Error java.lang.NullPointerException while calling class method -

im attempting make program takes names, dates of start, salary of employees. ive made several classes including calling class:

testworker.java

public class testworker {     public static void main(string[] args) {         worker w1, w2, w3;         w1 = new worker ("robert  william hunter", "23/10/2005", 35000.00);         w2 = new worker ("john smith", "15/11/2005", 25000.00);         w3 = new worker ("mary jane hull", "06/09/2007");         w2. setsalary(20000.00);         w2.setsupervisor(w1);         w3.setsupervisor(w1);         system.out.println("number of workers = " + worker.gethowmanyworkers() +" \n");         system.out.println("supervisor of john " + w2.getsupervisorname());         system.out.println(w1.tostring()+" \n");         system.out.println(w2.tostring()+" \n");         system.out.println(w3.tostring()+" \n");     } } 

worker.java

public class worker {     private name workername;     private mydate datejoiningcompany;     private worker supervisor;     public worker(string name, string date, double salary) {         workername = new name(name);         datejoiningcompany = new mydate(date);         salary = (float)salary;     }     public worker(string name, string date) {         workername = new name(name);         datejoiningcompany = new mydate(date);     }      public void setsupervisor(worker supervisor) {         if(supervisor != null) {             supervisor.workername = supervisor.workername;  //error here             supervisor.datejoiningcompany = supervisor.datejoiningcompany;         }         else {             system.out.println("the person trying assign supervisor has no supervisor");         }     }     public string tostring() {         if(supervisor != null) {             return (workernumber + " " + workername.tostring() + " " + datejoiningcompany.tostring() + " " + supervisor.workername.tostring() + " " + salary);         }         else {             return (workernumber + " " + workername.tostring() + " " + datejoiningcompany.tostring() + " " + salary);         }     } } 

name.java

import java.util.stringtokenizer;  public class name {     private string firstname;     private string middlename;     private string lastname;     public name(string name) {         stringtokenizer tokens;         tokens = new stringtokenizer(name," ");         int numtokens = tokens.counttokens();         if(numtokens == 2) {             firstname = tokens.nexttoken();             middlename = null;             lastname = tokens.nexttoken();         }         else if(numtokens == 3) {             firstname = tokens.nexttoken();             middlename = tokens.nexttoken();             lastname = tokens.nexttoken();         }         else {             system.out.println("that not valid input");             return;         }     }     public name(name name) {         if(name != null) {             firstname = name.firstname;             middlename = name.middlename;             lastname = name.lastname;            }     } 

when run error on line supervisor.workername = supervisor.workername;. understand error reading answer https://stackoverflow.com/a/10464598/5339899 cant understand why supervisor.workername null.

i trimmed out of code in classes felt irrelevant question if seems missing important please comment , update desired code

you never initialize supervisor variable in class, you're getting null pointer exception.

one more thing- common convention class name starts uppercase , variables lowercase, supervisor name variable not good. not recommended have 2 variables same name, 1 uppercase , 1 lower case, makes lines 1 confusing: supervisor.workername = supervisor.workername;