java null pointer by accessing the second class -

hi have problem java coding. cannot figure out how point object.

the code looks this. have 2 classes. user , users. users class, manages user list linked list, class adds, removes , on. have code method sends message 1 user user.

the method live in user class , send method located in users class called user - send() access list. since main constructor of users set drive whole program. have write constructor. stuck here.

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.send();     }  } 

second class

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();     }      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()     {      }      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)     {         (user user: users)             if (user.matches(name))                 return user;         return null;     }      private string readname()     {         system.out.print("  names: ");         return in.nextline();     } } 

in first file (class) need initialize variable. private users users = new users();

that way, send method won't accessing null object.

looking @ code, though, have bigger problem: user accesses users , vice versa.

your problem not null pointer, problem program isn't correctly designed. if classa uses classb, classb should not use classa if ever possible. in case, need rethink logic.

to use suggested solution, when instance user in second file should pass reference current object. this

  private void add()   {     user user = new user(this);     ...   } 

and this

 private void use() {     user user = new user(this);     ... }