null pointerException handling

[craig long]

I am new to java and am doing self study using Lawrenceville press Guide to Java. In this last exercise I am working on, the classes compile fine but I am getting a nullpointer error referencing the lines - "while (current.getNext() != null)" - and - "anObject.enqueue((String)entry);". I am having trouble getting around it. I have tried to handling the exceptions using throws, catch etc. but I am not setting the code up right or something in order to get the program to accept the input. What can I do to get the program to accept the user input? thank you.

class QueueList {
	private Node head;
	
	public QueueList(){
		String head = null;
	} 
	
	public void enqueue(Object anObject) {
		//Node current = head;
		Node current = new Node();		  
		current = head;
		Node newNode = new Node(anObject);
		while (current.getNext() != null) {
			current = current.getNext();
		}
		current.setNext(newNode);
		newNode.setNext(null);
	}
	
	public Object dequeue() {
		Node current = head;
		head = current.getNext();
		return current.getData();
	}
	public Object front() {
		return head.getData();
	}
	public int size(){
		int sizeAmt = 0;
		Node current = head;
		while (current != null) {
			sizeAmt++;
			current = current.getNext();
		}
		return sizeAmt;
	}
	private class Node {
		private Object data;
		private Node next;
	
		public Node(Object newData) {
			data = newData;
			next = null;
		}
		public Node() {
			next = null;
		}
		
		public Node getNext() {
			return(next);
		}
	
		public void setNext(Node newNode) {
			next = newNode;
		}
		
		public Object getData() {
			return(data);
		}
	}
}

 import java.util.Scanner;
public class Exercise4Try2_QueueList {
    
    public static void main(String[] args) {
    	Scanner input = new Scanner(System.in);
    	String choice;
    	String entry;
    	
    	QueueList anObject = new QueueList();
    	//anObject.enqueue("SomeText");
    	
    	do {
    		System.out.print("Add/Remove/Count/Front/Quit: ");
    		choice = input.next();
    		if (choice.equalsIgnoreCase("a")) {
    			System.out.println("Enter a string to add: ");
    			entry = input.next();
    			anObject.enqueue((String)entry);
    		} else if (choice.equalsIgnoreCase("r")) {
    			System.out.println("Dequeueing: " + (String)anObject.dequeue());
    		} else if (choice.equalsIgnoreCase("c")) {
    			System.out.println("Items in queue: " + anObject.size());
    		} else if (choice.equalsIgnoreCase("f")) {
    			System.out.println("Front of queue: " + (String)anObject.front());
    		} else if (choice.equalsIgnoreCase("q")) {
    			System.out.println("Quitting...");
    		} else {
    			System.out.println("There is not a choice linked to that submission.");
    		}
    	} while (!choice.equalsIgnoreCase("q"));
    }
}


class Node {
}

[W. Joe Smith]

Welcome to JavaRanch!

Please edit your post to place the code inside code tags. It makes the code much easeir to read, you can retain formatting, and will increase the chances of getting a helpful response.

[David Newton]

You set current to a new node, then to head, which is null:

Node current = new Node();          
current = head;  

One or both of those statements are wrong... even if head is the correct value, why set current to something only to throw it away on the next line?

[craig long]

Thank you. I am not sure why I did that. Followng too close to examples in the book I guess. I removed the line and now it works great. Thank you again.