java - Nullpointer exception when iterating over a list -

i have arraylist of user objects.

private arraylist<user> users; 

this constructor of class contains array list "users" :

public communicationthread(socket socket, arraylist<user> users) {     super("communicationthread");      try {         // initialize data fields          this.users = users;          system.out.println("client connected : " + client_socket.getinetaddress().tostring());     } catch(ioexception e) {         system.out.println("could not initialize communication properly. -- communicationthread.\n");         e.printstacktrace(client_out); // inform client.     } } 

i wrote procedure "search_user" searches user given username.

private user search_user(string username) {     for(int = 0; < this.users.size(); i++) {         user found_user = this.users.get(i);         if(found_user.username.equals(username))             return found_user;     }            return new usernotfound(); } 

when i'm trying search user gives me nullpointer exception.

exception in thread "communicationthread" java.lang.nullpointerexception     @ directoryservice.communicationthread.search_user(communicationthread.java:92) 

the "users" list empty when i'm searching, should return new "usernotfound" object , not supposed throw nullpointer exception.

looks you're using different constructor create object of communicationthread. instead doing useless null checks , uglyfy code, recommend initialize list when declaring field:

public communicationthread {      //updated declaration of users interface list      private list<user> users = new arraylist<>();      //rest of code... }