调用类方法时发生错误java.lang.NullPointerException

我正在尝试制作一个使用姓名，开始日期，员工薪水的程序。因此，香港专业教育学院开设了几门课，包括我的电话课：

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 is " + 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 you are trying to assign a Supervisor to 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 was not a valid input");
            return;
        }
    }
    public Name(Name name) {
        if(name != null) {
            firstName = name.firstName;
            middleName = name.middleName;
            lastName = name.lastName;   
        }
    }

当我运行它时，我会得到错误提示Supervisor.workerName = supervisor.workerName;。我通过阅读答案https://stackoverflow.com/a/10464598/5339899了解错误，但我无法理解为什么supervisor.workerName为null。

我删节了与该问题无关的类中的一些代码，因此，如果它似乎缺少重要的内容，请发表评论，我将使用所需的代码进行更新