++;
static class Node {
int [] dist;
int [] previous;
String name;
Node(String theName){
dist = new int [NUMBEROFNODES];
previous = new int [NUMBEROFNODES];
name = theName;
public String toString(){
String myString = name;
myString = myString.concat(" Dist ");
for(int i = 0; i < NUMBEROFNODES ; i++){
myString = myString.concat(String.valueOf(dist[i]) + ", ");
myString = myString.concat("Prev ");
for(int i = 0; i < NUMBEROFNODES; i++){
myString = myString.concat(String.valueOf(previous[i]) + ", ");
return myString;
class Queue {
     LinkedList list;
     // Queue constructor
     Queue()
          // Create a new LinkedList.
          list = new LinkedList();
     boolean isEmpty()
     // Post: Returns true if the queue is empty. Otherwise, false.
          return (list.size() == 0);
     void enqueue(Object item)
     // Post: An item is added to the back of the queue.
          // Append the item to the end of our linked list.
          list.add(item);
     Object dequeue()
     // Pre: this.isEmpty() == false
     // Post: The item at the front of the queue is returned and
     // deleted from the queue. Returns null if precondition
     // not met.
          // Store a reference to the item at the front of the queue
          // so that it does not get garbage collected when we
          // remove it from the list.
          // Note: list.get(...) returns null if item not found at
          // specified index. See postcondition.
          Object item = list.getFirst();
          // Remove the item from the list.
          // My implementation of the linked list is based on the
          // J2SE API reference. In both, elements start at 1,
          // unlike arrays which start at 0.
          list.removeFirst();
          // Return the item
          return item;
     Object peek()
     // Pre: this.isEmpty() == false
     // Post: The item at the front of the queue is returned and
     // deleted from the queue. Returns null if precondition
     // not met.
          // This method is very similar to dequeue().
          // See Queue.dequeue() for comments.
          return list.getFirst();
Here is the output
run:
A Dist 0, 10000, 10000, 10000, Prev 99, 99, 99, 99,
B Dist 0, 10000, 10000, 10000, Prev 99, 99, 99, 99,
C Dist 0, 10000, 10000, 10000, Prev 99, 99, 99, 99,
D Dist 0, 10000, 10000, 10000, Prev 99, 99, 99, 99,
BUILD SUCCESSFUL (total time: 2 seconds)