Thread: Java nulpointer errsors

Hi I have some problem with my java coding. I cannot figure out hot to point the an object. the code looks like this. I have two classes. User and Users. Users class, which manages a user list as Linked List, is the class that adds, removes and so on. I have to code a method as well than sends message from one user to another user. the method live in User class and the also another send method is located in Users class which will be called by User - send() to access the list. since main constructor of Users is set to drive whole program. I have to write another constructor. But I am stuck here.

public class User{
        private Users users;
        private String name;
        ...
        public void send(){
            users.send();
        }
    }
 
    public class Users{
        private LinkedList<Users> users = new LinkedList<User>();
        public static void main(String[] args){
            new Users();
    }
        private void send(){
        }
    }
I am getting Nullpointer errsors[COLOR="Silver"]
 
--- Update ---
 
[/COLOR]package assignment;
 
import java.util.*;
 
public class Users 
{
	public static void main(String[] args)
	{
		new Users();
	}
 
	private LinkedList<User> users = new LinkedList<User>();
	private LinkedList<Email> emails = new LinkedList<Email>();
 
	public Users()
	{		
		menu();
	}
	public Users(User user)
	{
		users.add(new User(this));
	}
 
	private void menu()
	{
		char c = readChoice();
		while (!isEnd(c))
		{
			execute(c);
			c = readChoice();
		}
	}
	private char readChoice()
	{
		System.out.print("Choice (a/d/g/u/x): ");
		return In.nextChar();
	}
	private boolean isEnd(char c)
	{
		return c == 'x';
	}
	private void execute(char c)
	{
		switch(c)
		{
		case 'a': add(); break;
		case 'd': delete(); break;
		case 'g': break;
		case 'u': use(); break;
		default : System.out.println("    Invalid choice");
		}
	}
 
	private void add()
	{
		User user = new User();
		if (!exists(user.getName()))
		{	
			users.add(user);
			System.out.println("    User " + getIndex(user) + ": " + user.getName());
		}
		else
			System.out.println("already exists");
	}
 
	private int getIndex(User user)
	{
		int index = users.indexOf(user) + 1;
		return index;
	}
 
	private void delete()
	{
		User user = user(readName());
		if (user != null)
			users.remove(user);
		else
			System.out.println("  No such name");
	}
 
	public void send()
	{
		//System.out.println("hi");
		if (user(readName()) != null)
			System.out.println("I am here");
		else
			System.out.println("sorry not here");
	}
 
	private void use()
	{
		User user = new User();
		if (exists(user.getName()))
			user.use();
		else
			System.out.println("    No such user");
	}
 
	private boolean exists(String name)
	{
		return user(name) != null;
	}
 
	private User user(String name)
	{
		for (User user: users)
			if (user.matches(name))
				return user;
		return null;
	}
 
	private String readName()
	{
		System.out.print("  Names: ");
		return In.nextLine();
	}
}[COLOR="Silver"]
 
--- Update ---
 
[/COLOR]package assignment;
 
import java.util.*;
 
public class User 
{
	private String name;
	private Users users;
 
	public User()
	{
		this.name = readName();
	}
	public User(Users users)
	{
		this.users = users;
	}
	private String readName()
	{
		System.out.print("  Name: ");
		return In.nextLine();
	}
	public boolean matches(String name)
	{
		return this.name.equals(name);
	}
	public String getName()
	{
		return this.name;
	}
 
	public void use()
	{
		char c = readChoice();
		while (!isEnd(c))
		{
			execute(c);
			c = readChoice();
		}
	}
	private char readChoice()
	{
		System.out.print("  Choice (l/r/s/d/x): ");
		return In.nextChar();
	}
	private boolean isEnd(char c)
	{
		return c == 'x';
	}
	private void execute(char c)
	{
		switch(c)
		{
		case 'l': look(); break;
		case 'r': read(); break;
		case 's': send(); break;
		default : System.out.println("    Invalid choice");
		}
	}
	private void look()
	{
 
	}
	private void read()
	{
 
	}
	private void send()
	{
		users = new Users(this);
		users.send();
	}
 
}

--- Update ---

these are the actual codes

when I try to use send method in the User class I am getting Null pointer exception as it is not initialized with new User(). The reason I have not done that is that new User bill bring me back to the begin of the program as it meant to be so. I'll have to write another constructor to point the users.send() from User class to Users class. However, I don't know how to do the second constructor.