Null pointer exception with static arrayList

[Adam Chalkley]

Hi guys,

It's been a while since I've coded in Java I seem to be getting a null pointer exception but can't seem to figure out why.

import java.util.Scanner;

public class PhoneBill {
	
	private String name;
	private static int ID = 0;
	private double total;
	private double totalAfterTax;
	private final double taxRate = 0.09;
	private int messagesSent = 0;
	private Scanner input = new Scanner(System.in);
	
	public PhoneBill(String name,int messagesSent) {
		
		this.name = name;
		this.messagesSent = messagesSent;
		total = 0;
		totalAfterTax = 0;
		ID++;
	}
	
	public String getName() {
		return name;
	}

	public int getID() {
		return ID;
	}

	public double getTotal() {
		return total;
	}

	public double getTotalAfterTax() {
		return totalAfterTax;
	}

	public int getMessagesSent() {
		return messagesSent;
	}

	public double calculatePrice() {

		for (int i = 0; i < messagesSent; i++) {

			total += 0.25;

		}

		totalAfterTax = total;
		totalAfterTax = (total * taxRate) / 100;

		return totalAfterTax;
	}

	public void displayPrice() {

		System.out.println("Number of message sent: " + messagesSent);
		System.out.println("Subtotal: $" + total);
		System.out.println("Tax: $" + taxRate);
		System.out.println("Total: $" + totalAfterTax);

	}
}

import java.util.ArrayList;
import java.util.Scanner;


public class Input {
	
	
	public Input() {}
	
	private Scanner input = new Scanner(System.in);
	
	public void promptUser(ArrayList<PhoneBill> users) {
		
		System.out.println("enter ID of user");
		int userID = input.nextInt();
		String str1 = input.nextLine();
		
	    for(int i = 0; i < users.size(); i++) {
	    	
	    	if(userID == users.get(i).getID()) {
	    		
	    		users.get(i).displayPrice();
	    	}
	    	
	    }
		
	}
	

}

import java.util.ArrayList;
import java.util.Scanner;


public class Main {
	
	static ArrayList<PhoneBill> users = new ArrayList<PhoneBill>();
	static Input input;
    static Scanner scanner = new Scanner(System.in);

	public static void main(String[] args) {
		
		
		boolean quit = false;
		
		while(!quit) {
			
			
			System.out.println("press 1 to add new user");
			System.out.println("press 2 to view users Bill");
			System.out.println("press 3 to quit");
			
			int choice = scanner.nextInt();
			String str1 = scanner.nextLine();
			
			switch(choice) {
			
			case 1:
				addUser();
				break;
			case 2:
				System.out.println("two");
				input.promptUser(users);
				break;
			case 3:
				quit = true;
				
			}
	
		}
	
	}
	
	public static void addUser() {
		
		System.out.println("enter Name of User");
		String name = scanner.nextLine();
		
		System.out.println("enter number of messages sent by user");
		int messages = scanner.nextInt();
		
		PhoneBill user = new PhoneBill(name,messages);
		users.add(user);
		
	}

}

any idea why this may be happening?

thanks

[Knute Snortum]

Well, the stack trace and the error message would really help...

[Adam Chalkley]

Knute Snortum wrote:Well, the stack trace and the error message would really help...

error message : twoException in thread "main"
java.lang.NullPointerException
at Main.main(Main.java:33)



the stack trace doesn't seem to be printing maybe I've got something wrong?

public class Main {
	
	static ArrayList<PhoneBill> users = new ArrayList<PhoneBill>();
	static Input input;
    static Scanner scanner = new Scanner(System.in);

	public static void main(String[] args) {
		
		
		boolean quit = false;
		
		while(!quit) {
			
			
			System.out.println("press 1 to add new user");
			System.out.println("press 2 to view users Bill");
			System.out.println("press 3 to quit");
			
			int choice = scanner.nextInt();
			String str1 = scanner.nextLine();
			
			switch(choice) {
			
			case 1:
				addUser();
				break;
			case 2:
				System.out.println("two");
				
				try {
				input.promptUser(users);
				}
				catch(Exception e) {
					
				   e.printStackTrace(System.out);
				}
				break;
			case 3:
				quit = true;
				
			}
	
		}
	
	}
	

[Knute Snortum]

What's line 33 in the original program?  It looks like you removed the import statements which changes all the line numbers below.

[Adam Chalkley]

also I tried a bit of debugging to see if the array list grew when I called the addUser() function and the size did increase so I have no idea why I would be getting a null pointer exception :/

[Knute Snortum]

I bet it's line 31.  You use a variable input but you never instantiate it.

[Adam Chalkley]

Knute Snortum wrote:What's line 33 in the original program?  It looks like you removed the import statements which changes all the line numbers below.

line 33 was

input.promptUser(users);

[Adam Chalkley]

wow that was the problem,such a simple mistake overlooked :/


thanks Knute

[Knute Snortum]

You bet.

And to get a stack trace, I would use just e.printStackTrace() with no argument.  That will send it to the standard error output.

[Adam Chalkley]

much appreciated Knute,it's been a while since I have touched Java