I wish to implement a Queue based in a simple linked list class, without using java.util.
When I call the addEnd method in List class through enqueue method, I receive a java.lang.NullPointerException, though I expect the second element.
Which solution can I take?
The node class
public class Node {
private int value;
private Node next;
public Node(int val) {
value = val;
}
public Node(int val, Node next) {
value = val;
this.next=next;
}
public Node(Node next) {
this.next=next;
}
public int getValue() {
return value;
}
public Node getNext() {
return next;
}
public void setNext(Node next) {
this.next = next;
}
public void displayNode() {
System.out.print(" "+value+" ");
}
}
My interface
public interface MyQueue {
void enqueue(int oVal);
int dequeue();
}
The List
public class List {
private Node first;
private Node last;
private int counter;
public List() {
first = null;
last = null;
}
public boolean isEmpty() {
return first==null;
}
public void addEnd(int val) {
Node n1 = new Node(val);
if( isEmpty() ) {
first = n1;
} else {
last.setNext(n1);
last = n1;
}
}
public int deleteStart() {
int temp = first.getValue();
if(first.getNext() == null){
last = null;
first = first.getNext();
}
return temp;
}
public void displayList() {
Node current = first;
while(current != null) {
current.displayNode();
current = current.getNext();
}
System.out.println("");
}
public int size() {
return counter;
}
}
The Queue
public class Queue implements MyQueue {
private List listQ;
public Queue() {
listQ = new List();
}
public boolean isEmpty() {
return listQ.isEmpty();
}
public void enqueue(int oVal) {
listQ.addEnd(oVal);
}
public int dequeue() {
return listQ.deleteStart();
}
public void displayQueue() {
System.out.print("Queue ");
listQ.displayQueue();
}
}
public class App {
public static void main(String[] args) {
Queue q1 = new Queue();
System.out.println("Two insertions");
q1.enqueue(4);
q1.enqueue(64);
q1.displayQueue();
System.out.println("Insert at the end : ");
q1.enqueue(23);
q1.displayQueue();
System.out.println("Delete an element at the begining of the queue");
q1.dequeue();
q1.displayQueue();
}
}
